synchronized

There are several ways to avoid race conditions, one of them is using syncrhonzied methods or blocks. synchronized ensures that only one thread can execute it at a time.

syncrhonized blocks

public class Main {
    private static int counter = 0;

    public static void main(String[] args) {
        Object lock = new Object();

        for (int i = 0; i < 5; i++) {
            Thread thread = new Thread(() -> {
                synchronized (lock) {
                    for (int j = 0; j < 10; j++) {
                        counter++;
                        System.out.println(
                            Thread.currentThread().getName() + " " + counter);
                        try {
                            Thread.sleep(50);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
            });
            thread.start();
        }
    }
}

syncrhonized methods

This is more effecient than the above example, because only the crucial part is synchronzied and the threads can sleep at the same time.

public class Main {
    private static int counter = 0;

    public synchronized static void count() {
        counter++;
    }

    public static void main(String[] args) {
        for (int i = 0; i < 5; i++) {
            Thread thread = new Thread(() -> {
                for (int j = 0; j < 10; j++) {
                    count();
                    System.out.println(
                        Thread.currentThread().getName() + " " + counter);
                    try {
                        Thread.sleep(50);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            });
            thread.start();
        }
    }
}