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Test Driven Development in Java

April 12, 2011 by Krishna Srinivasan Leave a Comment

Introduction

Test driven development a software development methodology which ensure at an early stage that the code written by the developer is what it is supposed to do, which eventually improves the design, produces testable code and reduces the amount of re-engineering that would have been necessary because of the wrong assumptions. Test Driven Development (TDD) is widely used in extreme Programming. This document gives an overview on TDD.

also read:

  • Java Tutorials
  • Java EE Tutorials
  • Design Patterns Tutorials
  • Java File IO Tutorials

Drawback with the traditional testing

  • Testing Poorly Written Code: Defects with the software system makes it unusable and the approach followed for getting rid of defect is by testing the code and trying to break it. But testing once the code is frozen is a lengthy, slow and a costly process to further develop it.
  • Maintenance: Poorly written code apart from having a bad design is also difficult to change, since it’s difficult to understand the code. Even if the modifications are incorporated there can be some sideeffects on other parts of the functionality.
  • Failure to meet the requirement: Failing to deliver as per the requirement of the customer on time would not only lead to a delayed delivery but also extends the deadline , the requirement of the customer also changes during this period and the process keeps on going

The Solution -Test Driven Development(TDD)

What is TDD?

“Before we write the code, think about what your code will do.Write a test that will use the methods that hasn’t been written yet.”

TDD completely twirl traditional development around Kent Beck, had popularized TDD in eXtreme Programming he defines 2 simple rules one is write new business code only when your automated test has failed. second ,as you keep changing remove any duplication that you find. Beck also explained how these two simple rules generate complex individual and group behavior

On a lower level we test-drive code using the technique we call TDD,on a higher level-that of features and funcunality of the system -we test drive the system using a similar technique we call a acceptance TDD.

The advantage of TDD is

High Quality with TDD:as this way of programming that encourages good design and is a disciplined process that helps us avoid programming errors.This is achieved by making us write small,automated tests,which eventually build up a small alarm system for protecting our code from regression.

The short development cycle that TDD promotes is a well geared toward writing high-quality code from the start. Due to the short cycle we deviate from the traditional development methodology.

i.e. Design->Implement->Test the implementation

instead we follow

Test->Code->Refractor

i.e write the test first and then write the code to see that the test passes and finally refractor to remove any duplication and taking the code toward the best design we can imagine.

Improves the Quality:Quality of a s/w can be measured in terms of the degree to which the s/w fulfils the users requirement as well as in terms of the internal quality of the software that translates to external qualities like cost of development ,maintenance etc.by following TDD ,its assured that there is no code in the system that is developed that is unwanted.

Less time in Fixing code: Finding and fixing defects at an early stage is what we achieve practicing TDD which invariably reduces the development time.

Acceptance TDD

TDD helps us to write code having high technical quality .since the correctness of the code we develop with TDD is for isolated block of code rather than for the feature or functionality of the software .Acceptance TDD bridges the gap between the coder and the customer .Acceptance TDD tests the behaviour of the system rather than an single Object.

TDD Cycle

As already discussed when following the TDD approach we write the test first and then code and finally, design also
called the refractor phase as shown in figure-1

Test :When we are writing the test ,we are actually designing the API envisioning what the functionality of our code could be.

Code: When we talk about code we try to write the code to ensure that our test pass, when the test passes it doesn’t mean that the code written by us is optimal,but we keep improving the design in the last step Refractor.

Refractor: In the final step we look into how we can make our code more optimal and invariantly the design as well.

Tools of TDD

Just like any other type of development we require tools and technique for TTD too and there are 3 different categories

  • Unit Testing Framework
  • Continuous integration and builds
  • Code coverage

Unit testing framework for TDD :There are frameworks available for unit testing our code.One of the most popular framework for Java is JUnit built around XUnit which provides us with base classes which is extended for writing our tests,different test runners that helps us execute and collect the test results.

When it come to framework for acceptance TDD we don’t have a base framework as such since the idea of test-driving on feature level is relatively new.

As we keep constantly changing our code as we try to improve it, there is constant requirement to integrate their changes more often thus we need to have to adopt a process to synchronize our changes to the source storehouse. Apart from integration we also need to check that the integrated source code works too.A common practice is to run a relevant subset of all test before checking in their changes and to delegate the running of the full test suite to a dedicated build server also known as Continuous build server which keeps polling for any changes in the source repository and only tests those relevant portions rather than testing the entire code which would take lot of time.There are tools available to write the build servers.

There are tools available to measure code coverage or test coverage which detect any anomalies in our code in terms of wrong usage of constructs etc.Code coverage measures how thoroughly the test cases look through our production code.

Examples of Unit Testing Framework

JUnit

  • Expected results can be tested with the help of assertions
  • During development of the applications itself we can code and test
  • Easily Understandable
  • Provision of a GUI enables the writing and testing of the code more easily and quickly
  • Helps the developer to write and execute repeatable automated tests
  • Eclipse IDE comes with both JUnit and a plug-in for creating and working with JUnit tests
  • Junit today is a de facto standard framework for developing unit tests in Java

Mock Objects

  • Developing around dependencies i.e. code depending on classes that have not yet been developed
  • Mock Objects can be used to simulate the functionality of a production class.
  • Use a mock object framework that allows you to specify the behavior of the dependant class through API.
  • How Mock objects are used
    • mock object instance to be created
    • name of the mock object to be set
    • expected behavior of the mock object to be set
    • Use the mock object as a parameter in the test
    • At the end of the test,verify the mock object was used correctly.
  • The idea behind all Mock Object frameworks is the same.

[code lang=”java”]Eg: EasyMock a framework
//instance of Mock Object
Interface is the name of the un implemented class.All the methods of the class is declared in
an interface.
MockControl control= MockControl.createControl(interface.class);
//Getting a mock object.i.e an dummy implementation of our interface
Interfaceimpl mockimpl=(interfacename) control.getMock();
//Training the mock object .i.e for a given method if a certain input is passed what should be expected
Mockimpl.method1(“arguments”);
Control.setReturnValue(“return value”);
//using the mock object
Control.replay();
//verifying the mock object
Control.verify();[/code]

HttpUnit

  • HttpUnit is a framework based on JUnit, which allows the creation of automated test cases for Web applications.
  • It is suitable for the implementation of automated functional tests, or acceptance tests

Cactus

  • Cactus is a simple test framework that implements an in-container strategy, meaning that tests are executed in the container .
  • Cactus is a framework for unit testing server-side java code (like Servlets, EJBs, Tag Libs, Filters, …).
  • HttpUnit does not do an in container testing.

StrutsTestCase

  • unit testing a part of your Struts application
  • StrutsTestCase extends the JUnit framework to allow testing the Action class.StrutsTestCase is an open source project from SourceForge.
  • It even offers two approaches: one that uses the servlet container and utilizes Cactus framework from Jakarta, and one that simulates the container.

Summary

TDD is an approach to build working software from day one. Enables developers to write reliable & error free code at the same time .It is a technique quickly being adopted by Agile software developers

Filed Under: Java Tagged With: Java

Treating POJOs like POGOs

April 8, 2011 by Krishna Srinivasan Leave a Comment

This article is based on Making Java Groovyand the book will release on January 2011. It is being reproduced here by permission from Manning Publications. Manning publishes MEAP (Manning Early Access Program,) ebooks and pbooks. MEAPs are sold exclusively through Manning.com. All print book purchases include an ebook free of charge. When mobile formats become available all customers will be contacted and upgraded. Visit Manning.com for more information.

also read:

  • Java Tutorials
  • Java EE Tutorials
  • Design Patterns Tutorials
  • Java File IO Tutorials

Introduction

POGOs have more capabilities than POJOs. For example, all POGOs have a map-based constructor that is very convenient for setting properties. The interesting thing is that, even if a class is written in Java, many of the same conveniences apply as long as it is accessed from Groovy. Consider a simple POJO representing a person, possibly created as part of a domain model in Java.

Listing 1 A simple POJO representing a person
[code lang=”java”]
public class Person {
private int id;
private String name;
public Person() {}
public Person(int id, String name) {
this.id = id;
this.name = name;
}
public void setId(int id) { this.id = id; }
public int getId() { return id; }
public void setName(String name) { this.name = name; }
public String getName() { return name; }
@Override
public String toString() {
return "Person [id=" + id + ", name=" + name + "]";
}
}
[/code]
The Person class contains attributes representing an id and a name, which are the only really significant pieces of information in the class. The rest of the class is made up of constructors, getters, and setters, and an override of the toString method to make it easier to print instances to the console. Any typical Java persistence layer has dozens of classes just like this, which easily map to relational database tables.

The interesting thing is that, if we instantiate this class from Groovy, we can use a map-based constructor to do so, even though the Java version already specifies two constructors and neither is the one we want. Let’s create some Person instances from Groovy using three different mechanisms, none of which appear in the Java class.

