how to set boolean flag of thread-1 from thread-2 in Java multithreading

Question

I am writing a simple multithreaded application that involves three threads: Thread-1, Thread-2 and main.

Thread-1 is a random number generator class that produces random doubles and feeds to Thread-2.

Thread-2 consumes the numbers to calculate the average .I have used PipedOutputStream that Thread-1 feeds with random numbers. Thread-2 uses PipedInputStream to eat up the random numbers.

The question is::

if the average exceeds 1E5 in Thread-2, I want to signal Thread-1 to stop producing numbers. I have a boolean flag in Thread-1 that needs to be turned on. How can I achieve this?

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.PipedInputStream;
import java.io.PipedOutputStream;
import java.util.Random;

//

class NumGen extends Thread {

    PipedOutputStream pos;
    DataOutputStream dos;
    boolean isDone=false;

    public NumGen(PipedOutputStream pos){
        this.pos=pos;
        dos=new DataOutputStream(pos);
    }

    public void run(){
        while (!isDone){
            Random rand = new Random();
            try {
                dos.writeDouble(rand.nextDouble()+100.0);
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }
}

class RunningAvg extends Thread {

    PipedInputStream pis;
    DataInputStream dis;
    Double avg;
    int count;
    Double runningTotal;

    public RunningAvg(PipedInputStream pis){
        this.pis=pis;
        dis=new DataInputStream(pis);
            runningTotal=0.0;
    avg=0.0;
    }

    public void run(){
        try {
        while (dis.available()>0){
            count+=1;
            runningTotal+=dis.readDouble();
                avg=runningTotal/count;
                System.out.printf("The average in count no : %s is %s%n",count,avg);
                //if avg>1E5
                 //set NumGen.isDone=true
        }
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }


public class InterThreadComm {

    public static void main(String[] args){


    try {
        PipedOutputStream pos= new PipedOutputStream();
        PipedInputStream pis = new PipedInputStream(pos);
        NumGen ng = new NumGen(pos);
        RunningAvg ra = new RunningAvg(pis);
        ng.start();
        ra.start();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    }


}

PS: The code as it is runs forever without printing anything on console which I am yet to figure out why!!

Answer 1

You could use AtomicBoolean and pass it to the both threads, Atomic types is accessable throw multithreading also thread safe.

first declare you flag isDone as AtomicBoolean

private AtomicBoolean isDone;

then declare one AtomicBoolean object and pass it to the both threads

PipedOutputStream pos= new PipedOutputStream();
PipedInputStream pis = new PipedInputStream(pos);
AtomicBoolean isDone = new AtomicBoolean();
NumGen ng = new NumGen(pos, isDone);
RunningAvg ra = new RunningAvg(pis, isDone);
ng.start();
ra.start();

finally once you want to stop generting numbers ask Thread-2 to set the isDone false.