Thread: The book I'm studying is wrong!

Hello,

I'm learning java on my own and the example in the book to demonstrate threads and synchronization is not working. I'm not able to figure out why the sample will not run. Any ideas?


public enum TrafficLightColor {
RED,
GREEN,
YELLOW;
}

***************>

public class TrafficLightSimulator implements Runnable {
private Thread thrd;
private TrafficLightColor tlc;
boolean stop = false;
boolean changed = false;

TrafficLightSimulator(TrafficLightColor init) {
tlc = init;

thrd = new Thread(this);
thrd.start();
}

public void run() {
while(!stop) {
try {
switch(tlc) {
case GREEN:
Thread.sleep(10000);
break;
case YELLOW:
Thread.sleep(2000);
break;
case RED:
Thread.sleep(12000);
break;
}
} catch(InterruptedException exc) {
System.out.println(exc);
}
changeColor();
}
}

synchronized void changeColor() {
switch(tlc) {
case RED:
tlc = TrafficLightColor.GREEN;
break;
case YELLOW:
tlc = TrafficLightColor.RED;
break;
case GREEN:
tlc = TrafficLightColor.YELLOW;
}

changed = true;
notify();
}

synchronized void waitForChange() {
try {
while(!changed);
wait();
changed = false;
} catch(InterruptedException exc) {
System.out.println(exc);
}
}

synchronized TrafficLightColor getColor() {
return tlc;
}

synchronized void cancel() {
stop = true;
System.out.println("after stop changed. stop = " + stop);
}
}



***************>

public class TrafficLightDemo {

public static void main(String[] args) {
// TODO Auto-generated method stub
TrafficLightSimulator t1 = new TrafficLightSimulator(TrafficLightColor.GREEN);

for(int i=0; i<9; i++) {
System.out.println(t1.getColor());
t1.waitForChange(); //hanging here?
}
t1.cancel();
}

}

The output is "GREEN". Then it hangs. There is no error message. The output should be GREEN, YELLOW, RED (3 times).

Sorry - first time posting. Thanks

public enum TrafficLightColor {
RED,
GREEN,
YELLOW;
}
 
/////////////////////////
 
public class TrafficLightSimulator implements Runnable {
private Thread thrd;
private TrafficLightColor tlc;
boolean stop = false;
boolean changed = false;
 
TrafficLightSimulator(TrafficLightColor init) {
tlc = init;
 
thrd = new Thread(this);
thrd.start();
}
 
public void run() {
while(!stop) {
try {
switch(tlc) {
case GREEN:
Thread.sleep(10000);
break;
case YELLOW:
Thread.sleep(2000);
break;
case RED:
Thread.sleep(12000);
break;
}
} catch(InterruptedException exc) {
System.out.println(exc);
}
changeColor();
}
}
 
synchronized void changeColor() {
switch(tlc) {
case RED:
tlc = TrafficLightColor.GREEN;
break;
case YELLOW:
tlc = TrafficLightColor.RED;
break;
case GREEN:
tlc = TrafficLightColor.YELLOW;
}
 
changed = true;
notify();
}
 
synchronized void waitForChange() {
try {
while(!changed);
wait();
changed = false;
} catch(InterruptedException exc) {
System.out.println(exc);
}
}
 
synchronized TrafficLightColor getColor() {
return tlc;
}
 
synchronized void cancel() {
stop = true;
System.out.println("after stop changed. stop = " + stop);
}
}
 
 
 
////////////////////////
 
public class TrafficLightDemo {
 
public static void main(String[] args) {
// TODO Auto-generated method stub
TrafficLightSimulator t1 = new TrafficLightSimulator(TrafficLightColor.GREEN);
 
for(int i=0; i<9; i++) {
System.out.println(t1.getColor());
t1.waitForChange(); //hanging here?
}
t1.cancel();
}
 
}

Code hangs almost immediately and doesn't even complete it's first loop see main class. Java hangs as if it's waiting form something and cpu remains high. I have to do a ctrl+alt+delete in order to stop the app from hanging. memory is 5,496K and CPU is 49

 
public enum TrafficLightColor {
	RED, 
	GREEN, 
	YELLOW;
}
 
public class TrafficLightSimulator implements Runnable {
	private Thread thrd;
	private TrafficLightColor tlc;
	boolean stop = false;
	boolean changed = false;
 
