Multithreading: wait() releases the lock while notify() wakes one waiter

wait() must be called while holding the monitor, and it gives that lock up until another thread calls notify() on the same object. The consumer here blocks in take() until the producer's put() flips the flag and notifies it.

Code

class Box {
    private String value;
    private boolean has = false;
    synchronized void put(String v) {
        value = v;
        has = true;
        notify();
    }
    synchronized String take() throws InterruptedException {
        while (!has) wait();
        has = false;
        return value;
    }
}
Box box = new Box();
String[] received = new String[1];
Thread consumer = new Thread(() -> {
    try {
        received[0] = box.take();
    } catch (InterruptedException e) { }
});
Thread producer = new Thread(() -> box.put("payload"));
consumer.start();
Thread.sleep(20);
producer.start();
producer.join();
consumer.join();
System.out.println("Consumer received: " + received[0]);

Output

Consumer received: payload