HashMap

A basic implementation of the Map interface in Java. Uses keys in the same way as an Array uses an index. Duplicate keys are not allowed. Existing value will be overridden when a key is reused.

Note

HashMaps are not ordered and not thread safe.

Creating a HashMap

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

Here K is type of key and V is type of value.

Adding elements to the HashMap

We can use put() method to add elements to the HashMap.

import java.util.HashMap;

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

    // create a hashmap
    HashMap<String, Integer> numbers = new HashMap<>();

    // put() method to add elements
    numbers.put("key1", 1);
    numbers.put("key2", 2);
    numbers.put("key3", 3);
    System.out.println("HashMap after put(): " + numbers);
  }
}

Output

HashMap after put(): {key1=1, key2=2, key3=3}

Changing the elements in HashMap

import java.io.*;
import java.util.*;
class ChangeElementsOfHashMap {
    public static void main(String args[])
    {
        HashMap<String, Integer> hm = new HashMap<String, Integer>();
        hm.put("key1", 1);
        hm.put("key2", 2);
        hm.put("key3", 3);

        System.out.println("Initial Map " + hm);
        //Changing the element
        hm.put("key2", 5);
        System.out.println("Updated Map " + hm);
    }
}

Output

Initial Map {key1=1, key2=2, key3=3}
Updated Map {key1=1, key2=5, key3=3}

Access the elements

import java.util.HashMap;

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

    // create a hashmap
    HashMap<String, Integer> numbers = new HashMap<>();

    // put() method to add elements
    numbers.put("key1", 1);
    numbers.put("key2", 2);
    numbers.put("key3", 3);

    // get the value of key2
    int value2 = numbers.get("key2");
    System.out.println(value2);
  }
}

Output

2

Remove the elements from HashMap

import java.util.HashMap;

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

    // create a hashmap
    HashMap<String, Integer> numbers = new HashMap<>();

    numbers.put("key1", 1);
    numbers.put("key2", 2);
    numbers.put("key3", 3);
    System.out.println("Before: "+numbers);
    //remove the element
    numbers.remove("key2");
    System.out.println("After: "+numbers);


  }
}

Output

Before: {key1=1, key2=2, key3=3}
After: {key1=1, key3=3}

Traversal of Java HashMap

import java.util.HashMap;
import java.util.Map;

public class Test {
    public static void main(String[] args)
    {
        // initialize a HashMap
        HashMap<String, Integer> numbers = new HashMap<>();

        numbers.put("key1", 1);
        numbers.put("key2", 2);
        numbers.put("key3", 3);

        for(Map.Entry<String,Integer> e:numbers.entrySet()){
            System.out.println("Key: "+e.getKey()+" Value: "+e.getValue());
        }
    }
}

Output

Key: key1 Value: 1
Key: key2 Value: 2
Key: key3 Value: 3