Figure 1 Groovy adds a map-based constructor to Java classes, regardless of what constructors are already included
[code lang=”java”]
def buffy = new Person(name:’Buffy’)
assert buffy.id == 0
assert buffy.name == ‘Buffy’
def faith = new Person(name:’Faith’,id:1)
assert faith.id == 1
assert faith.name == ‘Faith’
def willow = [name:’Willow’,id:2] as Person
assert willow.id == 2
assert willow.name == ‘Willow’
[/code]
The instances buffy and faith are created using the map-based constructor, first setting only the name and then setting both the name and the id. I’m then able to verify, using Groovy’s built-in assert method (omitting its optional parentheses) that the person’s properties are set correctly.

Incidentally, all of the assert statements that seem to be accessing private properties of the class directly really aren’t. Groovy goes through the getter and setter methods provided in the Java class when it looks like properties are being accessed or assigned. We can prove this by modifying the implementation of the getter method to return more than just the name:

[code lang=”java”]
public String getName() {
return "from getter: " + name;
}
[/code]
Now we have to modify each of the asserts to include the string “from getter: ” for them to still return true.

The third person, willow, is constructed using the “as” operator in Groovy. This operator has several uses, one of which is to “coerce” a map into an object, as shown here. In this case, the operator instantiates a person and supplies the map as properties for the resulting instance.

Moving on, we can also add our person instances to a Groovy collection, which isn’t all that surprising but has some nice additional benefits. For example, Groovy collections support operator overloading, making it easy to add additional persons and have additional methods for searching.
[code lang=”java”]
def slayers = [buffy, faith]
assert [‘Buffy’,’Faith’] == slayers*.name
assert slayers.class == java.util.ArrayList
def characters = slayers + willow
assert [‘Buffy’,’Faith’,’Willow’] == characters*.name
def doubles = characters.findAll { it.name =~ /([a-z])\1/ }
assert [‘Buffy’,’Willow’] == doubles*.name
[/code]
Groovy has a native syntax for collections, which simplifies Java code. Putting the references inside square brackets creates an instance of the java.util.ArrayList class and adds each element to the collection. Then,in the assert statement, I used the so-called spread-dot operator to extract the name property from each instance and return a list of the results (in other words, the spread-dot operator behaves the same way collect does). By the way, I restored the getName method to its original form, which returns just the attribute value.

I was able to use operator overloading to add willow to the slayers collection, resulting in the characters collection. Finally, I took advantage of the fact that in Groovy, the java.util.Collections interface has been augmented to have a findAll method that returns all instances in the collection matching the condition in the provided closure. In this case, the closure contains a regular expression, which matches any repeated lowercase letter.

Summary

Many existing Java applications have extensive domain models. As you can see, Groovy code can work with them directly, even treating them as POGOs and giving you a poor-man’s search capability.

Filed Under: Java Tagged With: Java

What is Broad Assertion in Unit Testing in Java?

April 5, 2011 by Krishna Srinivasan Leave a Comment

This article is based on Unit Testing in Java, to be published on Fall 2011. It is being reproduced here by permission from Manning Publications. Manning publishes MEAP (Manning Early Access Program,) ebooks and pbooks. MEAPs are sold exclusively through Manning.com. All print book purchases include an ebook free of charge. When mobile formats become available all customers will be contacted and upgraded. Visit Manning.com for more information.

also read:

  • Java Tutorials
  • Java EE Tutorials
  • Design Patterns Tutorials
  • Java File IO Tutorials

Introduction

A broad assertion is one that is so scrupulous in nailing down every little detail about the behavior it is checking that it becomes brittle and hides its intent under its overwhelming breadth and depth. When you encounter a broad assertion, it’s hard to say what exactly is it supposed check and, when you step back to observe, that test is probably breaking far more frequently than the average because it’s so picky that any change whatsoever will cause a difference in the expected output.

Let’s make this discussion a bit more concrete again by looking at an example test that suffers from this condition.

Example

The following example is my very own doing. I wrote it some years back as part of a sales presentation tracking system for a medical corporation. The corporation wanted to gather data on how the various sales presentations were carried out by the sales fleet that visited doctors to push their products. Essentially, they wanted a log of which salesman showed which slide of which presentation for how many seconds before moving on.

The solution involved a number of components. There was a little plug-in in the actual presentation file, triggering events when starting a new slide show, entering a slide, and so forth—each with a timestamp to signify when that particular event happened. Those events were pushed to a background application that appended them into a log file. Before synchronizing that log file with the centralized server, however, we transformed the log file into another format, preprocessing it a bit to make it easier for the centralized server to chomp the log file and dump the numbers into a central database. Essentially, we calculated the slide durations from timestamps.

The object responsible for this transformation was called a LogFileTransformer and, being test-infected as I was, I had written some tests for it. Listing 1 presents one of those tests—the one that suffered from broad assertion—along with the relevant setup. Have a look at it and see if you can detect the broad assertion.

Listing 1 Broad assertion makes a test brittle and opaque

[code lang=”java”]public class LogFileTransformerTest {
private String expectedOutput;
private String logFile;
@Before
public void setUpBuildLogFile() {
StringBuilder lines = new StringBuilder();
appendTo(lines, "[2005-05-23 21:20:33] LAUNCHED");
appendTo(lines, "[2005-05-23 21:20:33] session-id###SID");
appendTo(lines, "[2005-05-23 21:20:33] user-id###UID");
appendTo(lines, "[2005-05-23 21:20:33] presentation-id###PID");
appendTo(lines, "[2005-05-23 21:20:35] screen1");
appendTo(lines, "[2005-05-23 21:20:36] screen2");
appendTo(lines, "[2005-05-23 21:21:36] screen3");
appendTo(lines, "[2005-05-23 21:21:36] screen4");
appendTo(lines, "[2005-05-23 21:22:00] screen5");
appendTo(lines, "[2005-05-23 21:22:48] STOPPED");
logFile = lines.toString();
}
@Before
public void setUpBuildTransformedFile() {
StringBuilder file = new StringBuilder();
appendTo(file, "session-id###SID");
appendTo(file, "presentation-id###PID");
appendTo(file, "user-id###UID");
appendTo(file, "started###2005-05-23 21:20:33");
appendTo(file, "screen1###1");
appendTo(file, "screen2###60");
appendTo(file, "screen3###0");
appendTo(file, "screen4###24");
appendTo(file, "screen5###48");
appendTo(file, "finished###2005-05-23 21:22:48");
expectedOutput = file.toString();
}
@Test
public void transformationGeneratesRightStuffIntoTheRightFile()
throws Exception {
TempFile input = TempFile.withSuffix(".src.log").append(logFile);
TempFile output = TempFile.withSuffix(".dest.log");
new LogFileTransformer().transform(input.file(), output.file());
assertTrue("Destination file was not created", output.exists());
assertEquals(expectedOutput, output.content());
}
// rest omitted for clarity
}[/code]

Did you see it? Did you see the broad assertion? You probably did—there are only two assertions in there. But, which of the two is the culprit here, and what makes it too broad?

The first assertion checks that the destination file was indeed created. The second assertion checks that the destination file’s content is what’s expected. Now, the value of the first assertion is questionable and it should probably be deleted. However, it’s the second assertion that’s our main concern—the broad assertion:

[code lang=”java”]assertEquals(expectedOutput, output.content());[/code]

This is quite a relevant assertion in the sense that it verifies exactly what the name of the test implies—that the right stuff ended up in the right file. The problem is really that the test is too broad, resulting in the assertion’s being a wholesale comparison of the whole log file. It’s a thick safety net, that’s for sure, as even the tiniest of changes in the output will fail the assertion. And therein lies the problem.

A test that has never failed is of little value—it’s probably not testing anything. In the other end of the spectrum, a test that always fails is a mere nuisance. What we’re looking for is a test that has failed in the past, proving that it is able to catch a deviation from the desired behavior of the code it’s testing and that it will break again if we make such a change to the code it’s testing.

The test in our example fails to fulfill this criterion by failing too easily, making it brittle and fragile. But that’s only a symptom of a more fundamental issue—the problem of being a broad assertion. The various small changes in the log file’s format or content that would break this test are valid reasons to fail the test. There’s nothing intrinsically wrong about the assertion. The problem lies in the test’s violation of a fundamental guiding principle for what constitutes a good test:

A test should have only one reason to fail.

If that principle seems familiar, it’s a variation of a well-known object-oriented design principle, the Single Responsibility Principle, which says, “A class should have one, and only one, reason to change.”1 Now let’s clarify why the principle of having only one reason to fail is so important.

Catching many kinds of changes in the generated output is good. However, when the test does fail, we want to know why. In our example it’s quite difficult to tell what happened if this test, transformationGeneratesRightStuffIntoTheRightFile, suddenly breaks. In practice, we’ll always have to look at the details to figure out what had changed and, consequently, broke the test. If the assertion is too broad, many of those details that break the test are in fact irrelevant.

