wait() & notify() of Object class - for ref: https://www.youtube.com/watch?v=A1tnVMpWHh8

WaitNotifyDemo.java

public class WaitNotifyDemo {
  
    public static void main(String[] args) {
        Q q = new Q();
        new Producer(q);
        new Consumer(q);
    }

    static class Q {
        private int num;

        private boolean valueSet = false;

        public synchronized void put(int num) {
            while (valueSet) {
                try {
                    wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.println("put: " + num);
            this.num = num;
            valueSet = true;
            notify();
        }

        public synchronized void get() {
            while (!valueSet) {
                try {
                    wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.println("get: " + num);
            valueSet = false;
            notify();
        }
    }

    static class Producer implements Runnable {

        private Q q;

        public Producer(Q q) {
            this.q = q;
            Thread thread = new Thread(this, "Producer");
            thread.start();
        }

        @Override
        public void run() {
            int i = 0;
            while (true) {
                q.put(i++);
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    static class Consumer implements Runnable {
        private Q q;

        public Consumer(Q q) {
            this.q = q;
            Thread thread = new Thread(this, "Consumer");
            thread.start();
        }

        @Override
        public void run() {
            while (true) {
                q.get();
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}