HashSet in Java

1. What is HashSet?

HashSet is a class in Java that implements the Set interface. It stores unique elements and does not guarantee insertion order.

Syntax:

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

Example:

import java.util.HashSet;

public class Main {
    public static void main(String[] args) {
        HashSet<String> fruits = new HashSet<>();

        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Apple");

        System.out.println(fruits);
    }
}

Output: The set contains Apple and Banana only. The order is not guaranteed.

2. Creating a HashSet

Creates an empty HashSet.

HashSet<Integer> numbers = new HashSet<>();

3. add()

Adds an element. Duplicate elements are ignored.

HashSet<Integer> numbers = new HashSet<>();
numbers.add(10);
numbers.add(20);
numbers.add(10);

System.out.println(numbers.size()); // 2

4. addAll()

Adds all elements from another collection.

HashSet<Integer> a = new HashSet<>();
a.add(10);
a.add(20);

HashSet<Integer> b = new HashSet<>();
b.add(30);

a.addAll(b);
System.out.println(a); // Contains 10, 20, 30

5. contains()

Checks whether an element exists.

HashSet<String> names = new HashSet<>();
names.add("Mani");

System.out.println(names.contains("Mani")); // true

6. remove()

Removes a specified element.

HashSet<Integer> numbers = new HashSet<>();
numbers.add(10);
numbers.add(20);

numbers.remove(10);
System.out.println(numbers); // Contains 20

7. size()

Returns the number of unique elements.

HashSet<Integer> numbers = new HashSet<>();
numbers.add(10);
numbers.add(20);

System.out.println(numbers.size()); // 2

8. isEmpty()

Checks whether the set is empty.

HashSet<Integer> numbers = new HashSet<>();

System.out.println(numbers.isEmpty()); // true

9. clear()

Removes all elements.

HashSet<Integer> numbers = new HashSet<>();
numbers.add(10);
numbers.add(20);

numbers.clear();
System.out.println(numbers.size()); // 0

10. Iterating a HashSet

Use an enhanced for loop to access elements.

HashSet<String> fruits = new HashSet<>();
fruits.add("Apple");
fruits.add("Mango");

for (String fruit : fruits) {
    System.out.println(fruit);
}

The iteration order is not guaranteed.

11. Duplicates in HashSet

HashSet does not allow duplicate elements.

HashSet<Integer> numbers = new HashSet<>();

numbers.add(10);
numbers.add(10);
numbers.add(20);

System.out.println(numbers.size()); // 2

12. Null Values in HashSet

HashSet allows one null element.

HashSet<String> names = new HashSet<>();

names.add(null);
names.add(null);

System.out.println(names.size()); // 1

13. HashSet with Integer Values

HashSet<Integer> numbers = new HashSet<>();

numbers.add(30);
numbers.add(10);
numbers.add(20);

System.out.println(numbers.contains(20)); // true

The elements are not automatically sorted.

14. Removing Duplicate Values from a List

HashSet can help remove duplicate elements from a list.

import java.util.*;

public class Main {
    public static void main(String[] args) {
        List<Integer> numbers =
                Arrays.asList(10, 20, 10, 30, 20);

        HashSet<Integer> uniqueNumbers =
                new HashSet<>(numbers);

        System.out.println(uniqueNumbers);
    }
}

Output: Contains 10, 20, and 30; order is not guaranteed.

15. HashSet vs ArrayList

16. HashSet vs LinkedHashSet vs TreeSet

17. Important HashSet Methods

18. Time Complexity

Note: HashSet operations can take O(n) in certain collision-heavy cases.

Conclusion

When Should You Use HashSet in Java?

HashSet is a Java Collection used to store unique elements without guaranteeing their order.

Key points to remember:

• Duplicate elements are ignored.

• One null element is allowed.

• Index-based access is not supported.

• add(), remove(), and contains() typically take O(1) average time.

• Use LinkedHashSet to preserve insertion order and TreeSet to maintain sorted order.