🔁 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

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📘 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.

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🔁 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.

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🔄 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.

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🔃 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.

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🚀 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+).

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📋 Comparison Table

Loop Type	Access Keys	Access Values	Access Entries	Java Version
keySet()	✅ Yes	✅ Yes (with get)	❌ No	Java 1.2+
values()	❌ No	✅ Yes	❌ No	Java 1.2+
entrySet()	✅ Yes	✅ Yes	✅ Yes	Java 1.2+
forEach() (Lambda)	✅ Yes	✅ Yes	✅ Yes	Java 8+

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📌 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.

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❓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();
    }
}

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