threads synchronization issue

Question

let's say i have 3 classes: 1. Storage which contains just one integer. 2. Counter which contains a thread inside who's responsible for counting (0,1,..,k) and stores each iteration of the loop index in Storage class. 3.Printer which contains a thread who's responsible for reading the value in class Storage and print it.

now i have to create a main class which creates these 3 objects runs the threads of Counter and Printer , and everynumber from(0,1,..,k) has to be printed just once and in the right order.

how do i synchronize the access to my Storage class so first i put a number inside Storage with Counter ,than print it with my Printer class ?

here's what i've wrote so far:

public  class Storage {
private int num;

public Storage(){
}

public synchronized void setNum(int num){
this.num = num;
}
public synchronized int getNum(){
return num;
}



public class Counter implements Runnable {
Storage s;
public Counter(Storage t){
    s  = t;
}
@Override
public void run() {
    int i = 0;
    while(true){
        s.setNum(i++);
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}


public class Printer implements Runnable {
Storage s;
public Printer(Storage s){
    this.s= s;
}
@Override
public void run() {
while(true){
    System.out.println(s.getNum());

    try {
        Thread.sleep(1000);
    } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

}
}




 public class mainProg {
public static void main(String[] args){
    Storage s = new Storage();
    Counter c = new Counter(s);
    Printer p = new Printer(s);
    Thread c1 = new Thread(c);
    Thread p2 = new Thread(p);
    c1.start();
    p2.start();

}
}

EDIT: i found out a solution, here it is:

public  class Storage {
private int num;
private boolean available = false;
public Storage(){
}

public synchronized void setNum(int num){
    while(available){
        try {
            wait();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
    available = true;
    notifyAll();
    this.num = num;
}
public synchronized int getNum(){
    while(!available){
        try {
            wait();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
    available = false;
    notifyAll();
    return num;
}
}

Answer 1

This approach won't work, because it's not guaranteed that for every cycle of Counter a cycle of Printer will be executed in a parallel thread. You need to be able to store more than a one value in your Storage . You can use BlockingQueue here and rewrite your Storage class like this:

public class Storage {

private BlockingQueue<Integer> numbers = new LinkedBlockingQueue<Integer>();

public void setNum(int num) {
    try {
        this.numbers.put(num);
    } catch (InterruptedException e) {
        throw new RuntimeException(e);
    }
}

public int getNum() {
    try {
        return numbers.take();
    } catch (InterruptedException e) {
        throw new RuntimeException(e);
    }
}
}

Note that if BlockingQueue is empty and Printer wants to get a new value, it will wait while a new element occurrs in the queue.