How should we go about improving this test, then?

What to do about it?

The first order of action when encountering an overly broad assertion is to identify irrelevant details and remove them from the test. In our example, we might look at the log file being transformed and try to reduce the number of lines.

We want it to represent a valid log file and be elaborate enough for the purposes of the test. For example, our log file has timings for five screens. Maybe two or three would be enough? Could we get by with just one?

This question brings us to the next improvement to consider—splitting the test. Asking ourselves how few lines in the log file we could get by with quickly leads to concerns about the test no longer testing this and that. Listing 2 presents one possible solution where each aspect of the log file and its transformation are extracted into separate tests.

Listing 2 More relaxed, semantics-oriented assertions reduce brittleness and improve readability

[code lang=”java”]public class LogFileTransformerTest {
private static final String END = "2005-05-23 21:21:37";
private static final String START = "2005-05-23 21:20:33";
private LogFile logFile;
@Before
public void setUp() {
logFile = new LogFile(START, END);
}
@Test #1
public void overallFileStructureIsCorrect()
throws Exception {
StringBuilder expected = new StringBuilder();
appendTo(expected, "session-id###SID");
appendTo(expected, "presentation-id###PID");
appendTo(expected, "user-id###UID");
appendTo(expected, "started###2005-05-23 21:20:33");
appendTo(expected, "finished###2005-05-23 21:21:37");
assertEquals(expected.toString(), transform(logFile.toString()));
}
@Test #2
public void screenDurationsGoBetweenStartedAndFinished()
throws Exception {
logFile.addContent("[2005-05-23 21:20:35] screen1"); String out =
transform(logFile.toString());
assertTrue(out.indexOf("started") < out.indexOf("screen1"));
assertTrue(out.indexOf("screen1") < out.indexOf("finished"));
}
@Test
public void screenDurationsAreRenderedInSeconds()
throws Exception {
logFile.addContent("[2005-05-23 21:20:35] screen1");
logFile.addContent("[2005-05-23 21:20:35] screen2");
logFile.addContent("[2005-05-23 21:21:36] screen3");
String output = transform(logFile.toString());
assertTrue(output.contains("screen1###0"));
assertTrue(output.contains("screen2###61"));
assertTrue(output.contains("screen3###1"));
}
// rest omitted for brevity
private String transform(String log) { … }
private void appendTo(StringBuilder buffer, String string) { … }
private class LogFile { … }
}
#1 Checks that common headers are placed correctly
#2 Checks screen durations’ place in the log
#3 Checks screen duration calculations[/code]

The solution above introduces a test helper class, LogFile, which establishes the standard “envelope”—the header and footer—for the log file being transformed based on the given starting and ending timestamps. This allows the second and the third test, screenDurationsGoBetweenStartedAndFinished and screenDurationsAreRenderedInSeconds, to append just the screen durations to the log, making the test more focused and easier to grasp. In other words, we delegate some of the responsibility for constructing the complete log file to LogFile. In order to ensure that that responsibility receives due diligence, the overall file structure is verified by the first test, overallFileStructureIsCorrect, in the context of the simplest possible scenario—an otherwise empty log file.

This refactoring has given us more focus by hiding the details from each test that are irrelevant for that particular test. That is also the downside of this approach—some of the details are hidden. In applying this technique, we must ask ourselves what we value more—being able to see the whole in one place or being able to see the essence of a test quickly.

I suggest that, most of the time, when speaking of unit tests, the latter is more desirable as the fine-grained, focused tests point us quickly to the root of the problem in case of a test failure. With all tests making assertions against the whole transformed log file, for example, a small change in the file syntax could easily break all of our tests making it more difficult to figure out what exactly broken—where’s the problem?

Summary

We can shoot ourselves in the proverbial foot by making too broad assertions. A broad assertion cuts out too large a chunk of output and side effects for bit-to-bit comparison, which makes it harmful due to the resulting brittleness—the assertion fails if any small detail changes, regardless of whether that change is relevant to the interests of this particular test.

A broad assertion also makes it difficult for the programmer to identify the intent and essence of the test. When you see a test that seems to bite off a lot, ask yourself what exactly do you want to verify? Then, try to formulate your assertion in those terms.

Filed Under: Java Tagged With: Java

Java Database Application with Apache Velocity

April 1, 2011 by Krishna Srinivasan Leave a Comment

Java Database Application with Apache Velocity

Introduction to Apache Velocity

Please Refer the article Spring with Apache Velocity

also read:

  • Java Tutorials
  • Java EE Tutorials
  • Design Patterns Tutorials
  • Java File IO Tutorials

Introduction to JDBC

JDBC stands for Java DataBase Connectivity. The JDBC API helps us to develop Java applications with data base connection. We can use any database like Oracle,MySql,MSAcess etc..

For developing JDBC Applications we have to depend many APIs from the package java.sql. There are certain steps to do this.

The steps are:

  1. Loads a DriverClass.forName(“sun.jdbc.odbc.JdbcOdbcDriver”)
  2. Connects to the databaseConnection con = DriverManager.getConnection(“jdbc:odbc:dsn”, “username”, “password”)
  3. Executes SQLStatement statmt = con.createStatement( );

    ResultSet rs = statmt.executeQuery(“SELECT * FROM ProductTable”);

  4. Retrieves the resultswhile(rs.next( )) {

    System.out.println(rs.getString(“pid”))

Example :JDBC Application with Apache Velocity

The application is using a Java file to retrieve the data from the database and the output is rendering with the help of a Velocity page.The java file uses an Arraylist to store the data from the resultset and the data is then add to the Velocity Context.Later the data is retrieving from the VelocityContext in the file Extract .vm page. The Database used in this example is Apache Derby.The below list contains all the related files and library details.

The files are :

[code]JDBCVelocityDemo.java
Extract.vm(Velocity file)
Derby database table Product with the fields product_ID,purchase_cost,..
Libraries: velocity-tools-view-1.4.jar ,velocity-dep-1.5.jar, velocity-tools-view-1.4.jar,derby.jar
Editor support :Netbeans 6.5 or above or any other java editor [/code]

JDBCVelocityDemo.java

[code lang=”java”]/**
*
* @author Rasmi_G
*/
import java.io.StringWriter;
import java.io.Writer;
import java.sql.*;
import java.util.*;
import org.apache.velocity.*;
import org.apache.velocity.app.Velocity;
import org.apache.velocity.app.VelocityEngine;
public class JDBCVelocityDemo {
static HashMap map;
public static void main(String[] args) throws Exception {
VelocityEngine ve = new VelocityEngine();
ve.init();
try {
//Derby database url
String url= "jdbc:derby://localhost:1527/sample";
//Database connection details with the username and password
Connection conn = DriverManager.getConnection(url,"app","app");
Statement stmt = conn.createStatement();
ResultSet rs;
//the product is table in the Derby Sample database
rs = stmt.executeQuery("SELECT * FROM PRODUCT");
String id ;
String cost1;
ArrayList list = new ArrayList();
//Retrieving the data and storing into a List in the form of map
while ( rs.next() ) {
map = new HashMap();
id = rs.getString("product_ID");
cost1=rs.getString("PURCHASE_COST");
map.put("Pid", id);
map.put("cost",cost1); }
list.add(map);
int count=list.size();
ArrayList l=new ArrayList();
l.add(list.get(count-1));
//For Loading the velocity page
Properties p = new Properties();
//loading the template engine path
p.setProperty("file.resource.loader.path", "D:/Learning Materials/2010-2011/AllDemoWorkFolder/Test11/src/java");
//Storing the data in the Velocity Context
VelocityContext context = new VelocityContext();
context.put("prdList", l);
//Initialize velocity run time engine through method init()
Velocity.init(p);
Template t = Velocity.getTemplate("Extract.vm");
StringWriter writer = new StringWriter();
//merge() is a method of the Template class.
//The usage of merge() is for merging the VelocityContext class object to produce the output.
t.merge(context, writer);
System.out.println(writer.toString());
}
conn.close();
catch (Exception e) {System.err.println("Exception");
System.err.println(e.getMessage());
}
}
}[/code]

Extract.vm

[code]The Details Are:
#foreach( $prd in $prdList )
Product id: $prd.Pid
Purchase Cost: $prd.cost
#end[/code]

After developing this application

  1. Start the Derby Database Server
  2. Compile and execute the JDBCVelocityDemo.java
  3. The contents in the Extrac.vm will display as the output.

The output :

[code]The Details Are:

Product id: 980001
Purchase Cost: 1095.00

The Details Are:

Product id: 980005
Purchase Cost: 11500.99

The Details Are:

Product id: 980025
Purchase Cost: 2095.99
[/code]

Conclusion

The article gives an introduction to JDBC and Apache Velocity .It contains a JDBC example which uses Apache velocity template engine for displaying the output page. Also this article gives an idea about how to develop this example and required libraries needed for this application.

