What Is Set Data Structure Java?


Scott Campbell

The Set data structure is a fundamental concept in Java programming. It represents a collection of unique elements, where each element can only occur once within the set.

In other words, a set does not allow duplicate values. This makes it a powerful tool for various operations such as eliminating duplicates, membership testing, and performing mathematical set operations like union, intersection, and difference.

Creating a Set

To create a set in Java, we can use the HashSet class from the java.util package. Here’s an example:

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

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

In this example, we created a set called “fruits” to store strings. We added three different fruits to the set using the add() method.

Operations on Sets

Sets provide various methods to perform operations like adding elements, removing elements, checking membership, and more. Here are some commonly used methods:

  • add(element): Adds an element to the set if it doesn’t already exist.
  • remove(element): Removes an element from the set.
  • contains(element): Checks if an element is present in the set.
  • size(): Returns the number of elements in the set.
  • isEmpty(): Checks if the set is empty.

Let’s see these methods in action:

// Adding elements to the set
fruits.add("apple"); // This won't be added since "apple" already exists

// Removing an element from the set

// Checking membership
System.out.println(fruits.contains("banana")); // Output: true

// Getting the size of the set
System.size()); // Output: 3

// Checking if the set is empty
System.isEmpty()); // Output: false

Set Operations

Sets also provide methods to perform mathematical set operations like union, intersection, and difference. These operations can be useful when dealing with multiple sets. Let’s see how to perform these operations using sets:


The union of two sets contains all the unique elements from both sets. In Java, we can use the addAll() method to perform the union operation.

Set set1 = new HashSet<>();

Set set2 = new HashSet<>();

Set union = new HashSet<>(set1);

System.println(union); // Output: [a, b, c]


The intersection of two sets contains only the elements that exist in both sets. In Java, we can use the retainAll() method to perform the intersection operation.

Set intersection = new HashSet<>(set1);

System.println(intersection); // Output: [b]


The difference of two sets contains the elements that exist in one set but not in the other. In Java, we can use the removeAll() method to perform the difference operation.

Set difference = new HashSet<>(set1);

System.println(difference); // Output: [a]


In summary, the Set data structure in Java provides a way to store unique elements and perform various operations on them. It is a powerful tool for eliminating duplicates and performing mathematical set operations. By understanding how to create and manipulate sets using Java’s HashSet class, you can effectively handle collections of unique elements in your programs.

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