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POWhy doesn't this thread example work? It all wait()'s
 

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  1. POWhy doesn't this thread example work? It all wait()'s
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    <p>The code below is to emulate a robotics simulator I'm working with. I'm not entirely sure why this doesn't work - I'm not very familiar with threads and even though I've tried reading plenty today, I don't seem to be making progress. The problem is that once pauseDistanceSensor() is called, it never wakes up.</p> <pre><code>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; } } </code></pre>
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