Java HashMap Loop – Iterate Over Keys, Values, and Entries

Introduction

HashMap is one of the most used data structures in Java. Whether you’re counting word frequencies, mapping users to settings, or processing key-value data, knowing how to loop through a HashMap efficiently is a must.

By the end of this article, you’ll know how to:

• Iterate over keys, values, and key-value pairs
• Use both classic and Java 8+ looping styles
• Choose the best loop for your use case

Sample HashMap

Let’s use this HashMap<String, Integer> for all examples:

import java.util.HashMap;

HashMap<String, Integer> map = new HashMap<>();
map.put("Apple", 10);
map.put("Banana", 20);
map.put("Mango", 15);
map.put("Grapes", 12);

This map stores fruit names and their quantities.

1. Loop Through Keys (keySet)

for (String key : map.keySet()) {
    System.out.println("Key: " + key + ", Value: " + map.get(key));
}

Explanation:

• map.keySet() returns a set of all keys.
• map.get(key) fetches the corresponding value.

Use this if you only need to process keys.

2. Loop Through Values (values)

for (Integer value : map.values()) {
    System.out.println("Value: " + value);
}

Explanation:

• map.values() returns all values.
• Doesn’t give access to keys.

Use this when keys are not needed.

3. Loop Through Entries (entrySet)

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

Explanation:

• entrySet() provides both key and value in each loop.
• entry.getKey() and entry.getValue() access the elements.

This is the most efficient and preferred method for full map traversal.

4. Java 8 forEach + Lambda

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

Explanation:

• Uses forEach() with a lambda expression.
• Clean and functional style.

Best for modern Java codebases (Java 8+).

Comparison Table

Summary

In this article, you learned:

• Multiple ways to loop through a HashMap in Java
• When to use keySet(), values(), entrySet(), and forEach()
• Best practices for modifying or filtering data

Knowing the right iteration method boosts performance and keeps your code clean.

FAQ – Java HashMap Looping

Which loop is the most efficient for HashMap?

entrySet() loop is generally the fastest and most memory-efficient when both keys and values are needed.

Can I modify values during looping?

Yes. Use entry.setValue(newValue) inside an entrySet() loop to modify values directly.

Can I remove elements while looping?

Use an Iterator to safely remove elements during iteration:

Iterator<Map.Entry<String, Integer>> iterator = map.entrySet().iterator();
while (iterator.hasNext()) {
    Map.Entry<String, Integer> entry = iterator.next();
    if (entry.getValue() < 15) {
        iterator.remove();
    }
}