Java HashMap Example

HashMap is collection which stores in the form of key-value pairs, where key is reference and value is actual data. HashMap always works on the principle of hashing to transform key-value pairs into the map.

  • It uses hash table to store the map.
  • It contains unique elements.
  • We cannot use duplicate data for the keys in the HashMap.
  • For inserting and locating pairs, HashMap provides constant – time performance.
  • The performance depends on the constructor which allows adjusting capacity and loading factor of the HashMap. HashMap supports both constructor and methods.

also read:

Collection Diagram of HashMap

Java HashMap Collection Diagram Example

HashMap Features

  • HashMap is equivalent to Hashtable.
  • HashMap is non-synchronized.
  • HashMap allows null values.
  • HashMap is case-sensitive.
  • HashMap will not keep keys and values in a proper order.

HashMap Syntax

HashMap<K, V>  map = new  HashMap<K, V> ( );

Where K specifies type of keys and V specifies type of values. We can create HashMap object by using new operator as shown in the syntax.

HashMap Constructors

  • HashMap ( ): It is a default hash map.
  • HashMap (Map m): It initializes the hash map by using the elements of m.
  • HashMap (int capacity): It is used to define the initial capacity of HashMap. The default initial capacity of HashMap is 16.
  • HashMap (int capacity, float fillRatio): It initializes both capacity and fill ratio by using its arguments.

HashMap Methods

MethodDescription
void clear( )Removes all the key-value pairs from the map.
Object clone( )Returns a duplicate copy of the map.
boolean containsValue(Object value)Returns the specified value of the map.
boolean containsKey( Object key)Returns the specified key of the map.
V get( Object key)Returns the value with the specified key of the mapping.
Set<Entry> entrySet( )Returns a set of key-value pairs contained in the map.
Set keySet ( )Returns a set of key contained in the map.
boolean isEmpty( )Returns true if the map contains no key or value pairs.
V put( K key, V value)Describes the specified value with the specified key in the map.
void putAll( Map map)It copies all the key-value pairs from the specified map to the required map.
int size( )Returns the number of key and value pairs in the map.
V remove( Object key)It removes the key-value pairs from the map.
Collection values( )Returns collection of values present in the map.

Simple Example of HashMap

package hashmap;

import java.util.*;

public class HashExample {
	public static void main(String[] args) {
	HashMap<String, String> hMap = new HashMap<String, String>();

		hMap.put("One", "Mahantesh");
		hMap.put("Two", "Maruti");

		Set set = hMap.entrySet();
		Iterator itr = set.iterator();

		while (itr.hasNext()) {
		Map.Entry m = (Map.Entry) itr.next();
		System.out.println(m.getKey() + " " + m.getValue());
		}
	}
}
  • In the above example, The HashMap interface which is used to store key values and creates HashMap object hMap by using new operator which takes parameter of type string.
  • In the next line, put the all elements to the map by using hMap.put( ) method.
  • The next line defines set of entries to the map by using entrySet( )method.
  • The iterator allows traversing of a object and the modification of the elements.
  • The itr.hasNext( ) returns true until it reaches at the end.
  • The getKey( ) and getValue( ) returns the value which the specified key is mapped in the hash map.

When you run the above example, you would get the following output:

Java Simple HashMap Example

Example using HashMap Methods


package hashmap;

import java.util.*;

public class HashMapMethodsExample {

	public static void main(String[] args) {
		HashMap<String, String> hMap = new HashMap<String, String>();

		hMap.put("One", "Mahantesh");
		hMap.put("Two", "Maruti");
		hMap.put("Three", "Ganesh");
		hMap.put("Four", "Vikrant");

		System.out.println("Before removing: "+ hMap);

		hMap.remove("Three");

		System.out.println("After removing: "+ hMap);

		boolean data=hMap.isEmpty();
		System.out.println("Is hash map empty: " + data );
		System.out.println("Size of the map: "+ hMap.size());

		System.out.println("value 'Maruti' exists: "+ hMap.containsValue("Maruti"));

        System.out.println("key 'Five' exists: "+ hMap.containsKey("Five"));

        System.out.println("Map elements: "+ hMap);

        hMap.clear();

        System.out.println("Map elements after clear: "+ hMap);

	}
}
  • In the above example we have used different HashMap Methods.
  • hMap.put( ); is used to specify value by using specified key in the map.
  • hMap.remove( ); is used to remove the elements from the map.
  • boolean isEmpty( ); is used to return true/false if the map contains any key-value pairs.
  • hMap.size( ); is used to represent the size of the map.
  • hMap.containsValue( ); is used to return true/false if the map contains any value.
  • hMap.containsKey( ); is used to return true/false if the map contains any key.
  • hMap.clear( ); is used to clear the HashMap.
  • System.out.println( ); statement is used to define the output of the used HashMap Methods.

When you run the above example, you would get the following output:

Java HashMap Methods Example

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He is Founder and Chief Editor of JavaBeat. He has more than 8+ years of experience on developing Web applications. He writes about Spring, DOJO, JSF, Hibernate and many other emerging technologies in this blog.

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