	TrafficLightSimulator(TrafficLightColor init)	{
		tlc = init;
 
		thrd = new Thread(this);
		thrd.start();
	}
 
	public void run() {
		while(!stop) {
			try {
				switch(tlc) {
				case GREEN:
					Thread.sleep(10000);
					break;
				case YELLOW:
					Thread.sleep(2000);
					break;
				case RED:
					Thread.sleep(12000);
					break;
			}
			} catch(InterruptedException exc)	{
				System.out.println(exc);
			}
			changeColor();
		}
	}
 
	synchronized void changeColor()	{
		switch(tlc)	{
		case RED:
			tlc = TrafficLightColor.GREEN;
			break;
		case YELLOW:
			tlc = TrafficLightColor.RED;
			break;
		case GREEN:
			tlc = TrafficLightColor.YELLOW;
		}
 
		changed = true;
		notify();
	}
 
		synchronized void waitForChange()	{
			try {
				while(!changed);
					wait();
				changed = false;
			} catch(InterruptedException exc) {
				System.out.println(exc);
			}
		}
 
		synchronized TrafficLightColor getColor() {
			return tlc;
		}
 
		synchronized void cancel()	{
			stop = true;
			System.out.println("after stop changed. stop =  " + stop);
	}
}
 
 
///////////////////////////Main PRG
 
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		 TrafficLightSimulator t1 =	new TrafficLightSimulator(TrafficLightColor.GREEN);
 
		 for(int i=0; i<9; i++) {
			System.out.println(t1.getColor() + "   loop "+ i);
			t1.waitForChange(); //hanging here. Doesn't complete loop.
					    //Doesn't go into waitForChange()
		 }
		 t1.cancel();
	}
 
}

Originally Posted by NormR

Ok, that formatting looks a lot better.
I see two while loops that don't have print statements before, inside and and after them to show when and where they are executing.
Be sure to add print statements around ALL the loops.


That says there is a loop that is running continuously without doing enough work to slow it down some. The print statements will help you find it by printing thousands of lines as the loop goes around.

Thanks. I assumed it was hanging on the 1st loop. I'll put print statements in all loops and get back to you.

Originally Posted by MsNovice

Thanks. I assumed it was hanging on the 1st loop. I'll put print statements in all loops and get back to you.

The following is the output when I run the code with messages within loops to output:

In !stop while loop
Sleeping in GREEN
Before changeColor

It hanges in or trying to go into changedColor. Possibly tlc's value is lost once in changedColor? If so, shouldn't I get an error? Value is seen and is GREEN prior to changeColor call.

 
public enum TrafficLightColor {
	RED, 
	GREEN, 
	YELLOW;
}
 
 
///////////////////////////
	public class TrafficLightSimulator implements Runnable {
	private Thread thrd;
	private TrafficLightColor tlc;
	boolean stop = false;
	boolean changed = false;
 
	TrafficLightSimulator(TrafficLightColor init)	{
		tlc = init;
 
		thrd = new Thread(this);
		thrd.start();
	}
 
	public void run() {
		while(!stop) {
			System.out.println("In !stop while loop");
			try {
				switch(tlc) {
				case GREEN:
					System.out.println("Sleeping in GREEN");
					Thread.sleep(100);
					break;
				case YELLOW:
					System.out.println("Sleeping in YELLOW");
					Thread.sleep(20);
					break;
				case RED:
					System.out.println("Sleeping in RED");
					Thread.sleep(120);
					break;
			}
			} catch(InterruptedException exc)	{
				System.out.println(exc);
			}
			System.out.println("Before changeColor. Value of tlc is " + tlc);
			changeColor();
		}
	}
 
	synchronized void changeColor()	{
		System.out.println("In changeColor");
		switch(tlc)	{
		case RED:
			System.out.println("Changing to GREEN");
			tlc = TrafficLightColor.GREEN;
			break;
		case YELLOW:
			System.out.println("Changing to RED");
			tlc = TrafficLightColor.RED;
			break;
		case GREEN:
			System.out.println("Changing to YELLOW");
			tlc = TrafficLightColor.YELLOW;
		}
 
		changed = true;
		System.out.println("Changed = " + changed);
		notify();
	}
 
		synchronized void waitForChange()	{
			try {
				while(!changed);
					System.out.println("In synchronized while loop !changed");
					wait();
				changed = false;
			} catch(InterruptedException exc) {
				System.out.println(exc);
			}
		}
 
		synchronized TrafficLightColor getColor() {
			return tlc;
		}
 
		synchronized void cancel()	{
			stop = true;
			System.out.println("after stop changed. stop =  " + stop);
	}
}
 
//////////////////////////////
 
public class TrafficLightDemo {
 
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		 TrafficLightSimulator t1 =	new TrafficLightSimulator(TrafficLightColor.GREEN);
 
		 for(int i=0; i<9; i++) {
			System.out.println(t1.getColor() + "   for loop of main "+ i);
			t1.waitForChange(); //hanging here. Doesn't complete loop.
		 }
		 t1.cancel();
	}
 
}

THAT WAS IT!!!!! Thanks so much for the debugging lesson and finding the error!!!!

I'm glad that's fixed your problem. Please go ahead and mark the thread as solved! Should be an option available to you, see this post http://www.javaprogrammingforums.com...ad-solved.html for guidance.