How to get keys from a HashMap in Java?

How to Get Keys from a HashMap in Java

In Java, HashMap is a part of the Java Collections Framework and is used to store key-value pairs. Sometimes, you might need to retrieve all the keys from a HashMap. This can be done in several ways using the keySet(), entrySet(), and forEach() methods provided by the HashMap class.

Using keySet()

The keySet() method returns a Set view of the keys contained in the HashMap. This is the most straightforward way to get all the keys.

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

public class Main {
    public static void main(String[] args) {
        // Create a HashMap
        Map<String, Integer> map = new HashMap<>();
        map.put("apple", 1);
        map.put("banana", 2);
        map.put("cherry", 3);

        // Get the keys using keySet()
        Set<String> keys = map.keySet();

        // Iterate over the keys
        for (String key : keys) {
            System.out.println("Key: " + key);
        }
    }
}

Using entrySet()

The entrySet() method returns a Set view of the mappings contained in the HashMap. You can iterate over this set and extract the keys from each entry.

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

public class Main {
    public static void main(String[] args) {
        // Create a HashMap
        Map<String, Integer> map = new HashMap<>();
        map.put("apple", 1);
        map.put("banana", 2);
        map.put("cherry", 3);

        // Get the entries using entrySet()
        Set<Map.Entry<String, Integer>> entries = map.entrySet();

        // Iterate over the entries and get the keys
        for (Map.Entry<String, Integer> entry : entries) {
            String key = entry.getKey();
            System.out.println("Key: " + key);
        }
    }
}

Using forEach()

Java 8 introduced the forEach() method, which allows you to perform an action for each entry in the HashMap. You can use this method to retrieve the keys.

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

public class Main {
    public static void main(String[] args) {
        // Create a HashMap
        Map<String, Integer> map = new HashMap<>();
        map.put("apple", 1);
        map.put("banana", 2);
        map.put("cherry", 3);

        // Use forEach to iterate over the keys
        map.forEach((key, value) -> System.out.println("Key: " + key));
    }
}

Summary

• keySet(): Returns a Set view of the keys contained in the HashMap. It’s the most direct way to get all the keys.
• entrySet(): Returns a Set view of the mappings contained in the HashMap. You can iterate over this set and extract the keys.
• forEach(): Allows you to perform an action for each entry in the HashMap. You can use it to retrieve and process the keys.

Example of All Methods Combined

Here’s a combined example demonstrating all the above methods:

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

public class Main {
    public static void main(String[] args) {
        // Create a HashMap
        Map<String, Integer> map = new HashMap<>();
        map.put("apple", 1);
        map.put("banana", 2);
        map.put("cherry", 3);

        // Using keySet()
        System.out.println("Using keySet():");
        Set<String> keys = map.keySet();
        for (String key : keys) {
            System.out.println("Key: " + key);
        }

        // Using entrySet()
        System.out.println("\nUsing entrySet():");
        Set<Map.Entry<String, Integer>> entries = map.entrySet();
        for (Map.Entry<String, Integer> entry : entries) {
            String key = entry.getKey();
            System.out.println("Key: " + key);
        }

        // Using forEach()
        System.out.println("\nUsing forEach():");
        map.forEach((key, value) -> System.out.println("Key: " + key));
    }
}

By using these methods, you can effectively retrieve and work with the keys in a HashMap in Java. For more in-depth knowledge and practical examples on Java collections and other programming concepts, consider exploring Grokking the Coding Interview on DesignGurus.io, which provides comprehensive courses on essential coding and interview techniques.