Communication between Threads

Frequently, threads will need to coordinate between themselves, especially when the output of one thread is used by another thread. Let us consider the classical “The Producer-Consumer Problem” in which one part of the program is writing data and another part of it is reading data – but sometimes the reading part gets ahead of the writing part. We can solve this problem by communicating between the threads. One way of coordinating threads is to use the wait(), notify(), and notifyAll() methods.

Here is an example. In this case, a writer thread will call an object’s write() method to write some data, and a reader thread will call the same object’s read() method to read the result. Obviously, we want the reader thread to wait until write() method is finished. Therefore, all we have to do is call wait() in read() method to make the reader thread wait and call notify() in write() method when the writer thread is finished and the data is ready to be read. See the code below:

InterThreadComn.java

public class InterThreadComn {

    public static void main(String[] args) {
        Share sh = new Share();
        Reader t1 = new Reader(sh, "1st");
        Writer t2 = new Writer(sh, "2nd");
    }
}

class Share {
    int data = 0;

    synchronized void write() {
        try {
            Thread.sleep(1000);
        } 
        catch (InterruptedException e) {
            System.out.println(e);
        }
        data = 100;
        notify();
    }

    synchronized int read() {
        try {
            wait();
        } 
        catch (InterruptedException e) {
            System.out.println(e);
        }
        return data;
    }
}

class Reader extends Thread {
    Share sh;

    public Reader(Share sh, String name) {
        super(name);
        this.sh = sh;
        start();
    }

    public void run() {
        System.out.println("Result: " + sh.read());
    }
}

class Writer extends Thread {
    Share sh;

    public Writer(Share sh, String name) {
        super(name);
        this.sh = sh;
        start();
    }

    public void run() {
        sh.write();
    }
}

Output:

Result: 100

Differences between wait() and sleep() methods

The differences between these two methods are listed below.