Filed Under: Java Tagged With: Apache Velocity, Java

Java Threads Interview Questions

August 5, 2010 by Krishna Srinivasan Leave a Comment

Java Threads Interview Questions – 1

1)What is threaded programming and when is it used?

Threaded programming is normally used when a program is required to do more than one task at the same time. Threading is often used in applications with graphical user interfaces; a new thread may be created to do some processor-intensive work while the main thread keeps the interface responsive to human interaction.
The Java programming language has threaded programming facilities built in, so it is relatively easy to create threaded programs. However, multi-threaded programs introduce a degree of complexity that is not justified for most simple command line applications.

2)Why are wait(), notify() and notifyall() methods defined in the Object class?

A: These methods are detailed on the Java Software Development Kit JavaDoc page for the Object class, they are to implement threaded programming for all subclasses of Object.

3)Why are there separate wait and sleep methods?

A: The static Thread.sleep(long) method maintains control of thread execution but delays the next action until the sleep time expires. The wait method gives up control over thread execution indefinitely so that other threads can run.

4) What’s the difference between Thread and Runnable types?

A: A Java Thread controls the main path of execution in an application. When you invoke the Java Virtual Machine with the java command, it creates an implicit thread in which to execute the main method. The Thread class provides a mechanism for the first thread to start-up other threads to run in parallel with it.
The Runnable interface defines a type of class that can be run by a thread. The only method it requires is run, which makes the interface very easy to to fulfil by extending existing classes. A runnable class may have custom constructors and any number of other methods for configuration and manipulation.

5)How does the run() method in Runnable work?

A: It may help to think of the run method like the main method in standard single threaded applications. The run method is a standard entry point to run or execute a class. The run method is normally only executed in the context of an independent Thread, but is a normal method in all other respects.

6)A Thread is runnable, how does that work?

A: The Thread class’ run method normally invokes the run method of the Runnable type it is passed in its constructor. However, it is possible to override the thread’s run method with your own.

7)Why not override Thread to make a Runnable?

A: There is little difference in the work required to override the Thread class compared with implementing the Runnable interface, both require the body of the run() method. However, it is much simpler to make an existing class hierarchy runnable because any class can be adapted to implement the run() method. A subclass of Thread cannot extend any other type, so application-specific code would have to be added to it rather than inherited.
Separating the Thread class from the Runnable implementation also avoids potential synchronization problems between the thread and the run() method. A separate Runnable generally gives greater flexibility in the way that runnable code is referenced and executed.

8)What’s the difference between a thread’s start() and run() methods?

A: The separate start() and run() methods in the Thread class provide two ways to create threaded programs. The start() method starts the execution of the new thread and calls the run() method. The start() method returns immediately and the new thread normally continues until the run() method returns.
The Thread class’ run() method does nothing, so sub-classes should override the method with code to execute in the second thread. If a Thread is instantiated with a Runnable argument, the thread’s run() method executes the run() method of the Runnable object in the new thread instead.
Depending on the nature of your threaded program, calling the Thread run() method directly can give the same output as calling via the start() method, but in the latter case the code is actually executed in a new thread.

9)Can I implement my own start() method?

A: The Thread start() method is not marked final, but should not be overridden. This method contains the code that creates a new executable thread and is very specialised. Your threaded application should either pass a Runnable type to a new Thread, or extend Thread and override the run() method.

10)Do I need to use synchronized on setValue(int)?

A: It depends whether the method affects method local variables, class static or instance variables. If only method local variables are changed, the value is said to be confined by the method and is not prone to threading issues.

11)How do I create a Runnable with inheritance?

A: To introduce a Runnable type to an existing class hierarchy, you need to create a sub-class that declares that it implements the Runnable interface, and provide a run method to fullfil the interface. This combination of interface and inheritance means that runnable implementations can be very minor extensions of existing classes, as in the example below…

12)What is the SwingUtilities.invokeLater(Runnable) method for?

A: The static utility method invokeLater(Runnable) is intended to execute a new runnable thread from a Swing application without disturbing the normal sequence of event dispatching from the Graphical User Interface (GUI). The method places the runnable object in the queue of Abstract Windowing Toolkit (AWT) events that are due to be processed and returns immediately. The runnable object’s run() method is only called when it reaches the front of the queue.
The deferred effect of the invokeLater(Runnable) method ensures that any necessary updates to the user interface can occur immediately, and the runnable work will begin as soon as those high priority events are dealt with. The invoke later method might be used to start work in response to a button click that also requires a significant change to the user interface, perhaps to restrict other activities, while the runnable thread executes.

13)What is the volatile modifier for?

A: The volatile modifier is used to identify variables whose values should not be optimized by the Java Virtual Machine, by caching the value for example. The volatile modifier is typically used for variables that may be accessed or modified by numerous independent threads and signifies that the value may change without synchronization.

14)Which class is the wait() method defined in?

A: The wait() method is defined in the Object class, which is the ultimate superclass of all others. So the Thread class and any Runnable implementation inherit this method from Object. The wait() method is normally called on an object in a multi-threaded program to allow other threads to run. The method should should only be called by a thread that has ownership of the object’s monitor, which usually means it is in a synchronized method or statement block.

15)What is a green thread?

A: A green thread refers to a mode of operation for the Java Virtual Machine (JVM) in which all code is executed in a single operating system thread. If the Java program has any concurrent threads, the JVM manages multi-threading internally rather than using other operating system threads.
There is a significant processing overhead for the JVM to keep track of thread states and swap between them, so green thread mode has been deprecated and removed from more recent Java implementations. Current JVM implementations make more efficient use of native operating system threads.

16)What is a working thread?

A: A working thread, more commonly known as a worker thread is the key part of a design pattern that allocates one thread to execute one task. When the task is complete, the thread may return to a thread pool for later use. In this scheme a thread may execute arbitrary tasks, which are passed in the form of a Runnable method argument, typically execute(Runnable). The runnable tasks are usually stored in a queue until a thread host is available to run them.
The worker thread design pattern is usually used to handle many concurrent tasks where it is not important which finishes first and no single task needs to be coordinated with another. The task queue controls how many threads run concurrently to improve the overall performance of the system. However, a worker thread framework requires relatively complex programming to set up, so should not be used where simpler threading techniques can achieve similar results.

Filed Under: Interview Questions Tagged With: Java

Java Interview Questions & Answers

February 13, 2009 by Krishna Srinivasan Leave a Comment

This article lists over 200+ questions for the Java interview. These questions are most frequently asked by the Java interviewers. If you are preparing for the Core java interview, this questions will be much helpful to understand the nature of questions. Note that this preparations would be very much suitable for the beginners and the intermediate level candidate, if you are more experienced then the questions listed here may not be suitable for your interview. Based on the feedback from the readers, we are adding more questions into the list and updating the questions with the most appropriate answers. We are looking for the feedback on the questions in this article. If you have any questions, please post it in the comments section. We will answer your questions immediately. The following are some of the other questions which may be useful for you:

  • Spring interview questions
  • Hibernate interview questions

Java Interview Questions

1) What is the difference between an Abstract class and Interface?

  • Abstract classes may have some executable methods and methods left unimplemented. Interfaces contain no implementation code.
  • An class can implement any number of interfaces, but subclass at most one abstract class.
  • An abstract class can have non abstract methods. All methods of an interface are abstract.
  • An abstract class can have instance variables. An interface cannot.
  • An abstract class can define constructor. An interface cannot.
  • An abstract class can have any visibility: public, protected, private or none (package). An interface’s visibility must be public or none (package).
  • An abstract class inherits from Object and includes methods such as clone() and equals().

java interview

2) What are checked and unchecked exceptions?

Java defines two kinds of exceptions :

  • Checked exceptions : Exceptions that inherit from the Exception class are checked exceptions. Client code has to handle the checked exceptions thrown by the API, either in a catch clause or by forwarding it outward with the throws clause. Examples – SQLException, IOxception.
  • Unchecked exceptions : RuntimeException also extends from Exception. However, all of the exceptions that inherit from RuntimeException get special treatment. There is no requirement for the client code to deal with them, and hence they are called unchecked exceptions. Example Unchecked exceptions are NullPointerException, OutOfMemoryError, DivideByZeroException typically, programming errors.

3) What is a user defined exception?

User-defined exceptions may be implemented by

  • defining a class to respond to the exception and
  • embedding a throw statement in the try block where the exception can occur or declaring that the method throws the exception (to another method where it is handled).

The developer can define a new exception by deriving it from the Exception class as follows:

public class MyException extends Exception { /* class definition of constructors (but NOT the exception handling code) goes here */ public MyException() { super(); } public MyException( String errorMessage ) { super( errorMessage ); } }

The throw statement is used to signal the occurance of the exception within a try block. Often, exceptions are instantiated in the same statement in which they are thrown using the syntax.

throw new MyException(“I threw my own exception.”)

