public class TestSync {
    private int value;
    private List<Integer> list = new ArrayList<>();


    public synchronized int increment() {
        for (int i = 0; i < 1000; i++) {
            list.add(value++);
        }
        return list.size();
    }

    public int getValue() {
        return this.value;
    }
}

 TestSync s1 = new TestSync();
        Callable<String> callable = () -> {
            // Perform some computation
            new Thread(s1::increment).start();
            new Thread(s1::increment).start();

            System.out.println("Result: " + s1.getValue());
            System.out.println("Result: " + s1.getValue());

            Thread.sleep(1000);
            return "Hello from Callable";
        };