In this article, we will learn to create an unmodifiable Set in Java 9. We will be learning about Set and unmodifiable Set, and will be learning different ways of creating an unmodifiable Set(Set.of() Method and unmodifiableSet() Method).

Set

In Java, a Set is an interface that inherits the Collection interface and contains no duplicate elements. A Set interface has methods to add, delete, and check the existence of an element.

Some of the implementations of the Set interface are:

What is an unmodifiable Set?

An unmodifiable set is an unchangeable collection of unique elements that doesn't allow any modification in the set. It means that elements can't be added, removed, or replaced from a Set. Calling any mutator method on the Set causes UnsupportedOperationException.

Set<String> names = Set.of("Adithya", "Bhavish", "Chaitanya", "Jai");
names.add("Raja");

Here, names is an immutable set, and trying to add "Raja" to the set will throw an UnsupportedOperationException error.

Different ways of creating an unmodifiable Set

The following are two different ways of creating an unmodifiable Set in Java 9:

Using Set.of() Method
Using unmodifiableSet() Method

Using Set.of() Method

The immutable static factory method Set.of() can provide an easy way to create unmodifiable sets in Java 9.

Syntax

Initialization of an unmodifiable set using the Set.of() method. The below Set.of() method accepts the null values only:

Set<Integer> immutableEmptySet = Set.of();

In addition to this method, the Set interface provides various polymorphic variants that accepts different parameters. Using these you can create an unmodifiable set with a single element, two elements, three elements ... up to 10 elements. We also have a variant that accepts variable number of elements (E... elements) as a parameters.

If we want to generate an immutable set of elements from multiple input elements(n number of elements), we can use:

static <E> Set<E> of(E... elements)

If we want to generate an immutable set of 2 elements where the input elements are e1 and e2, we can use:

static <E> Set<E> of(E e1, E e2)

An instance of a set created by using the Set.of() method has the following characteristics:

An immutable set can be created using static factory methods that don't allow null elements. If we are trying to create with null elements, it throws a NullPointerException.
It rejects duplicate elements at the time of immutable set creation. The duplicate elements passed to a static factory method result in an IllegalArgumentException.
The order of iteration of set elements is unspecified and subject to change.

Example

Below is an example of creating an unmodifiable Set in Java 9 using the Set.of() method:

import java.util.Set;
public class SetOfMethodTest {
   public static void main(String args[]) {
      Set<String> names = Set.of("Adithya", "Bhavish", "Chaitanya", "Jai");
      System.out.println("Names: " + names);
   }
}

Output

Names - [Bhavish, Adithya, Jai, Chaitanya]

Using unmodifiableSet() Method

The standard way to create an unmodifiable set was by wrapping a mutable set using Collections.unmodifiableSet() in Java 9.

Syntax

Initialization of an unmodifiable Set using the Set.of() method:

Set<Integer> unmodifiableSet = Collections.unmodifiableSet(mutableSet);

An instance of a set created by using the unmodifiableSet() method has the following characteristics:

In contrast with Set.of(), unmodifiableSet() supports null values if the input set has them.
No IllegalArgumentException is raised for duplicates before making.
The iteration order is not guaranteed and depends on the original set.

Example

Below is an example of creating an unmodifiable Set in Java 9 using the unmodifiableSet() method:

import java.util.*;
public class UnmodifiableSetMethodTest {
    public static void main(String[] args) {
        Set<String> names = new HashSet<>();
        names.add("Adithya");
        names.add("Bhavish");
        names.add("Chaitanya");
        names.add("Jai");
        Set<String> unmodifiableNames = Collections.unmodifiableSet(names);
        System.out.println("Names: " + unmodifiableNames);
    }
}

Output

Names: [Chaitanya, Bhavish, Jai, Adithya]

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