To handle the exception within the method where it is thrown, a catch statement that handles MyException, must follow the try block. If the developer does not want to handle the exception in the method itself, the method must pass the exception using the syntax:

public myMethodName() throws MyException

4) What is the difference between C++ & Java?

Well as Bjarne Stroustrup says “..despite the syntactic similarities, C++ and Java are very different languages. In many ways, Java seems closer to Smalltalk than to C++..”. Here are few I discovered:

  • Java is multithreaded
  • Java has no pointers
  • Java has automatic memory management (garbage collection)
  • Java is platform independent (Stroustrup may differ by saying “Java is a platform”
  • Java has built-in support for comment documentation
  • Java has no operator overloading
  • Java doesn’t provide multiple inheritance
  • There are no destructors in Java

5) What are statements in JAVA ?

Statements are equivalent to sentences in natural languages. A statement forms a complete unit of execution. The following types of expressions can be made into a statement by terminating the expression with a semicolon

  • Assignment expressions
  • Any use of ++ or —
  • Method calls
  • Object creation expressions

These kinds of statements are called expression statements. In addition to these kinds of expression statements, there are two other kinds of statements. A declaration statement declares a variable. A control flow statement regulates the order in which statements get executed. The for loop and the if statement are both examples of control flow statements.

6) What is JAR file?

JavaARchive files are a big glob of Java classes, images, audio, etc., compressed to make one simple, smaller file to ease Applet downloading. Normally when a browser encounters an applet, it goes and downloads all the files, images, audio, used by the Applet separately. This can lead to slower downloads.

7)What is JNI?

JNI is an acronym of Java Native Interface. Using JNI we can call functions which are written in other languages from Java. Following are its advantages and disadvantages.

Advantages:

  • You want to use your existing library which was previously written in other language.
  • You want to call Windows API function.
  • For the sake of execution speed.
  • You want to call API function of some server product which is in c or c++ from java client.

Disadvantages:

  • You can’t say write once run anywhere.
  • Difficult to debug runtime error in native code.
  • Potential security risk.
  • You can’t call it from Applet.

8) What is serialization?

Quite simply, object serialization provides a program the ability to read or write a whole object to and from a raw byte stream. It allows Java objects and primitives to be encoded into a byte stream suitable for streaming to some type of network or to a file-system, or more generally, to a transmission medium or storage facility. A seralizable object must implement the Serilizable interface. We use ObjectOutputStream to write this object to a stream and ObjectInputStream to read it from the stream.

9) Why there are some null interface in java ? What does it mean ? Give me some null interfaces in JAVA?

Null interfaces act as markers..they just tell the compiler that the objects of this class need to be treated differently..some marker interfaces are : Serializable, Remote, Cloneable

10) Is synchronised a modifier?indentifier??what is it??

It’s a modifier. Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method’s object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

11) What is singleton class?where is it used?

Singleton is a design pattern meant to provide one and only one instance of an object. Other objects can get a reference to this instance through a static method (class constructor is kept private). Why do we need one? Sometimes it is necessary, and often sufficient, to create a single instance of a given class. This has advantages in memory management, and for Java, in garbage collection. Moreover, restricting the number of instances may be necessary or desirable for technological or business reasons–for example, we may only want a single instance of a pool of database connections.

12) What is a compilation unit?

The smallest unit of source code that can be compiled, i.e. a .java file.

13) Is string a wrapper class?

String is a class, but not a wrapper class. Wrapper classes like (Integer) exist for each primitive type. They can be used to convert a primitive data value into an object, and vice-versa.

14) Why java does not have multiple inheritance?

The Java design team strove to make Java:

  • Simple, object oriented, and familiar
  • Robust and secure
  • Architecture neutral and portable
  • High performance
  • Interpreted, threaded, and dynamic

The reasons for omitting multiple inheritance from the Java language mostly stem from the “simple, object oriented, and familiar” goal. As a simple language, Java’s creators wanted a language that most developers could grasp without extensive training. To that end, they worked to make the language as similar to C++ as possible (familiar) without carrying over C++’s unnecessary complexity (simple).

In the designers’ opinion, multiple inheritance causes more problems and confusion than it solves. So they cut multiple inheritance from the language (just as they cut operator overloading). The designers’ extensive C++ experience taught them that multiple inheritance just wasn’t worth the headache.

15) Why java is not a 100% oops?

Many people say this because Java uses primitive types such as int, char, double. But then all the rest are objects. Confusing question..

16) What is a resource bundle?

In its simplest form, a resource bundle is represented by a text file containing keys and a text value for each key.

17) What is transient variable?

Transient variable can’t be serialize. For example if a variable is declared as transient in a Serializable class and the class is written to an ObjectStream, the value of the variable can’t be written to the stream instead when the class is retrieved from the ObjectStream the value of the variable becomes null.

18) What is Collection API?

The Collection API is a set of classes and interfaces that support operation on collections of objects. These classes and interfaces are more flexible, more powerful, and more regular than the vectors, arrays, and hashtables if effectively replaces.

Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap.

Example of interfaces: Collection, Set, List and Map.

19) Is Iterator a Class or Interface? What is its use?

Iterator is an interface which is used to step through the elements of a Collection.

20) What is similarities/difference between an Abstract class and Interface?

Differences are as follows:

  • Interfaces provide a form of multiple inheritance. A class can extend only one other class.
  • Interfaces are limited to public methods and constants with no implementation. Abstract classes can have a partial implementation, protected parts, static methods, etc.
  • A Class may implement several interfaces. But in case of abstract class, a class may extend only one abstract class.
  • Interfaces are slow as it requires extra indirection to to find corresponding method in in the actual class. Abstract classes are fast.

Similarities:

  • Neither Abstract classes or Interface can be instantiated.

21) What is a transient variable?

A transient variable is a variable that may not be serialized.

22) Which containers use a border Layout as their default layout?

The window, Frame and Dialog classes use a border layout as their default layout.

23) Why do threads block on I/O?

Threads block on i/o (that is enters the waiting state) so that other threads may execute while the i/o Operation is performed.

24) How are Observer and Observable used?

Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.

25) What is synchronization and why is it important?

With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object’s value. This often leads to significant errors.

26) Can a lock be acquired on a class?

Yes, a lock can be acquired on a class. This lock is acquired on the class’s Class object..

27) What’s new with the stop(), suspend() and resume() methods in JDK 1.2?

The stop(), suspend() and resume() methods have been deprecated in JDK 1.2.

28) Is null a keyword?

The null value is not a keyword.

29) What is the preferred size of a component?

The preferred size of a component is the minimum component size that will allow the component to display normally.

30) What method is used to specify a container’s layout?

The setLayout() method is used to specify a container’s layout.

31) Which containers use a FlowLayout as their default layout?

The Panel and Applet classes use the FlowLayout as their default layout.

32) What state does a thread enter when it terminates its processing?

When a thread terminates its processing, it enters the dead state.

33) What is the Collections API?

The Collections API is a set of classes and interfaces that support operations on collections of objects.

34) Which characters may be used as the second character of an identifier, but not as the first character of an identifier?

The digits 0 through 9 may not be used as the first character of an identifier but they may be used after the first character of an identifier.

35) What is the List interface?

The List interface provides support for ordered collections of objects.

36) How does Java handle integer overflows and underflows?

It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.

37) What is the Vector class?

The Vector class provides the capability to implement a growable array of objects

38) What modifiers may be used with an inner class that is a member of an outer class?

A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.

39) What is an Iterator interface?

The Iterator interface is used to step through the elements of a Collection.

40) What is the difference between the >> and >>> operators?

The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that have been shifted out.

41) Which method of the Component class is used to set the position and size of a component?

setBounds()

42) How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?

Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits. UTF-8 represents characters using 8, 16, and 18 bit patterns. UTF-16 uses 16-bit and larger bit patterns.

43) What is the difference between yielding and sleeping?

When a task invokes its yield() method, it returns to the ready state. When a task invokes its sleep() method, it returns to the waiting state.

44) Which java.util classes and interfaces support event handling?

The EventObject class and the EventListener interface support event processing.

45) Is sizeof a keyword?

The sizeof operator is not a keyword.

46) What are wrapped classes?

Wrapped classes are classes that allow primitive types to be accessed as objects.

47) Does garbage collection guarantee that a program will not run out of memory?

Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection

48) What restrictions are placed on the location of a package statement within a source code file?

A package statement must appear as the first line in a source code file (excluding blank lines and comments).

49) Can an object’s finalize() method be invoked while it is reachable?

An object’s finalize() method cannot be invoked by the garbage collector while the object is still reachable. However, an object’s finalize() method may be invoked by other objects.

50) What is the immediate superclass of the Applet class?

Panel

51) What is the difference between preemptive scheduling and time slicing?

Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

52) Name three Component subclasses that support painting.

The Canvas, Frame, Panel, and Applet classes support painting.

53) What value does readLine() return when it has reached the end of a file?

The readLine() method returns null when it has reached the end of a file.

54) What is the immediate superclass of the Dialog class?

Window

55) What is clipping?

