Immutable Set in Java

beginner· Collections · Immutability

Problem

Set.of() (Java 9+) builds a Set that can never be modified after creation, the same immutability guarantee List.of() provides for lists.

Create a Set of elements that can never be added to or removed from after it's built.

Input

Set.of("Apple", "Banana", "Cherry")

Output

Contains Banana: true

Java Program

Java

import java.util.Set;

public class ImmutableSetDemo {
    public static void main(String[] args) {
        Set<String> fruits = Set.of("Apple", "Banana", "Cherry");
        System.out.println("Contains Banana: " + fruits.contains("Banana"));
        // Set.of("Apple", "Apple") would throw IllegalArgumentException — duplicates aren't allowed
    }
}

▶ Run Code

Output

Contains Banana: true

Core Logic

Building the set through the Set.of() factory method returns an immutable implementation, the same idea as List.of() but for sets.

How It Works

1Set.of("Apple", "Banana", "Cherry") returns a Set already populated with those three elements.
2Reading from it — contains(), iteration, size — works exactly like any other Set.
3Calling a mutating method like .add(...) on it throws UnsupportedOperationException, the same as List.of().
4Set.of() does not iterate elements in insertion order — the order elements print in isn't guaranteed, unlike List.of(), which does preserve the order given.

Set.of("Apple", "Banana", "Cherry").contains("Banana") returns true, read like any normal Set.

💡

Key Point: Set.of() rejects duplicate arguments outright — passing the same element twice throws IllegalArgumentException at the moment the set is built, rather than silently keeping just one copy the way a mutable HashSet's add() would.

Complexity

Time Complexity: O(n)Space Complexity: O(n)

Why: Set.of() stores each of the n given elements once, after checking for duplicates among them.

Key Concepts

Set.of()immutabilityduplicate rejection

Approach 2: Java 8

Java

import java.util.Set;
import java.util.stream.Collectors;

public class ImmutableSetStream {
    public static void main(String[] args) {
        Set<String> source = Set.of("Apple", "Banana", "Cherry");

        // Transforms each element, then collects the result into a genuinely immutable set
        Set<String> upper = source.stream()
            .map(String::toUpperCase)
            .collect(Collectors.toUnmodifiableSet());

        System.out.println("Contains BANANA: " + upper.contains("BANANA"));
        // upper.add("DATE") here would throw UnsupportedOperationException
    }
}

▶ Run Code

Output

Contains BANANA: true

Core Logic

Collectors.toUnmodifiableSet() builds an immutable set from a stream pipeline directly, for the common case where the elements are derived from existing data rather than being known ahead of time as literals.

How It Works

1source.stream() opens a stream over an existing mutable set.
2.map(String::toUpperCase) transforms each element — something Set.of() can't express, since it just takes fixed elements directly.
3.collect(Collectors.toUnmodifiableSet()) gathers the transformed elements into a genuinely immutable set, the same UnsupportedOperationException-on-mutation guarantee Set.of() provides.
4Calling .add(...) on the result throws exactly as it would on a Set.of() result.

Upper-casing {Apple, Banana, Cherry} and collecting produces the immutable set {APPLE, BANANA, CHERRY}, so contains("BANANA") returns true.

💡

Key Point: Set.of() is for elements already known as literals; Collectors.toUnmodifiableSet() is for the more common real-world case — an immutable set built from data that was transformed or derived from another collection first.

Complexity

Time Complexity: O(n)Space Complexity: O(n)

Why: map() visits each of the n source elements once, and collect() builds a new immutable set to hold the transformed elements.

Key Concepts

StreamCollectors.toUnmodifiableSet()

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