public class BlockingQueue<T> {
private Queue<T> queue = new LinkedList<>();
private final int capacity;

public BlockingQueue(int capacity) {
super();
this.capacity = capacity;
    }

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

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

public int size() {
synchronized (queue) {
return queue.size();
        }
    }


public static void main(String[] args) throws InterruptedException {
BlockingQueue<Integer> queue = new BlockingQueue<>(3);
Random random = new Random();
Runnable p = () -> {
try {
while (true) {
int element = random.nextInt(10);
queue.put(element);
System.out.println("Produced " + element + " and current size  " + queue.size());
Thread.sleep(random.nextInt(3) * 1000 );
                }
            } catch (InterruptedException e) {
e.printStackTrace();
            }
        };

Runnable c = () -> {
try {
while (true) {
int element = queue.take();
System.out.println("Consumed " + element + " and current size  " + queue.size());
Thread.sleep(random.nextInt(3) * 1000 );
                }
            } catch (InterruptedException e) {
e.printStackTrace();
            }
        };

Thread producerThread = new Thread(p);
Thread consumerThread = new Thread(c);
producerThread.start();
consumerThread.start();

    }
}

Consumed 3 and current size  0

Produced 3 and current size  0

Produced 8 and current size  1

Produced 7 and current size  2

Consumed 8 and current size  1

Produced 5 and current size  2

Consumed 7 and current size  1

Produced 1 and current size  2

Produced 5 and current size  3

Consumed 5 and current size  2

Consumed 1 and current size  1

Produced 7 and current size  2

Consumed 5 and current size  1