Clipping is the process of confining paint operations to a limited area or shape.

56) What is a native method?

A native method is a method that is implemented in a language other than Java.

57) Can a for statement loop indefinitely?

Yes, a for statement can loop indefinitely. For example, consider the following:

for(;;) ;

58) What are order of precedence and associativity, and how are they used?

Order of precedence determines the order in which operators are evaluated in expressions. Associatity determines whether an expression is evaluated left-to-right or right-to-left

59) When a thread blocks on I/O, what state does it enter?

A thread enters the waiting state when it blocks on I/O.

60) To what value is a variable of the String type automatically initialized?

The default value of an String type is null.

61) What is the catch or declare rule for method declarations?

If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.

62) What is the difference between a MenuItem and a CheckboxMenuItem?

The CheckboxMenuItem class extends the MenuItem class to support a menu item that may be checked or unchecked.

63) What is a task’s priority and how is it used in scheduling?

A task’s priority is an integer value that identifies the relative order in which it should be executed with respect to other tasks. The scheduler attempts to schedule higher priority tasks before lower priority tasks.

64) What class is the top of the AWT event hierarchy?

The java.awt.AWTEvent class is the highest-level class in the AWT event-class hierarchy.

65) When a thread is created and started, what is its initial state?

A thread is in the ready state after it has been created and started.

66) Can an anonymous class be declared as implementing an interface and extending a class?

An anonymous class may implement an interface or extend a superclass, but may not be declared to do both.

67) What is the range of the short type?

The range of the short type is -(2^15) to 2^15 – 1.

68) What is the range of the char type?

The range of the char type is 0 to 2^16 – 1.

69) In which package are most of the AWT events that support the event-delegation model defined?

Most of the AWT-related events of the event-delegation model are defined in the java.awt.event package. The AWTEvent class is defined in the java.awt package.

70) What is the immediate superclass of Menu?

MenuItem

71) What is the purpose of finalization?

The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.

72) Which class is the immediate superclass of the MenuComponent class.

Object

73) What invokes a thread’s run() method?

After a thread is started, via its start() method or that of the Thread class, the JVM invokes the thread’s run() method when the thread is initially executed.

74) What is the difference between the Boolean & operator and the && operator?

If an expression involving the Boolean & operator is evaluated, both operands are evaluated. Then the & operator is applied to the operand. When an expression involving the && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. The && operator is then applied to the first and second operands. If the first operand evaluates to false, the evaluation of the second operand is skipped.

75) Name three subclasses of the Component class.

Box.Filler, Button, Canvas, Checkbox, Choice, Container, Label, List, Scrollbar, or TextComponent

76) What is the GregorianCalendar class?

The GregorianCalendar provides support for traditional Western calendars.

77) Which Container method is used to cause a container to be laid out and redisplayed?

validate()

78) What is the purpose of the Runtime class?

The purpose of the Runtime class is to provide access to the Java runtime system.

79) How many times may an object’s finalize() method be invoked by the garbage collector?

An object’s finalize() method may only be invoked once by the garbage collector.

80) What is the purpose of the finally clause of a try-catch-finally statement?

The finally clause is used to provide the capability to execute code no matter whether or not an exception is thrown or caught.

81) What is the argument type of a program’s main() method?

A program’s main() method takes an argument of the String[] type.

82) Which Java operator is right associative?

The = operator is right associative.

83) What is the Locale class?

The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.

84) Can a double value be cast to a byte?

Yes, a double value can be cast to a byte.

85) What is the difference between a break statement and a continue statement?

A break statement results in the termination of the statement to which it applies (switch, for, do, or while). A continue statement is used to end the current loop iteration and return control to the loop statement.

86) What must a class do to implement an interface?

It must provide all of the methods in the interface and identify the interface in its implements clause.

87) What method is invoked to cause an object to begin executing as a separate thread?

The start() method of the Thread class is invoked to cause an object to begin executing as a separate thread.

88) Name two subclasses of the TextComponent class.

TextField and TextArea

89) What is the advantage of the event-delegation model over the earlier eventinheritance model?

The event-delegation model has two advantages over the event-inheritance model. First, it enables event handling to be handled by objects other than the ones that generate the events (or their containers). This allows a clean separation between a component’s design and its use. The other advantage of the event-delegation model is that it performs much better in applications where many events are generated. This performance improvement is due to the fact that the event-delegation model does not have to repeatedly process unhandled events, as is the case of the event-inheritance model.

90) Which containers may have a MenuBar?

Frame

91) How are commas used in the intialization and iteration parts of a for statement?

Commas are used to separate multiple statements within the initialization and iteration parts of a for statement.

92) What is the purpose of the wait(), notify(), and notifyAll() methods?

The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads to wait for a shared resource. When a thread executes an object’s wait() method, it enters the waiting state. It only enters the ready state after another thread invokes the object’s notify() or notifyAll() methods..

93) What is an abstract method?

An abstract method is a method whose implementation is deferred to a subclass.

94) How are Java source code files named?

A Java source code file takes the name of a public class or interface that is defined within the file. A source code file may contain at most one public class or interface. If a public class or interface is defined within a source code file, then the source code file must take the name of the public class or interface. If no public class or interface is defined within a source code file, then the file must take on a name that is different than its classes and interfaces. Source code files use the .java extension.

95) What is the relationship between the Canvas class and the Graphics class?

A Canvas object provides access to a Graphics object via its paint() method.

96) What are the high-level thread states?

The high-level thread states are ready, running, waiting, and dead.

97) What value does read() return when it has reached the end of a file?

The read() method returns -1 when it has reached the end of a file.

98) Can a Byte object be cast to a double value?

No, an object cannot be cast to a primitive value.

99) What is the difference between a static and a non-static inner class?

A non-static inner class may have object instances that are associated with instances of the class’s outer class. A static inner class does not have any object instances.

100) What is the difference between the String and StringBuffer classes?

String objects are constants. StringBuffer objects are not.

101) If a variable is declared as private, where may the variable be accessed?

A private variable may only be accessed within the class in which it is declared.

102) What is an object’s lock and which object’s have locks?

An object’s lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object’s lock. All objects and classes have locks. A class’s lock is acquired on the class’s Class object.

103) What is the Dictionary class?

The Dictionary class provides the capability to store key-value pairs.

104) How are the elements of a BorderLayout organized?

The elements of a BorderLayout are organized at the borders (North, South, East, and West) and the center of a container.

105) What is the % operator?

It is referred to as the modulo or remainder operator. It returns the remainder of dividing the first operand by the second operand.

106) When can an object reference be cast to an interface reference?

An object reference be cast to an interface reference when the object implements the referenced interface.

107) What is the difference between a Window and a Frame?

The Frame class extends Window to define a main application window that can have a menu bar.

108) Which class is extended by all other classes?

The Object class is extended by all other classes.

109) Can an object be garbage collected while it is still reachable?

A reachable object cannot be garbage collected. Only unreachable objects may be garbage collected..

110) Is the ternary operator written x : y ? z or x ? y : z ?

It is written x ? y : z.

111) What is the difference between the Font and FontMetrics classes?

The FontMetrics class is used to define implementation-specific properties, such as ascent and descent, of a Font object.

112) How is rounding performed under integer division?

The fractional part of the result is truncated. This is known as rounding toward zero.

113) What happens when a thread cannot acquire a lock on an object?

If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object’s lock, it enters the waiting state until the lock becomes available.

114) What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy?

The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented.

115) What classes of exceptions may be caught by a catch clause?

A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error and Exception types.

116) If a class is declared without any access modifiers, where may the class be accessed?

A class that is declared without any access modifiers is said to have package access. This means that the class can only be accessed by other classes and interfaces that are defined within the same package.

117) What is the SimpleTimeZone class?

The SimpleTimeZone class provides support for a Gregorian calendar.

118) What is the Map interface?

The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.

119) Does a class inherit the constructors of its superclass?

A class does not inherit constructors from any of its superclasses.

120) For which statements does it make sense to use a label?

The only statements for which it makes sense to use a label are those statements that can enclose a break or continue statement.

121) What is the purpose of the System class?

The purpose of the System class is to provide access to system resources.

122) Which TextComponent method is used to set a TextComponent to the read-only state?

setEditable()

123) How are the elements of a CardLayout organized?

The elements of a CardLayout are stacked, one on top of the other, like a deck of cards.

124) Is &&= a valid Java operator?

No, it is not.

125) Name the eight primitive Java types?

The eight primitive types are byte, char, short, int, long, float, double, and boolean.

126) Which class should you use to obtain design information about an object?

The Class class is used to obtain information about an object’s design.

127) What is the relationship between clipping and repainting?

When a window is repainted by the AWT painting thread, it sets the clipping regions to the area of the window that requires repainting.

128) Is “abc” a primitive value?

The String literal “abc” is not a primitive value. It is a String object.

129) What is the relationship between an event-listener interface and an event-adapter class?

