I do not understand how multi-thread part of program works

[Truck35]

I am working on this project in my java text book and I am unable to figure out how the multi thread part of the program works. The program that I am working on is a traffic light simulation program. I have a little understanding how multi thread works. I am particularly baffled about the waitForChange() method. I just cant figure out how it knows that the simulated light has changed. I understand that the method is syncronized so, when the wait() method is called, it waits until it is notified to start again. I understand how it works a little bit but, not enough to do the exercise in my text book. could someone please help me?

Thanks,
Truck35

// An improved version of the traffic light simulation that 
// stores the light delay in TrafficLightColor. 
 
// An enumeration of the colors of a traffic light. 
enum TrafficLightColor {  
  RED(12000), GREEN(10000), YELLOW(2000); 
 
  private int delay; 
 
  TrafficLightColor(int d) { 
    delay = d; 
  } 
 
  int getDelay() { return delay; } 
} 
 
// A computerized traffic light. 
class TrafficLightSimulator implements Runnable { 
  private Thread thrd; // holds the thread that runs the simulation 
  private TrafficLightColor tlc; // holds the current traffic light color 
  boolean stop = false; // set to true to stop the simulation 
  boolean changed = false; // true when the light has changed
 
  TrafficLightSimulator(TrafficLightColor init) {  
    tlc = init; 
 
    thrd = new Thread(this); 
    thrd.start(); 
  } 
 
  TrafficLightSimulator() {  
    tlc = TrafficLightColor.RED; 
 
    thrd = new Thread(this); 
    thrd.start(); 
  } 
 
  // Start up the light. 
  public void run() { 
    while(!stop) { 
 
      // Notice how this code has been simplified! 
      try { 
        Thread.sleep(tlc.getDelay()); 
      } catch(InterruptedException exc) { 
        System.out.println(exc); 
      } 
 
      changeColor(); 
    }  
  } 
 
  // Change color. 
  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(); // signal that the light has changed 
  } 
 
  // Wait until a light change occurs. 
  synchronized void waitForChange() { 
    try { 
      while(!changed)
        wait(); // wait for light to change 
      changed = false;
    } catch(InterruptedException exc) { 
      System.out.println(exc); 
    } 
  } 
 
  // Return current color. 
  synchronized TrafficLightColor getColor() { 
    return tlc; 
  } 
 
  // Stop the traffic light. 
  synchronized void cancel() { 
    stop = true; 
  } 
}  
 
class TrafficLightDemo {  
  public static void main(String args[]) {  
    TrafficLightSimulator tl =
      new TrafficLightSimulator(TrafficLightColor.GREEN); 
 
    for(int i=0; i < 9; i++) { 
      System.out.println(tl.getColor()); 
      tl.waitForChange(); 
    } 
 
    tl.cancel(); 
  }  
}


[Norm]

Try debugging the code by adding println statements that print messages as the code is executed so you can see when and where events happen.

[Truck35]

Try debugging the code by adding println statements that print messages as the code is executed so you can see when and where events happen.

Thanks for your reply.

Truck35

[vishal_165]

Hello, It's the same problem I'm facing, and thanks for sharing this question.