How to Iterate HashSet in Java: Simple Examples and Tips

To iterate over a HashSet in Java, you can use a for-each loop, an Iterator, or the forEach method with a lambda expression. These methods allow you to access each element in the set one by one.

Syntax

Here are common ways to iterate a HashSet:

• For-each loop: Simple and readable way to access each element.
• Iterator: Gives more control, allows safe removal during iteration.
• forEach method: Uses lambda expressions introduced in Java 8 for concise iteration.

java

HashSet<Type> set = new HashSet<>();

// For-each loop
for (Type item : set) {
    // use item
}

// Iterator
Iterator<Type> iterator = set.iterator();
while (iterator.hasNext()) {
    Type item = iterator.next();
    // use item
}

// forEach method (Java 8+)
set.forEach(item -> {
    // use item
});

Example

This example shows how to create a HashSet of strings and iterate over it using all three methods.

java

import java.util.HashSet;
import java.util.Iterator;

public class HashSetIterationExample {
    public static void main(String[] args) {
        HashSet<String> fruits = new HashSet<>();
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Cherry");

        System.out.println("Using for-each loop:");
        for (String fruit : fruits) {
            System.out.println(fruit);
        }

        System.out.println("\nUsing Iterator:");
        Iterator<String> iterator = fruits.iterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.next());
        }

        System.out.println("\nUsing forEach method:");
        fruits.forEach(fruit -> System.out.println(fruit));
    }
}

Output

Using for-each loop:
Apple
Banana
Cherry

Using Iterator:
Apple
Banana
Cherry

Using forEach method:
Apple
Banana
Cherry

Common Pitfalls

Some common mistakes when iterating a HashSet include:

• Trying to access elements by index (HashSet does not support indexing).
• Modifying the set directly inside a for-each loop, which can cause ConcurrentModificationException.
• Not using an Iterator when removing elements during iteration.

Always use an Iterator if you need to remove elements while looping.

java

import java.util.HashSet;
import java.util.Iterator;

public class HashSetRemoveExample {
    public static void main(String[] args) {
        HashSet<String> set = new HashSet<>();
        set.add("One");
        set.add("Two");
        set.add("Three");

        // Wrong: Removing inside for-each loop causes error
        // for (String s : set) {
        //     if (s.equals("Two")) {
        //         set.remove(s); // Causes ConcurrentModificationException
        //     }
        // }

        // Right: Use Iterator to remove safely
        Iterator<String> it = set.iterator();
        while (it.hasNext()) {
            if (it.next().equals("Two")) {
                it.remove();
            }
        }

        System.out.println(set); // Output: [One, Three]
    }
}

Output

[One, Three]

Quick Reference

Remember these tips when iterating a HashSet:

• Use for-each loop for simple read-only iteration.
• Use Iterator when you need to remove elements safely during iteration.
• Use forEach method with lambda for concise code in Java 8 and later.
• HashSet does not maintain order, so iteration order is unpredictable.

Key Takeaways

Use a for-each loop to easily iterate over all elements in a HashSet.

Use an Iterator to safely remove elements while iterating.

The forEach method with lambda expressions offers concise iteration in Java 8+.

HashSet does not support indexing or ordered access.

Avoid modifying the HashSet directly inside a for-each loop to prevent errors.