An event-listener interface defines the methods that must be implemented by an event handler for a particular kind of event. An event adapter provides a default implementation of an event-listener interface.

130) What restrictions are placed on the values of each case of a switch statement?

During compilation, the values of each case of a switch statement must evaluate to a value that can be promoted to an int value.

131) What modifiers may be used with an interface declaration?

An interface may be declared as public or abstract.

132) Is a class a subclass of itself?

A class is a subclass of itself.

133) What is the highest-level event class of the event-delegation model?

The java.util.EventObject class is the highest-level class in the event-delegation class hierarchy.

134) What event results from the clicking of a button?

The ActionEvent event is generated as the result of the clicking of a button.

135) How can a GUI component handle its own events?

A component can handle its own events by implementing the required event-listener interface and adding itself as its own event listener.

136) What is the difference between a while statement and a do statement?

A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.

137) How are the elements of a GridBagLayout organized?

The elements of a GridBagLayout are organized according to a grid. However, the elements are of different sizes and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes.

138) What advantage do Java’s layout managers provide over traditional windowing systems?

Java uses layout managers to lay out components in a consistent manner across all windowing platforms. Since Java’s layout managers aren’t tied to absolute sizing and positioning, they are able to accomodate platform-specific differences among windowing systems.

139) What is the Collection interface?

The Collection interface provides support for the implementation of a mathematical bag – an unordered collection of objects that may contain duplicates.

140) What modifiers can be used with a local inner class?

A local inner class may be final or abstract.

141) What is the difference between static and non-static variables?

A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

142) What is the difference between the paint() and repaint() methods?

The paint() method supports painting via a Graphics object. The repaint() method is used to cause paint() to be invoked by the AWT painting thread.

143) What is the purpose of the File class?

The File class is used to create objects that provide access to the files and directories of a local file system.

144) Can an exception be rethrown?

Yes, an exception can be rethrown.

145) Which Math method is used to calculate the absolute value of a number?

The abs() method is used to calculate absolute values.

146) How does multithreading take place on a computer with a single CPU?

The operating system’s task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.

147) When does the compiler supply a default constructor for a class?

The compiler supplies a default constructor for a class if no other constructors are provided.

148) When is the finally clause of a try-catch-finally statement executed?

The finally clause of the try-catch-finally statement is always executed unless the thread of execution terminates or an exception occurs within the execution of the finally clause.

149) Which class is the immediate superclass of the Container class?

Component

150) If a method is declared as protected, where may the method be accessed?

A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared.

151) How can the Checkbox class be used to create a radio button?

By associating Checkbox objects with a CheckboxGroup.

152) Which non-Unicode letter characters may be used as the first character of an identifier?

The non-Unicode letter characters $ and _ may appear as the first character of an identifier

153) What restrictions are placed on method overloading?

Two methods may not have the same name and argument list but different return types.

154) What happens when you invoke a thread’s interrupt method while it is sleeping or waiting?

When a task’s interrupt() method is executed, the task enters the ready state. The next time the task enters the running state, an InterruptedException is thrown.

155) What is casting?

There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.

156) What is the return type of a program’s main() method?

A program’s main() method has a void return type.

157) Name four Container classes.

Window, Frame, Dialog, FileDialog, Panel, Applet, or ScrollPane

158) What is the difference between a Choice and a List?

A Choice is displayed in a compact form that requires you to pull it down to see the list of available choices. Only one item may be selected from a Choice. A List may be displayed in such a way that several List items are visible. A List supports the selection of one or more List items.

159) What class of exceptions are generated by the Java run-time system?

The Java runtime system generates RuntimeException and Error exceptions.

160) What class allows you to read objects directly from a stream?

The ObjectInputStream class supports the reading of objects from input streams.

161) What is the difference between a field variable and a local variable?

A field variable is a variable that is declared as a member of a class. A local variable is a variable that is declared local to a method.

162) Under what conditions is an object’s finalize() method invoked by the garbage collector?

The garbage collector invokes an object’s finalize() method when it detects that the object has become unreachable.

163) How are this() and super() used with constructors?

this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.

164) What is the relationship between a method’s throws clause and the exceptions that can be thrown during the method’s execution?

A method’s throws clause must declare any checked exceptions that are not caught within the body of the method.

165) What is the difference between the JDK 1.02 event model and the event-delegation model introduced with JDK 1.1?

The JDK 1.02 event model uses an event inheritance or bubbling approach. In this model, components are required to handle their own events. If they do not handle a particular event, the event is inherited by (or bubbled up to) the component’s container. The container then either handles the event or it is bubbled up to its container and so on, until the highest-level container has been tried..

In the event-delegation model, specific objects are designated as event handlers for GUI components. These objects implement event-listener interfaces. The event-delegation model is more efficient than the event-inheritance model because it eliminates the processing required to support the bubbling of unhandled events.

166) How is it possible for two String objects with identical values not to be equal under the == operator?

The == operator compares two objects to determine if they are the same object in memory. It is possible for two String objects to have the same value, but located indifferent areas of memory.

167) Why are the methods of the Math class static?

So they can be invoked as if they are a mathematical code library.

168) What Checkbox method allows you to tell if a Checkbox is checked?

getState()

169) What state is a thread in when it is executing?

An executing thread is in the running state.

170) What are the legal operands of the instanceof operator?

The left operand is an object reference or null value and the right operand is a class, interface, or array type.

171) How are the elements of a GridLayout organized?

The elements of a GridBad layout are of equal size and are laid out using the squares of a grid.

172) What an I/O filter?

An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.

173) If an object is garbage collected, can it become reachable again?

Once an object is garbage collected, it ceases to exist. It can no longer become reachable again.

174) What is the Set interface?

The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.

175) What classes of exceptions may be thrown by a throw statement?

A throw statement may throw any expression that may be assigned to the Throwable type.

176) What are E and PI?

E is the base of the natural logarithm and PI is mathematical value pi.

177) Are true and false keywords?

The values true and false are not keywords.

178) What is a void return type?

A void return type indicates that a method does not return a value.

179) What is the purpose of the enableEvents() method?

The enableEvents() method is used to enable an event for a particular object. Normally, an event is enabled when a listener is added to an object for a particular event. The enableEvents() method is used by objects that handle events by overriding their eventdispatch methods.

180) What is the difference between the File and RandomAccessFile classes?

The File class encapsulates the files and directories of the local file system. The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file.

181) What happens when you add a double value to a String?

The result is a String object.

182) What is your platform’s default character encoding?

If you are running Java on English Windows platforms, it is probably Cp1252. If you are running Java on English Solaris platforms, it is most likely 8859_1..

183) Which package is always imported by default?

The java.lang package is always imported by default.

184) What interface must an object implement before it can be written to a stream as an object?

An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.

185) How are this and super used?

this is used to refer to the current object instance. super is used to refer to the variables and methods of the superclass of the current object instance.

186) What is the purpose of garbage collection?

The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources may be reclaimed and reused.

187) What is a compilation unit?

A compilation unit is a Java source code file.

188) What interface is extended by AWT event listeners?

All AWT event listeners extend the java.util.EventListener interface.

189) What restrictions are placed on method overriding?

  • Overridden methods must have the same name, argument list, and return type.
  • The overriding method may not limit the access of the method it overrides.
  • The overriding method may not throw any exceptions that may not be thrownby the overridden method.

190) How can a dead thread be restarted?

A dead thread cannot be restarted.

191) What happens if an exception is not caught?

An uncaught exception results in the uncaughtException() method of the thread’s ThreadGroup being invoked, which eventually results in the termination of the program in which it is thrown.

192) What is a layout manager?

A layout manager is an object that is used to organize components in a container.

193) Which arithmetic operations can result in the throwing of an ArithmeticException?

Integer / and % can result in the throwing of an ArithmeticException.

194) What are three ways in which a thread can enter the waiting state?

A thread can enter the waiting state by invoking its sleep() method, by blocking on I/O, by unsuccessfully attempting to acquire an object’s lock, or by invoking an object’s wait() method. It can also enter the waiting state by invoking its (deprecated) suspend() method.

195) Can an abstract class be final?

An abstract class may not be declared as final.

196) What is the ResourceBundle class?

The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program’s appearance to the particular locale in which it is being run.

197) What happens if a try-catch-finally statement does not have a catch clause to handle an exception that is thrown within the body of the try statement?

The exception propagates up to the next higher level try-catch statement (if any) or results in the program’s termination.

198) What is numeric promotion?

Numeric promotion is the conversion of a smaller numeric type to a larger numeric type, so that integer and floating-point operations may take place. In numerical promotion, byte, char, and short values are converted to int values. The int values are also converted to long values, if necessary. The long and float values are converted to double values, as required.

199) What is the difference between a Scrollbar and a ScrollPane?

A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane handles its own events and performs its own scrolling.

200) What is the difference between a public and a non-public class?

A public class may be accessed outside of its package. A non-public class may not be accessed outside of its package.

