Java Program to Iterate over a HashMap

Java Program to Iterate over a HashMap

In this example, we will learn to iterate over keys, values, and key/value mappings of a Java HashMap.

To understand this example, you should have the knowledge of the following Java programming topics:


In Java HashMap, we can iterate through its keys, values, and key/value mappings.

Example 1: Iterate through HashMap using the forEach loop

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

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

    // Creating a HashMap
    HashMap<String, String> languages = new HashMap<>();
    languages.put("Java", "Enterprise");
    languages.put("Python", "ML/AI");
    languages.put("JavaScript", "Frontend");
    System.out.println("HashMap: " + languages);

    // iterating through key/value mappings
    System.out.print("Entries: ");
    for(Entry<String, String> entry: languages.entrySet()) {
      System.out.print(entry);
      System.out.print(", ");
    }

    // iterating through keys
    System.out.print("\nKeys: ");
    for(String key: languages.keySet()) {
      System.out.print(key);
      System.out.print(", ");
    }

    // iterating through values
    System.out.print("\nValues: ");
    for(String value: languages.values()) {
      System.out.print(value);
      System.out.print(", ");
    }
  }
}

Output

HashMap: {Java=Enterprise, JavaScript=Frontend, Python=ML/AI}
Entries: Java=Enterprise, JavaScript=Frontend, Python=ML/AI, 
Keys: Java, JavaScript, Python,
Values: Enterprise, Frontend, ML/AI,

In the above example, we have created a hashmap named languages. Here, we have used the forEach loop to iterate through the elements of the hashmap.

Notice that we are independently iterating through the keys, values, and key/value mappings.

Note: We have used the Map.Entry class. It is the nested class that returns a view of the map.


Example 2: Iterate through HashMap using iterator()

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

class Main {
  public static void main(String[] args) {
    // create a HashMap
    HashMap<String, String> languages = new HashMap<>();
    languages.put("Java", "Enterprise");
    languages.put("Python", "ML/AI");
    languages.put("JavaScript", "Frontend");
    System.out.println("HashMap: " + languages);

    // create an object of Iterator
    Iterator<Entry<String, String>> iterate1 = languages.entrySet().iterator();

    // iterate through key/value mappings
    System.out.print("Entries: ");
    while(iterate1.hasNext()) {
      System.out.print(iterate1.next());
      System.out.print(", ");
    }

    // iterate through keys
    Iterator<String> iterate2 = languages.keySet().iterator();
    System.out.print("\nKeys: ");
    while(iterate2.hasNext()) {
      System.out.print(iterate2.next());
      System.out.print(", ");
    }

    // iterate through values
    Iterator<String> iterate3 = languages.values().iterator();
    System.out.print("\nValues: ");
    while(iterate3.hasNext()) {
      System.out.print(iterate3.next());
      System.out.print(", ");
    }
  }
}

Output

HashMap: {Java=Enterprise, JavaScript=Frontend, Python=ML/AI}
Entries: Java=Enterprise, JavaScript=Frontend, Python=ML/AI, 
Keys: Java, JavaScript, Python,
Values: Enterprise, Frontend, ML/AI,

In the above example, we are iterating through keys, values, and key/value mappings of the hash map. We have used the iterator() method to iterate over the hashmap. Here,

  • hasNext() - returns true if there is next element in the hashmap
  • next() - returns the next element of the hashmap

Note: We can also use the HashMap forEach() method to iterate over the hashmap.