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Java Set Interface

The Java Set Interface is part of the Java Collection Framework. It extends the Collection interface and is an unordered collection of objects, in which duplicate values cannot be stored. It is an interface that implements a mathematical set. This interface contains the methods inherited from the Collection interface and adds a feature that restricts the insertion of duplicate elements.

Let's dive into the specifics of the Set interface and how to use it effectively in your Java programs.

Declaring a Set​

In Java, you can declare a Set like this:

Set<data-type> set = new HashSet<data-type>();

Here data-type can be any Java data type, and HashSet is a class that implements Set. There are multiple classes in Java that implements Set interface, and we will look into those later.

Set Interface Methods​

The Set interface includes all the methods of the Collection interface. Some of the commonly used methods are:

  • add(element): This method is used to add elements to the set. The function returns true if the element is not already present in the set, otherwise false.
  • addAll(collection): This method is used to add all the elements from the specified collection to the set.
  • remove(element): This method is used to remove an element from the set.
  • removeAll(collection): This method is used to remove all the elements from the set which are present in the specified collection.
  • contains(element): This method is used to check whether an element is present in the set or not.
  • size(): This method is used to get the size of the set.
  • isEmpty(): This method is used to check if the set is empty.

Implementations of Set Interface​

There are three classes that implement the Set interface in Java:

  • HashSet: This class implements the Set interface, backed by a hash table (actually a HashMap instance). It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time.

  • LinkedHashSet: This class is an ordered version of HashSet that maintains a doubly-linked List across all elements.

  • TreeSet: This class implements the Set interface that uses a tree for storage. Objects are stored in a sorted and ascending order.

Set Interface Example​

Let's look at a simple example demonstrating the use of Set:

import java.util.*;

class Main {
  public static void main(String[] args) {
    Set<String> set = new HashSet<String>();
    set.add("Hello");
    set.add("World");
    set.add("!");
    System.out.println("Set: " + set);
    System.out.println("Size of Set: " + set.size());
  
    if(set.contains("Hello")) {
        System.out.println("Hello is in the set.");
    }

    set.remove("Hello");
    System.out.println("After removing 'Hello': " + set);
  }
}

In this tutorial, we have seen the basics of the Set interface in Java, its methods and its different implementations. It is a powerful interface that allows you to manipulate collections of unique elements. Make use of it in your programs whenever you need a collection that cannot contain duplicate elements. Happy Coding!