Affecting variables in different classes in Java

Question

I have two classes that I'm trying to manipulate one variable with, as an example

public class A {

    public static void main(String[] args) {

        while(game_over[0] == false) {
            System.out.println("in the while-loop");
        }
        System.out.println("out of the while-loop");
    }

    static boolean[] game_over = {false};
}

and

public class B {

    public boolean[] game_over;

    public printBoard(boolean[] game_over) {

        this.game_over = game_over;
    }

    public void run() {

        for (int i = 0; i < 10; i++) {
            // do something
        }
        game_over[0] = true;
        System.out.println("GAME OVER");
    }
}

The code snippets provided are not meant to be actual workable code, I'm more concerned with the concept. In my program, class A creates a thread that utilizes class B, and I want class B to affect the variable 'game_over' such that the while-loop in class A will be affected by the change... any idea how I can successfully update the variable? Thanks.

Answer 1

Don't use an array for this, that makes it harder to ensure a data-race free application.

Since you want to be able to pass around the game_over flag as an independent object, the easiest way to achieve a correct multi-threaded application is to use the AtomicBoolean class.

import java.util.concurrent.atomic.AtomicBoolean;

class B {
    private AtomicBoolean game_over;

    public B(AtomicBoolean game_over) {
        this.game_over = game_over;
    }

    public void run() {
        // do stuff
        game_over.set(true);
    }
}

and in your class A:

public class A {
    static AtomicBoolean game_over = new AtomicBoolean();

    public static void main(String[] args) {
        B b = new B();
        Thread t = new Thread(b);
        t.start();

        while (!game_over.get()) {
            System.out.println("in the while-loop");
        }
        System.out.println("out of the while-loop");
    }
}