Creating user defined exceptions

Though Java provides an extensive set of in-built exceptions, there are cases in which we may need to define our own exceptions in order to handle the various application specific errors that we might encounter.

also read:

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

While defining an user defined exception, we need to take care of the following aspects:

• The user defined exception class should extend from Exception class.
• The toString() method should be overridden in the user defined exception class in order to display meaningful information about the exception.

Let us see a simple example to learn how to define and make use of user defined exceptions.

NegativeAgeException.java
[code lang=”java”]
public class NegativeAgeException extends Exception {

private int age;

public NegativeAgeException(int age){
this.age = age;
}

public String toString(){
return "Age cannot be negative" + " " +age ;
}
}
[/code]

CustomExceptionTest.java
[code lang=”java”]
public class CustomExceptionTest {

public static void main(String[] args) throws Exception{

int age = getAge();

if (age < 0){
throw new NegativeAgeException(age);
}else{
System.out.println("Age entered is " + age);
}
}

static int getAge(){
return -10;
}
}
[/code]

In the CustomExceptionTest class, the age is expected to be a positive number. It would throw the user defined exception NegativeAgeException if the age is assigned a negative number.

At runtime, we get the following exception since the age is a negative number.

[code lang=”java”]
Exception in thread "main" Age cannot be negative -10
at tips.basics.exception.CustomExceptionTest.main(CustomExceptionTest.java:10)

[/code]