Multithreading (монитор) !

public class Operation {
 
    public void transfer(String  a1, String  a2, int amount) throws InterruptedException {
        synchronized (a1){
            Thread.sleep(100);
            System.out.println(Thread.currentThread().getName() + " has entered in a1");
            System.out.println(Thread.currentThread().getName() + " waked up");
            synchronized (a2){
                System.out.println(Thread.currentThread().getName() + " has entered in a2");
            }
        }
    }
 
    private void sleep(long millis) {
        try {
            Thread.sleep(millis);
        } catch (InterruptedException e) {
            /* NOP */
        }
    }
}

public class DeadlockTest {
    public static void main(String[] arg) throws InterruptedException {
        final String a1 = new String ("A");
        final String a2 = new String ("B");
        final Operation operation = new Operation();
 
        Thread t0 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    operation.transfer(a1, a2, 200);
                } catch (InterruptedException e) {
                }
            }
        });
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    operation.transfer(a2, a1, 100);
                } catch (InterruptedException e) {
                }
            }
        });
        t0.start();
        t1.start();
        t0.join();
        t1.join();
        System.out.println(a1);
        System.out.println(a2);
    }
}

System.out.println(Thread.currentThread().getName() + " waked up");

synchronized (a2){
                System.out.println(Thread.currentThread().getName() + " has entered in a2");
            }

 System.out.println(Thread.currentThread().getName() + " has entered in a2");