Overview of the Set interface in Java


The Set interface in java is a part of the collection framework.

Here is the overview of the classes and interfaces in this interface:

The Set interface in Java
The Set interface in Java

Here are some characteristics of a set:

  • You cannot store duplicate values in a set
  • As a result of characteristic #1, at most 1 null value is allowed (some implementations of the Set interface in java don’t allow adding null value)

Creating a set

Java, especially since version 8, provides many convenient ways to create a set. Let’s consider some of them here:

    private static void createSets() {
        //Create an empty set
        Set<String> name = Set.of();
        //Create a set with some values
        Set<String> classmate = Set.of("Jane", "Jake");

        //Create a set with just one element
        Set<Integer> houseNumber = Collections.singleton(111);       
        //Create a HashSet
        Set<String> friends = new HashSet<>();

Basic Set operation

Here we explore the basic operations you can do on a Set in Java

Add elements

        Set<String> mySet = new HashSet<>();
        //add one element

        //add many elements
        mySet.addAll(List.of("Leo", "Neo", "Jake"));

Remove elements

        //Remove one element

        //Remove multiple elements
        mySet.removeAll(List.of("Leo", "Neo"));

        //empty the set

        //Use retainAll to keep only the selected elements
        mySet.addAll(List.of("Jim", "Derek", "Cat", "Horseman"));

        mySet.retainAll(List.of("Derek", "Cat"));

        System.out.println("Now the set is: " + mySet);

The output of the following code is:

From the code above, the retainAll method removed all other elements and only kept the elements specified in the passed-in list.

Inspecting a Set

Java provides some convenient methods to inspect the status of a set:

    private static void inspectSet() {

        Set<String> mySet = new HashSet<>(List.of("Jake", "Luis", "Liam"));

        //get number of items in the set
        System.out.println("Number of elements: " + mySet.size());

        //check if a set contain an element
        System.out.println("Contains Luis? " + mySet.contains("Luis"));


The output of that function when executed is:


This post provides a quick overview of the Set interface in Java. We will dig deeper in the sub interfaces and classes in other posts.

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