Java: How to Loop Through a HashMap

A HashMap in Java is a collection that stores key-value pairs. It’s widely used for fast data retrieval based on keys. In this tutorial by The Coding College, we’ll explore multiple ways to iterate through a HashMap effectively.

Example HashMap Setup

Let’s start with a simple example HashMap to use in all our demonstrations:

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        HashMap<String, Integer> people = new HashMap<>();
        people.put("Alice", 25);
        people.put("Bob", 30);
        people.put("Charlie", 35);

        // Use the examples below to loop through this HashMap.
    }
}

Method 1: Using a For-Each Loop for EntrySet

The entrySet() method returns a set of key-value pairs, which can be iterated with a for-each loop.

Example Code

for (HashMap.Entry<String, Integer> entry : people.entrySet()) {
    System.out.println("Name: " + entry.getKey() + ", Age: " + entry.getValue());
}

Output:

Name: Alice, Age: 25  
Name: Bob, Age: 30  
Name: Charlie, Age: 35

Method 2: Using a For-Each Loop for Keys and Values

You can iterate through the keys and fetch corresponding values using keySet().

Example Code

for (String name : people.keySet()) {
    System.out.println("Name: " + name + ", Age: " + people.get(name));
}

Output:

Name: Alice, Age: 25  
Name: Bob, Age: 30  
Name: Charlie, Age: 35

Method 3: Using an Iterator

An Iterator provides another way to traverse the entries in the HashMap.

Example Code

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

Iterator<Map.Entry<String, Integer>> iterator = people.entrySet().iterator();
while (iterator.hasNext()) {
    Map.Entry<String, Integer> entry = iterator.next();
    System.out.println("Name: " + entry.getKey() + ", Age: " + entry.getValue());
}

Output:

Name: Alice, Age: 25  
Name: Bob, Age: 30  
Name: Charlie, Age: 35

Method 4: Using Java Streams

Introduced in Java 8, Streams offer a modern and concise way to loop through a HashMap.

Example Code

people.forEach((name, age) -> System.out.println("Name: " + name + ", Age: " + age));

Output:

Name: Alice, Age: 25  
Name: Bob, Age: 30  
Name: Charlie, Age: 35

Method 5: Using a While Loop

If you prefer a while loop for better control over the iteration, you can combine it with an Iterator.

Example Code

Iterator<String> keyIterator = people.keySet().iterator();
while (keyIterator.hasNext()) {
    String name = keyIterator.next();
    System.out.println("Name: " + name + ", Age: " + people.get(name));
}

Output:

Name: Alice, Age: 25  
Name: Bob, Age: 30  
Name: Charlie, Age: 35

Method 6: Iterating Over Values Only

To iterate over values alone, use the values() method.

Example Code

for (Integer age : people.values()) {
    System.out.println("Age: " + age);
}

Output:

Age: 25  
Age: 30  
Age: 35

Best Practices

1. Choose the Right Method:
   • Use entrySet() for both keys and values.
   • Use keySet() if you only need keys or need to fetch values dynamically.
   • Use forEach for modern, concise code.
2. Thread Safety: Use ConcurrentHashMap instead of HashMap in multithreaded environments to avoid ConcurrentModificationException.
3. Performance: Iterating over entrySet is generally faster than fetching values repeatedly with keySet.

Conclusion

Java offers a variety of ways to loop through a HashMap, making it flexible for different use cases. Choose the method that best suits your specific requirements.

For more in-depth tutorials, visit The Coding College and take your coding skills to the next level!