Looping Through a HashMap in Java: Methods and Examples

Introduce Java How To Loop Through a HashMap and give examples

In Java, a HashMap is a part of the Java Collections Framework and is used to store key-value pairs. Looping through a HashMap can be done in several ways, depending on what you want to achieve. Below are some common methods to iterate through a HashMap along with examples.

1. Using for-each Loop with entrySet()

The entrySet() method returns a set view of the mappings contained in the map. You can then use a for-each loop to iterate through the entries.

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

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

        // Loop through the HashMap using entrySet()
        for (Map.Entry<String, Integer> entry : map.entrySet()) {
            System.out.println("Key: " + entry.getKey() + ", Value: " + entry.getValue());
        }
    }
}

2. Using for-each Loop with keySet()

You can also iterate through the keys of the HashMap using the keySet() method and then retrieve the corresponding values.

import java.util.HashMap;

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

        // Loop through the HashMap using keySet()
        for (String key : map.keySet()) {
            System.out.println("Key: " + key + ", Value: " + map.get(key));
        }
    }
}

3. Using forEach Method (Java 8 and above)

Java 8 introduced the forEach method, which allows you to iterate through the HashMap using a lambda expression.

import java.util.HashMap;

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

        // Loop through the HashMap using forEach
        map.forEach((key, value) -> {
            System.out.println("Key: " + key + ", Value: " + value);
        });
    }
}

4. Using Iterator

You can also use an Iterator to loop through the HashMap. This is useful if you need to remove entries while iterating.

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

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

        // Loop through the HashMap using an Iterator
        Iterator<Map.Entry<String, Integer>> iterator = map.entrySet().iterator();
        while (iterator.hasNext()) {
            Map.Entry<String, Integer> entry = iterator.next();
            System.out.println("Key: " + entry.getKey() + ", Value: " + entry.getValue());
        }
    }
}

Summary

These are some of the common ways to loop through a HashMap in Java. Depending on your specific needs (e.g., whether you need to modify the map during iteration), you can choose the method that best suits your requirements.