Why does this thread example not work? All this is wait ()

Question

Below is the code to emulate the robotics simulator I'm working with. I'm not quite sure why this is not working. I am not very fluent in streams, and although I tried to read a lot today, I do not seem to have made progress. The problem is that as soon as pauseDistanceSensor () is called, it never wakes up.

import java.util.Random;

public class TestThreads
{
    private DistanceSensor dist;
    private Thread distanceThread;
    public TestThreads()
    {
        this.dist = new DistanceSensor();
        this.distanceThread = new Thread(this.dist);
        this.distanceThread.start();
    }

    public int getDistance()
    {
        return this.dist.getMeasurement();
    }

    public void pauseDistanceSensor()
    {
        synchronized(this.dist)
        {
            try {
                this.dist.wait();
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }

    public void resumeDistanceSensor()
    {
        synchronized(this.dist)
        {
            this.dist.notify();
        }
    }

    public static void main(String[] args)
    {
        TestThreads test = new TestThreads();
        long timestamp = System.currentTimeMillis();
        System.out.println("Starting at "+timestamp);
        System.out.println("1: "+test.getDistance());
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println("2: "+test.getDistance());
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println("waiting distance sensor and making 3 getDistance calls then sleeping main thread for 1 second - all 3 getDistance calls should be printed when the sleep ends");
        test.pauseDistanceSensor();
        System.out.println("3: "+test.getDistance());
        System.out.println("4: "+test.getDistance());
        System.out.println("5: "+test.getDistance());
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println("Alive! Notifying the thread");
        test.resumeDistanceSensor();
        System.out.println("Done at "+System.currentTimeMillis());
    }
}

class DistanceSensor implements Runnable
{
    private final Random gen = new Random(54);
    private int currentVal;
    public DistanceSensor()
    {
        this.currentVal = this.gen.nextInt(1500);
    }

    public void run()
    {
        this.currentVal = this.gen.nextInt(1500);
    }

    public int getMeasurement()
    {
        return this.currentVal;
    }
}

Answer 1

Your call to pauseDistanceSensor blocks the main thread when you wait.

Also, your sensor start method only sets the sensor value once; the start method should loop, setting the value each time.

, , .

import java.util.Random;

public class TestThreads
{
    private DistanceSensor dist;
    private Thread distanceThread;
    public TestThreads()
    {
        this.dist = new DistanceSensor();
        this.distanceThread = new Thread(this.dist);
        this.distanceThread.start();
    }

    public int getDistance()
    {
        return this.dist.getMeasurement();
    }

    public void pauseDistanceSensor()
    {
      dist.setPaused(true);
    }

    public void resumeDistanceSensor()
    {
      dist.setPaused(false);
    }

    public void finish() {
      dist.setDone();
    }

    public static void main(String[] args)
    {
        TestThreads test = new TestThreads();
        long timestamp = System.currentTimeMillis();
        System.out.println("Starting at "+timestamp);
        System.out.println("1: "+test.getDistance());
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println("2: "+test.getDistance());
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println("waiting distance sensor and making 3 getDistance calls then sleeping main thread for 1 second - all 3 getDistance calls should be printed when the sleep ends");
        test.pauseDistanceSensor();
        System.out.println("3: "+test.getDistance());
        System.out.println("4: "+test.getDistance());
        System.out.println("5: "+test.getDistance());
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println("Alive! Notifying the thread");
        test.resumeDistanceSensor();
        System.out.println("Done at "+System.currentTimeMillis());
        test.finish();
    }
}

class DistanceSensor implements Runnable
{
    private final Random gen = new Random(54);
    private int currentVal;
    private boolean done = false, paused = false;
    public DistanceSensor()
    {
        this.currentVal = this.gen.nextInt(1500);
    }

    public void run()
    {
        while(!getDone()) {
          if(!getPaused()) synchronized(this) {this.currentVal = this.gen.nextInt(1500);}
          synchronized(this) {
            try {
              wait(500);
            } catch(InterruptedException ex) {
              ex.printStackTrace();
            }
          }
        }
    }

    public synchronized int getMeasurement()
    {
        return this.currentVal;
    }

    public synchronized boolean getPaused() {return paused;}
    public synchronized boolean getDone() {return done;}
    public synchronized void setPaused(boolean p) {paused = p;}
    public synchronized void setDone() {done = true;notify();}
}