Iterate Over a HashMap Using Lambda Expressions in Java

📘 Premium Read: Access my best content on Medium member-only articles — deep dives into Java, Spring Boot, Microservices, backend architecture, interview preparation, career advice, and industry-standard best practices.

✅ Some premium posts are free to read — no account needed. Follow me on Medium to stay updated and support my writing.

🎓 Top 10 Udemy Courses (Huge Discount): Explore My Udemy Courses — Learn through real-time, project-based development.

▶️ Subscribe to My YouTube Channel (172K+ subscribers): Java Guides on YouTube

Lambda expressions, introduced in Java 8, provide a concise way to iterate over collections, including HashMap. By using lambda expressions, you can make your code more readable and concise. This guide will cover how to iterate over a HashMap using lambda expressions, explain how they work, and provide examples to demonstrate their functionality.

Table of Contents

1. Introduction
2. Using forEach with Lambda Expressions
3. Examples
   • Iterating Over Key-Value Pairs
   • Iterating Over Keys
   • Iterating Over Values
4. Real-World Use Case
5. Conclusion

Introduction

A HashMap in Java is a part of the Java Collections Framework and is used to store key-value pairs. Iterating over a HashMap can be done in multiple ways, but using lambda expressions makes the code more concise and readable. The forEach method in HashMap accepts a BiConsumer, which can be implemented using a lambda expression.

Using forEach with Lambda Expressions

The forEach method in HashMap allows you to iterate over the entries of the map using a BiConsumer. This method takes a lambda expression that defines the action to be performed for each entry.

Syntax

map.forEach((key, value) -> {
    // Action to be performed on each key-value pair
});

Examples

Iterating Over Key-Value Pairs

The following example demonstrates how to iterate over the key-value pairs in a HashMap using a lambda expression:

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

public class IterateHashMapExample {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();
        map.put("Apple", 1);
        map.put("Banana", 2);
        map.put("Orange", 3);

        // Iterate over key-value pairs
        map.forEach((key, value) -> {
            System.out.println("Key: " + key + ", Value: " + value);
        });
    }
}

Output:

Key: Apple, Value: 1
Key: Banana, Value: 2
Key: Orange, Value: 3

Iterating Over Keys

If you only need to iterate over the keys of the HashMap, you can use the keySet() method along with a lambda expression:

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

public class IterateKeysExample {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();
        map.put("Apple", 1);
        map.put("Banana", 2);
        map.put("Orange", 3);

        // Iterate over keys
        map.keySet().forEach(key -> {
            System.out.println("Key: " + key);
        });
    }
}

Output:

Key: Apple
Key: Banana
Key: Orange

Iterating Over Values

Similarly, if you only need to iterate over the values of the HashMap, you can use the values() method along with a lambda expression:

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

public class IterateValuesExample {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();
        map.put("Apple", 1);
        map.put("Banana", 2);
        map.put("Orange", 3);

        // Iterate over values
        map.values().forEach(value -> {
            System.out.println("Value: " + value);
        });
    }
}

Output:

Value: 1
Value: 2
Value: 3

Real-World Use Case

Processing User Data

In a user management system, you might store user information in a HashMap where the key is the user ID and the value is the user object. You can use lambda expressions to iterate over the map and perform operations such as printing user information or updating user statuses.

Example

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

class User {
    String id;
    String name;
    boolean active;

    User(String id, String name, boolean active) {
        this.id = id;
        this.name = name;
        this.active = active;
    }

    @Override
    public String toString() {
        return "User{id='" + id + "', name='" + name + "', active=" + active + "}";
    }
}

public class UserManagement {
    public static void main(String[] args) {
        Map<String, User> users = new HashMap<>();
        users.put("1", new User("1", "Alice", true));
        users.put("2", new User("2", "Bob", false));
        users.put("3", new User("3", "Charlie", true));

        // Print user information
        users.forEach((id, user) -> {
            System.out.println("User ID: " + id + ", " + user);
        });

        // Deactivate all users
        users.forEach((id, user) -> {
            user.active = false;
        });

        // Print updated user information
        System.out.println("Updated user information:");
        users.forEach((id, user) -> {
            System.out.println("User ID: " + id + ", " + user);
        });
    }
}

Output:

User ID: 1, User{id='1', name='Alice', active=true}
User ID: 2, User{id='2', name='Bob', active=false}
User ID: 3, User{id='3', name='Charlie', active=true}
Updated user information:
User ID: 1, User{id='1', name='Alice', active=false}
User ID: 2, User{id='2', name='Bob', active=false}
User ID: 3, User{id='3', name='Charlie', active=false}

Conclusion

Using lambda expressions to iterate over a HashMap in Java provides a concise and readable way to perform operations on each entry in the map. The forEach method in HashMap allows you to define actions to be performed on each key-value pair, key, or value. This approach is particularly useful in real-world applications where you need to process or manipulate data stored in maps. By leveraging lambda expressions, you can make your code more concise and maintainable.