201) To what value is a variable of the boolean type automatically initialized?

The default value of the boolean type is false.

202) Can try statements be nested?

Try statements may be tested.

203) What is the difference between the prefix and postfix forms of the ++ operator?

The prefix form performs the increment operation and returns the value of the increment operation. The postfix form returns the current value all of the expression and then performs the increment operation on that value.

204) What is the purpose of a statement block?

A statement block is used to organize a sequence of statements as a single statement group.

205) What is a Java package and how is it used?

A Java package is a naming context for classes and interfaces. A package is used to create a separate name space for groups of classes and interfaces. Packages are also used to organize related classes and interfaces into a single API unit and to control accessibility to these classes and interfaces.

206) What modifiers may be used with a top-level class?

A top-level class may be public, abstract, or final.

207) What are the Object and Class classes used for?

The Object class is the highest-level class in the Java class hierarchy. The Class class is used to represent the classes and interfaces that are loaded by a Java program..

208) How does a try statement determine which catch clause should be used to handle an exception?

When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exception is executed. The remaining catch clauses are ignored.

209) Can an unreachable object become reachable again?

An unreachable object may become reachable again. This can happen when the object’s finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.

210) When is an object subject to garbage collection?

An object is subject to garbage collection when it becomes unreachable to the program in which it is used.

211) What method must be implemented by all threads?

All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.

212) What methods are used to get and set the text label displayed by a Button object?

getLabel() and setLabel()

213) Which Component subclass is used for drawing and painting?

Canvas

214) What are synchronized methods and synchronized statements?

Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method’s object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

215) What are the two basic ways in which classes that can be run as threads may be defined?

A thread class may be declared as a subclass of Thread, or it may implement the Runnable interface.

216) What are the problems faced by Java programmers who don’t use layout managers?

Without layout managers, Java programmers are faced with determining how their GUI will be displayed across multiple windowing systems and finding a common sizing and positioning that will work within the constraints imposed by each windowing system.

217) What is the difference between an if statement and a switch statement?

The if statement is used to select among two alternatives. It uses a boolean expression to decide which alternative should be executed. The switch statement is used to select among multiple alternatives. It uses an int expression to determine which alternative should be executed.

218) What happens when you add a double value to a String?

The result is a String object.

219) What is the List interface?

The List interface provides support for ordered collections of objects.

Filed Under: Interview Questions Tagged With: Java

Overriding the toString() method in Object class

August 21, 2007 by Krishna Srinivasan Leave a Comment

The toString() method in the Object class is used to display some information regarding any object. If any code needs some information of an object of a class, then it can get it by using this method. The toString() method of an object gets invoked automatically, when an object reference is passed in the System.out.println() method. The following code illustrates this,

also read:

  • hascode and equals method
  • A static utility for objects introduced in Java 7
  • Reading file asynchronously in Java
  • Virtual Extension Methods(or Defender Methods) in Java 8

ToStringMethodTest.java
[code lang=”java”]
public class ToStringMethodTest {
private String companyName;
private String companyAddress;
public ToStringMethodTest(String companyName, String companyAddress) {
this.companyName = companyName;
this.companyAddress = companyAddress;
}
public static void main(String[] args) {
ToStringMethodTest test = new ToStringMethodTest("ABC private Ltd","10, yy Street, CC Town");
System.out.println(test);
}
}[/code]
This output seems a bit weird. If we analyze it closely, we can find that the output is nothing but the Class name ToStringMethodTest and then the ‘@’ symbol is followed by 18d107f which is the hashcode of the object.

In case we would like to display some meaningful details of an object, we can override the toString() and thereby achieve it. Consider the following code,
ToStringMethodTest.java

[code lang=”java”]
public class ToStringMethodTest {
private String companyName;
private String companyAddress;
public ToStringMethodTest(String companyName, String companyAddress) {
this.companyName = companyName;
this.companyAddress = companyAddress;
}
public static void main(String[] args) {
ToStringMethodTest test = new ToStringMethodTest("ABC private Ltd","10, yy Street, CC Town");
System.out.println(test);
}

public String toString() {
return ("Company Name: " + companyName + "n" +
"Company Address: " + companyAddress);
}
}[/code]
The result of the above code is,
[code]
Company Name: ABC private Ltd
Company Address: 10, yy Street, CC Town
[/code]

What we have just now seen is just a sample of how a meaningful override of the toString() method would prove to be of great use in displaying an object’s information when we try printing an object using the System.out.println statement during debugging process.

also read:

  • hascode and equals method
  • A static utility for objects introduced in Java 7
  • Reading file asynchronously in Java
  • Virtual Extension Methods(or Defender Methods) in Java 8

Filed Under: Java Tagged With: Core Java, Java

HashCode and equals methods

August 21, 2007 by Krishna Srinivasan Leave a Comment

HashTable, HashMap and HashSet are the Collection classes in java.util package that make use of hashing algorithm to store objects. In all these Collection classes except HashSet, objects are stored as key-value pairs. For the storage and the retrieval of any user-defined objects it is a good practice to override the following methods which is mentioned below,

  • hashCode()
  • equals()

These methods are available in the Object class and hence available to all java classes.Using these two methods, an object can be stored or retrieved from a Hashtable, HashMap or HashSet.

also read:

  • A static utility for objects introduced in Java 7
  • Reading file asynchronously in Java
  • Virtual Extension Methods(or Defender Methods) in Java 8

hashCode() method

This method returns a hashcode value as an int for the object. Default implementation for hashcode() should be overridden in order to make searching of data faster. The implementation of hashCode() method for an user-defined object should be calculated based on the properties of the class which we wish to consider.

equals() method

This method returns a boolean which specifies whether two objects are equal or not. The default implementation of equals() method given by the Object Class uses the ‘==’ operator to compare two object references, and returns true only if they refer to the same object. But, we can meaningfully re-define this equals() method to have en equality check based on our own criteria.
Consider the following code, which defines two user defined classes Employee and EmployeeId which are supposed to be stored in a Map.

Employee.java
[code lang=”java”]public class Employee {
private String name;

public Employee(String name) {
this.name = name;
}

public String toString() {
return name;
}
}[/code]

EmployeeId.java
[code lang=”java”]public class EmployeeId {
private String id;

public EmployeeId(String id) {
this.id = id;
}

public String toString() {
return id;
}
}[/code]
The following class makes use of the above classes by storing it in a Map for later retrieval. We are adding Employee objects into the Map keyed with Employee Id.
HashCodeTest.java

[code lang=”java”]public class HashCodeTest {
public static void main(String[] args) {
Map<employeeId, Employee>
employees = new HashMap<employeeId, Employee>();
employees.put(new EmployeeId("111"), new Employee("Johny"));
employees.put(new EmployeeId("222"), new Employee("Jeny")); // Line A
employees.put(new EmployeeId("333"), new Employee("Jessie"));
Employee emp = employees.get(new EmployeeId("222"));// Line B
System.out.println(emp); // Line C
}
}
}
}[/code]
In Line B, we try to retrieve the Employee object who has Employee Id with a value of 222. We expect the output to be ‘Jeny’, because the Employee with Employee Id (222) was already there in the Collection, but surprisingly, the output of the above code is null. The reason is that we did not override the equals() method for EmployeeId and Employee classes because the default implementation of equals() in the Object class considers the new EmployeeId("222") in the put statement and new EmployeeId("222") in the get statement as two different instances, and hence the call to get() in Line B returns null.
Let us look at how the same code works when we provide our desired implementation for hashcode() and equals() methods. We basically override hashcode() here just to make the object to be searched fast.

Employee.java
[code lang=”java”]public class Employee {
private String name;

public Employee(String name) {
this.name = name;
}

public String toString() {
return name;
}

@Override
public boolean equals(Object obj) {
if (obj == null) {
return false;
}
if (obj.getClass() != getClass()) {
return false;
}
Employee emp = (Employee) obj;
if (this.name == emp.name) {
return true;
}
return false;
}

@Override
public int hashCode() {
return name.hashCode();
}
}[/code]

EmployeeId.java
[code lang=”java”]public class EmployeeId {
private String id;

public EmployeeId(String id) {
this.id = id;
}

public String toString() {
return id;
}

public boolean equals(Object obj) {
if (obj == null)
return false;
if (obj.getClass() != getClass()) {
return false;
}
EmployeeId empId = (EmployeeId) obj;
if (this.id == empId.id) {
return true;
}
return false;
}

@Override
public int hashCode() {
return id.hashCode();
}
}[/code]

Now, we get the desired output ‘Jeny’, because as per our implementation for the equals() method, the new EmployeeId("222") in the put statement and new EmployeeId("222") in the get statement are considered one and the same.

also read:

  • A static utility for objects introduced in Java 7
  • Reading file asynchronously in Java
  • Virtual Extension Methods(or Defender Methods) in Java 8

Filed Under: Java Tagged With: Core Java, Java

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