HashSet

The HashSet Class in Java

The HashSet class is one of the most commonly used implementations of the Set interface in the Java Collection Framework. It represents a collection that does not allow duplicate elements and provides constant-time performance for basic operations like add, remove, and contains. Internally, HashSet is backed by a hash table, which ensures fast lookups and insertions.

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Key Characteristics of HashSet

• Uniqueness: Does not allow duplicate elements. If a duplicate is added, add() returns false.
• No Guaranteed Order: Iteration order is not guaranteed.
• Null Handling: Allows one null element.
• Performance: Basic operations are typically O(1) assuming good hash distribution.

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Common Use Cases

• Storing unique elements.
• Fast lookup operations.
• Removing duplicates from collections.
• Membership checking (contains).

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Important Methods

Method	Description
boolean add(E e)	Adds element if not present
boolean remove(Object o)	Removes element
boolean contains(Object o)	Checks existence
int size()	Returns number of elements
boolean isEmpty()	Checks if set is empty
void clear()	Removes all elements

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Example 1: Basic Operations

import java.util.HashSet;
import java.util.Set;

public class HashSetExample {

    public static void main(String[] args) {

        Set<String> fruits = new HashSet<>();

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

        System.out.println("Fruits: " + fruits);

        System.out.println("Contains Banana? " + fruits.contains("Banana"));

        fruits.remove("Cherry");

        System.out.println("After removal: " + fruits);

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

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Example 2: Removing Duplicates from List

import java.util.*;

public class RemoveDuplicatesExample {

    public static void main(String[] args) {

        List<String> fruits = new ArrayList<>();
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Apple");
        fruits.add("Cherry");

        System.out.println("Original List: " + fruits);

        Set<String> unique = new HashSet<>(fruits);

        System.out.println("Unique Fruits: " + unique);
    }
}

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Example 3: Handling Null Values

import java.util.HashSet;
import java.util.Set;

public class HashSetNullExample {

    public static void main(String[] args) {

        Set<String> colors = new HashSet<>();

        colors.add("Red");
        colors.add(null);
        colors.add("Blue");
        colors.add(null);

        System.out.println(colors);
    }
}

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Performance Characteristics

Operation	Complexity
add()	O(1)
remove()	O(1)
contains()	O(1)
iteration	O(n)

Performance depends on good hashCode() implementation.

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When to Use HashSet

Use HashSet when:

• You need unique elements
• Ordering does not matter
• Fast lookups are required

Avoid when:

• Ordering matters → use LinkedHashSet
• Sorting required → use TreeSet

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Comparison: HashSet vs LinkedHashSet vs TreeSet

Feature	HashSet	LinkedHashSet	TreeSet
Order	No order	Insertion order	Sorted
Performance	O(1)	O(1) slightly slower	O(log n)
Null Support	One null	One null	No null
Structure	Hash table	Hash table + linked list	Red‑Black tree