Question

Odd even number printing using thread.Create one thread class, two instance of the thread. One will print the odd number and the other will print the even number.

I did the following coding. But it comes to dead lock state. Can some one please explain what might be the reason for that?

public class NumberPrinter implements Runnable{
private String type;
private static boolean oddTurn=true;


public NumberPrinter(String type){
    this.type=type;
}
public void run() {
    int i=type.equals("odd")?1:2;
    while(i<10){
        if(type.equals("odd"))
            printOdd(i);
        if(type.equals("even"))
            printEven(i);
        i=i+2;
    }

}

private synchronized void printOdd(int i){
    while(!oddTurn){
        try {
            wait();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
    System.out.println(type + i);
    oddTurn=false;
    notifyAll();
}

private synchronized  void printEven(int i){
    while(oddTurn){
        try {
            wait();
        } catch (InterruptedException e) {
            e.printStackTrace(); 
        }
    }
    System.out.println(type + i);
    oddTurn=true;
    notifyAll();

}

public static void main(String[] s){
    Thread odd=new Thread(new NumberPrinter("odd"));
    Thread even=new Thread(new NumberPrinter("even"));
    odd.start();
    even.start();

}
}

Out Put:
odd1
even2

---

then comes to deadlock!!!!!!

Thanks for your help.

Answer 1

You’re waiting and notifying different objects (monitors).

The idea is that you can call obj.wait() to wait for someone to do obj.notify(), while you’re doing objA.wait() and objB.notify().

Change your printOdd method to something like

private void printOdd(int i) {
    synchronized (lock) {                        // <-------
        while (!oddTurn) {
            try {
                lock.wait();                     // <-------
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println(type + i);
        oddTurn = false;
        lock.notifyAll();                        // <-------
    }
}

and the printEven method similarly.

Then provide the NumberPrinter with a lock object:

Object lock = new Object();
Thread odd = new Thread(new NumberPrinter("odd", lock));
Thread even = new Thread(new NumberPrinter("even", lock));

Output:

odd1
even2
odd3
even4
odd5
even6
odd7
even8
odd9