package demo1;
import java.util.Queue;

public class Consumer extends Thread {
    private final Queue sharedQ;

    public Consumer(Queue sharedQ) {
        super("Consumer");
        this.sharedQ = sharedQ;
    }

    @Override
    public void run() {
        while (true) {
            synchronized (sharedQ) {
                while (sharedQ.size() == 0) {
                    try {
//                        System.out.println("Queue is empty, waiting");
                        sharedQ.wait();
                    } catch (InterruptedException ex) {
                        ex.printStackTrace();
                    }
                }
                int number = (int) sharedQ.poll();
                System.out.println("consuming : " + number);
                sharedQ.notify();
                if (number == 3) {
                    break;
                }
            }
        }
    }
}

package demo1;
import java.util.Queue;

public class Producer extends Thread {
    private final Queue sharedQ;

    public Producer(Queue sharedQ) {
        super("Producer");
        this.sharedQ = sharedQ;
    }

    @Override
    public void run() {

        for (int i = 0; i < 4; i++) {
            synchronized (sharedQ) {
                while (sharedQ.size() >= 1) {
                    try {
//                        System.out.println("Queue is full, waiting");
                        sharedQ.wait();
                    } catch (InterruptedException ex) {
                        ex.printStackTrace();
                    }
                }
                System.out.println("producing : " + i);
                sharedQ.add(i);
                sharedQ.notify();
            }
        }
    }
}

package demo1;
import java.util.LinkedList;
import java.util.Queue;

public class Test {
    public static void main(String[] args) {
        final Queue sharedQ = new LinkedList();
        Thread producer = new Producer(sharedQ);
        Thread consumer = new Consumer(sharedQ);

        producer.start();
        consumer.start();
    }
}