java.util.HashMap class inherits AbstractMap class and implements the Map interface. HashMap values store on the basis of key and value pair where each pair is known as an Entry.

• K: It’s the type of Key in HashMap
• V: It’s a type of value with respect to Key.

HashMap Class Declaration

public class HashMap extends implements Map, Cloneable, Serializable  

See Also:

• Java: How HashMap works?
• Java: Difference between HashMap and Hashtable

Points to Remember:

• HashMap uses data structure as a Hash Table.
• HashMap store values based on keys.
• HashMap contains unique keys.
• HashMap allows duplicate values.
• HashMap doesn’t maintain order.
• HashMap class allows only one null key and multiple null values.
• HashMap is not synchronized.
• HashMap initial default capacity is 16 elements with a load factor of 0.75.

Difference between HashSet and HashMap

HashSet Class contains only values whereas HashMap Class contains an entry (key and value pair).

HashMap Class Constructors

HashMap Class Methods

Example: add elements

Here, you will learn different ways to insert elements.

import java.util.*;

class HashMapExample1 {
	public static void main(String args[]) {
		HashMap<Integer,String> hm = new HashMap<Integer,String>();
		System.out.println("Initial hash map elements: " + hm );
		hm.put(20, "Anuj");
		hm.put(21, "Virendra");
		hm.put(22, "Raghav");

		System.out.println("\nAfter invoking put() method");
		for (Map.Entry m : hm.entrySet()) {
			System.out.println(m.getKey() + " " + m.getValue());
		}

		hm.putIfAbsent(23, "Gaurav");
		System.out.println("\nAfter invoking putIfAbsent() method");
		for (Map.Entry m : hm.entrySet()) {
			System.out.println(m.getKey() + " " + m.getValue());
		}
		HashMap map = new HashMap();
		map.put(24, "Ravi");
		map.putAll(hm);
		System.out.println("\nAfter invoking putAll() method ");
		for (Map.Entry m : map.entrySet()) {
			System.out.println(m.getKey() + " " + m.getValue());
		}
	}
}

Output :

Initial hash map elements: {}

After invoking put() method
20 Anuj
21 Virendra
22 Raghav

After invoking putIfAbsent() method
20 Anuj
21 Virendra
22 Raghav
23 Gaurav

After invoking putAll() method 
20 Anuj
21 Virendra
22 Raghav
23 Gaurav
24 Ravi

HashMap Example : remove()

Here you will see different ways to remove elements from HashMap

import java.util.*;
import java.util.HashMap;

public class HashMapExample2 {
	public static void main(String args[]) {
		HashMap<Integer,String> map = new HashMap<Integer,String>();
map.put(20, "Anuj");
		map.put(21, "Virendra");
		map.put(22, "Raghav");
		map.put(23, "Gaurav");
		System.out.println("Initial hash map elements: " + map);
		// key-based removal
		map.remove(20);
		System.out.println("\nUpdated hash map  elements: " + map);
		// value-based removal
		map.remove(21);
		System.out.println("\nUpdated hash map  elements: " + map);
		// key-value pair based removal
		map.remove(22, "Raghav");
		System.out.println("\nUpdated hash map  elements: " + map);
	}
}

Output :

Initial hash map elements: {20=Anuj, 21=Virendra, 22=Raghav, 23=Gaurav}

Updated hash map  elements: {21=Virendra, 22=Raghav, 23=Gaurav}

Updated hash map  elements: {22=Raghav, 23=Gaurav}

Updated hash map  elements: {23=Gaurav}

HashMap Example : replace()

Here you will see different ways to replace() elements in HashMap

import java.util.*;

class HashMapExample3 {
	public static void main(String args[]) {
		HashMap<Integer,String> hm = new HashMap<Integer,String>();
		hm.put(20, "Anuj");
		hm.put(21, "Virendra");
		hm.put(22, "Raghav");
		System.out.println("Initial hash map elements:");
		for (Map.Entry m : hm.entrySet()) {
			System.out.println(m.getKey() + " " + m.getValue());
		}
		System.out.println("\nUpdated hash map elements:");
		hm.replace(22, "Gaurav");
		for (Map.Entry m : hm.entrySet()) {
			System.out.println(m.getKey() + " " + m.getValue());
		}
		System.out.println("\nUpdated hash map elements:");
		hm.replace(21, "Virendra", "Ravi");
		for (Map.Entry m : hm.entrySet()) {
			System.out.println(m.getKey() + " " + m.getValue());
		}
		System.out.println("\nUpdated hash map elements:");
		hm.replaceAll((k, v) -> "Anuj");
		for (Map.Entry m : hm.entrySet()) {
			System.out.println(m.getKey() + " " + m.getValue());
		}
	}
}

Output :

Initial hash map elements:
20 Anuj
21 Virendra
22 Raghav

Updated hash map elements:
20 Anuj
21 Virendra
22 Gaurav

Updated hash map elements:
20 Anuj
21 Ravi
22 Gaurav

Updated hash map elements:
20 Anuj
21 Anuj
22 Anuj

HashMap Example: Objects handling

import java.util.*;
public class Magzine {
	int id;
	String name,author,publisher;
	int quantity;
	public Magzine(int id, String name, String author, String publisher, int quantity) {
	    this.id = id;
	    this.name = name;
	    this.author = author;
	    this.publisher = publisher;
	    this.quantity = quantity;
	}    

}
public class MapExample {
import java.util.HashMap;
import java.util.Map;

public class HashMapWithObjectsExample {

	public static void main(String[] args) {
	    //Creating map of Magzines
	    Map<Integer,Magzine> map=new HashMap<Integer,Magzine>();
	    //Creating Books
	    Magzine m1=new Magzine(21,"The Sun","Sy Sunfranchy","The Sun Company",8);
	    Magzine m2=new Magzine(22,"Glimmer Trains","Unknown","Glimmer Train Press",4);
	    Magzine m3=new Magzine(23,"Crazy horse","Brett Lot","College of Charleston",6);
	    //Adding Magzines to map
	    map.put(1,m1);
	    map.put(2,m2);
	    map.put(3,m3);  

	    //Traversing map
	    for(Map.Entry<Integer,Magzine> entry:map.entrySet()){
	        int key=entry.getKey();
	        Magzine m=entry.getValue();
	        System.out.println("\nMagzine "+key+" Details:");
	        System.out.println(m.id+" "+m.name+" "+m.author+" "+m.publisher+" "+m.quantity);
	    }
	}
}

Output :

Magzine 1 Details:
21 The Sun Sy Sunfranchy The Sun Company 8

Magzine 2 Details:
22 Glimmer Trains Unknown Glimmer Train Press 4

Magzine 3 Details:
23 Crazy horse Brett Lot College of Charleston 6

References

• https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html