HashMap in Java

HashMap uses the hash table to implement the Map interface. Map is key-value pair where key is the reference and value is the actual data. HashMap is very extensively used in the projects when key-value type data is stored. This doesn’t ensure the order of insertion. This class extends the AbstractMap and implements the Map interface.

HaspMap supports the four types of constructors:

• HashMap( )
• HashMap(Map m)
• HashMap(int capacity)
• HashMap(int capacity, float fillRatio)

HashMap Methods

• void clear() –> Clears all entries in this map.
• Object clone() –> Returns a clone of the map object
• boolean containsKey(Object key) –> Will return true if the map contains the key
• boolean containsValue(Object value) –> Will return true if this map maps one or more keys to the specified value.
• Set entrySet() –> Will return true if a collection of the mappings found this map.
• Object get(Object key) –> Will return the value to which the passed key is mapped in this identity hash map, or null if the map contains no mapping for the passed key.
• boolean isEmpty() –> Will return true if this map contains no entries
• Set keySet() –> Will return the set of keys in the map object
• Object put(Object key, Object value) –> Will link the key and value passed
• putAll(Map m) –> Copy all of the amppings to the another map
• Object remove(Object key) –> Will remove the passed key-value from the map
• int size() –> Will return the size of the map
• Collection values() –> Will retrun the collection view of the map values. It may be a list of values

HashMap Example

package javabeat.net.core;

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;

public class HashMapExample {
	public static void main(String args[]) {

		// Create a default hash map
		HashMap<String,String> hashmap = new HashMap<String,String>();
		// Add elements to the map
		hashmap.put("1", "Entry 1");
		hashmap.put("2", "Entry 2");
		hashmap.put("3", "Entry 3");

		// Get a set of the entries
		Set<Entry<String, String>> set = hashmap.entrySet();

		// Get an iterator
		Iterator<Entry<String, String>> i = set.iterator();

		// Display elements
		while (i.hasNext()) {
			Map.Entry<String,String> mapEntry = i.next();
			System.out.print(mapEntry.getKey() + ": ");
			System.out.println(mapEntry.getValue());
		}
	}
}

The output for the above example will be:

3: Entry 3
2: Entry 2
1: Entry 1