Implement a bounded blocking queue with wait and notify.

class BoundedQueue<T> {
    private final Queue<T> items = new ArrayDeque<>();
    private final int capacity;

    BoundedQueue(int capacity) {
        this.capacity = capacity;
    }

    synchronized void put(T item) throws InterruptedException {
        while (items.size() == capacity) {
            wait();
        }
        items.add(item);
        notifyAll();
    }

    synchronized T take() throws InterruptedException {
        while (items.isEmpty()) {
            wait();
        }
        T item = items.remove();
        notifyAll();
        return item;
    }
}

Expect the follow-up: why notifyAll and not notify? Because producers and consumers wait on the same monitor, so notify can wake a producer when only a consumer can proceed, and the wakeup is lost. The better version uses a ReentrantLock with two Conditions, notFull and notEmpty, so each signal reaches a thread that can actually run. That is exactly what ArrayBlockingQueue does.