Defensive copies

Definition

A defensive copy means copying a mutable object when it goes into or comes out of your class. Without it, the caller keeps a reference to your internal data and can change it behind your back, even if the field is final.

Example code

Copying a list on the way in, and exposing a read-only view

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

final class Team {
    private final List<String> members;

    Team(List<String> members) {
        this.members = new ArrayList<>(members);               // copy on the way in
    }

    List<String> members() {
        return Collections.unmodifiableList(members);          // read-only view on the way out
    }
}

class Demo {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>(List.of("Asha", "Ravi"));
        Team team = new Team(names);

        names.add("Hacker");                                    // the caller changes its own list
        System.out.println(team.members());                     // the team is not affected

        try {
            team.members().add("Hacker");
        } catch (UnsupportedOperationException e) {
            System.out.println("cannot change the team from outside");
        }
    }
}

// Output:
// [Asha, Ravi]
// cannot change the team from outside