Set in Java

Set in Java

In Java, a Set is a collection that:

  • Stores unique elements (no duplicates allowed).
  • Does not guarantee order (depending on the implementation).
  • Is part of the Java Collections Framework (java.util package).

🔑 Main Set Implementations

1. HashSet

  • Backed by a hash table.
  • Elements are unordered.
  • Fast for add(), remove(), contains() (O(1) on average).
import java.util.*;

public class HashSetExample {
    public static void main(String[] args) {
        Set<String> set = new HashSet<>();
        set.add("Apple");
        set.add("Banana");
        set.add("Orange");
        set.add("Apple"); // Duplicate ignored

        System.out.println(set); // Output: [Apple, Banana, Orange] (order may vary)
    }
}

2. LinkedHashSet

  • Maintains insertion order.
  • Slightly slower than HashSet due to extra ordering overhead.
Set<String> set = new LinkedHashSet<>();
set.add("One");
set.add("Two");
set.add("Three");
System.out.println(set); // Output: [One, Two, Three]

3. TreeSet

  • Implements SortedSet (elements stored in ascending order).
  • Backed by a Red-Black Tree.
  • Operations like add(), remove(), and contains() are O(log n).
Set<Integer> set = new TreeSet<>();
set.add(30);
set.add(10);
set.add(20);
System.out.println(set); // Output: [10, 20, 30]

🔧 Common Methods of Set

  • add(E e) → Adds element if not already present.
  • remove(Object o) → Removes specified element.
  • contains(Object o) → Checks if element exists.
  • size() → Returns number of elements.
  • isEmpty() → Checks if set is empty.
  • clear() → Removes all elements.
  • addAll(Collection c) → Union of sets.
  • retainAll(Collection c) → Intersection of sets.
  • removeAll(Collection c) → Difference of sets.

✅ Example: Union, Intersection, Difference

import java.util.*;

public class SetOperations {
    public static void main(String[] args) {
        Set<Integer> set1 = new HashSet<>(Arrays.asList(1, 2, 3, 4));
        Set<Integer> set2 = new HashSet<>(Arrays.asList(3, 4, 5, 6));

        // Union
        Set<Integer> union = new HashSet<>(set1);
        union.addAll(set2);
        System.out.println("Union: " + union); // [1, 2, 3, 4, 5, 6]

        // Intersection
        Set<Integer> intersection = new HashSet<>(set1);
        intersection.retainAll(set2);
        System.out.println("Intersection: " + intersection); // [3, 4]

        // Difference
        Set<Integer> difference = new HashSet<>(set1);
        difference.removeAll(set2);
        System.out.println("Difference: " + difference); // [1, 2]
    }
}

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