How to create an immutable class in Java?

1. Declare the class final (no subclass can break it).
2. Make all fields private final.
3. Do not write setters.
4. Set all fields in the constructor.
5. For mutable fields (like a list or a date) make a defensive copy when you get it and when you return it.

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Main {
    static final class Team {
        private final String name;
        private final List<String> members;

        Team(String name, List<String> members) {
            this.name = name;
            this.members = List.copyOf(members);           // copy in
        }

        String name() { return name; }
        List<String> members() { return members; }          // List.copyOf is already unmodifiable
    }

    public static void main(String[] args) {
        List<String> list = new ArrayList<>(List.of("Riya", "Aman"));
        Team t = new Team("Java", list);
        list.add("Hacker");                                 // does not change the team
        System.out.println(t.members());
        try {
            t.members().add("Another");
        } catch (UnsupportedOperationException e) {
            System.out.println("Cannot change the members");
        }
    }
}

A record is the shortest way, if you only need the data and you copy the mutable parts yourself. See Java Records.

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