Can't get notify() to work

[Jared Folino]

Basically the problem is that the last print statement "Code Block 4" does not print. This is because the main thread that calls notify() completes before the second thread (Thread-1) begins. As such when Thread-1 is called it permanently blocked as their is no active thread to call notify().

  
public class Play extends Thread{
    
    private Object o;
    
    public Play(Object o) {
        this.o = o;
    }
    public void run(){
        try {
            synchronized(o){
                System.out.println("Code Block 3");
                o.wait();
                System.out.println("Code Block 4");
                System.exit(0);
            }
        } catch (InterruptedException e){}
        
 
    }
    
    
    public static void main(String[] args) throws Exception{
        Object o = new Object();
        Thread t = new Play(o);
        t.start();
        synchronized(o){
            System.out.println("Code Block 1");
            System.out.println("Code Block 2");
           // sleep(5000);
            o.notifyAll();
        }
       
    }
    
}
  

[Edward Harned]

This is because the main thread that calls notify() completes before the second thread (Thread-1) begins.

How true.

You need to communicate between the threads, back and forth. Read the tutorial and follow the instructions.
Concurrency

[Stan James]

See how this variation is different:

public class Play extends Thread{
 
    private Object o;
 
    public Play(Object o) {
        this.o = o;
    }
    public void run(){
        try {
            System.out.println("run before wait");
            synchronized(o){
                o.wait();
            }
            System.out.println("run after wait");
            System.exit(0);
        } catch (InterruptedException e){}
    }
 
    public static void main(String[] args) throws Exception{
        Object o = new Object();
        Thread t = new Play(o);
        t.start();
        System.out.println("main before sleep");
        sleep(5000);
        System.out.println("main before notify");
        synchronized(o){
            o.notifyAll();
        }
        System.out.println("main after notify");
    }
}

I narrowed the scope of the synchronized blocks so they only control the wait & notify. Work through what was synchronized in your version and see if that explains your results.