Java Multithreading : How to make a thread wait ?

Question

I want to make a plane (Thread) fly from one airfield to another. When a plane is starting the airfield is blocked for 5 sec. (if another plane wants to land or take off - must wait). If a plane reach its destination it lands (if airfield is not blocked by another plane) and then takes a random airfield and fly there and so on.... I have questions in comments - how to make a thread wait ? And what else is wrong with my code ? Here is my class Samolot aka Plane:

     public class Samolot extends Thread{ 

        int id;
        double paliwo;
        Lotnisko source; //Lotnisko aka airfield
        Lotnisko dest;
      double xPosition;
      double yPosition;
        double xTarget;
      double yTarget;

    public Samolot(Lotnisko source, Lotnisko dest) {


        this.source = source;
        this.dest = dest;
        paliwo = 100;

    }
public void run(){
         while(true){

            tryStart();


         }
     }


     public void tryStart(){
         if(source.pas == true){  // if true the airfield is not blocked and we can go
            source.timer();      // its a method in class Lotnisko, makes pas = false for 8sec 
            lecimy(source, dest);
         }
         else if(source.pas == false){
             // how to make a thread wait ? 
         }
     }

public void tryLadowanie(){
         if(dest.pas == true){
             dest.timer();

            source = dest;
            dest = Rand(source);
            tryStart();
        } 
         else if(dest.pas == false){
             //how to make a thread wait ?
         }



     }
     public void lecimy(Lotnisko source, Lotnisko dest){

        xPosition = source.coords.getX();
        yPosition = source.coords.getY();
        xTarget = dest.coords.getX();
        yTarget = dest.coords.getY();


        while( (xPosition != xTarget) && (yPosition != yTarget) ){
            update();

            try{

                sleep(100);// ok 
                }
            catch (InterruptedException e) {
                System.out.println("Error");
                                    }
     }
     tryLadowanie();
     }



    public void update() {

        paliwo -= 0.05;
        double dx = xTarget - xPosition;
        double dy = yTarget - yPosition;
        double length = sqrt(dx*dx+dy*dy);

        dx /= length;
        dy /= length;

        if (Math.abs(dest.coords.getX() - source.coords.getX()) < 1)
            dx = 0;
        if (Math.abs(dest.coords.getY() - source.coords.getY()) < 1)
            dy = 0;
            xPosition += dx;
            yPosition += dy;
  }

    public Point getPositions() {
        Point curPos = new Point((int)xPosition, (int)yPosition);

        return curPos;
    }

Answer 1

OK, so your plane is a thread and your airfields are the shared resource. So to make your plane (thread) wait, you'll need to synchronized on the shared resource (airfield). You'll probably do something like this.

For take off,

public void tryStart() {
    synchronized(source) { // try to obtain source lock
        try {
            Thread.sleep(5000); // lock it for 5 seconds.
        }
        catch(Exception ignore) {
            // You'll need to decide what to do if something interrupts the thread while it's "sleeping" (ie. locked) on the source airfield. Normally, this should not happen.
        }
    }
    // After 5 seconds, releases lock on airfield "source" and Plane starts flying
}

For landing,

public void tryLadowanie() {
    synchronized(dest) { // try to obtain dest lock
        // successfully obtained lock.
        // perform your landing
    }
    // plane landed and releases "dest" resource for other planes (ie. threads)
}

For a more complete picture of plane flying.

public void run(){
    while(true){
        tryStart(); // take off from source
        lecimy();   // fly the plane. Method will only return when plane reaches destination.
        tryLadowanie(); // land the plane
        source = dest;
        dest = Rand(source); // returns a new destination but can't be the same as source
    }
}


public void tryStart(){
    synchronized(source) {
        try {
            Thread.sleep(5000);
        }
        catch(Exception ignore) { }
    }
}

public void tryLadowanie(){
    synchronized(dest) {
        // land the plane
    }
}

Answer 2

Not a complete answer but wait method is used to wait the thread and notify is used to wake up the thread that is in wait.

Check this for more info on wait and notify method: http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html#wait() http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html#notify()

Answer 3

Use general java object exp. Object lock=new Object(); To wait : Synchronized(lock) { lock.wait(); } To release or free the lock from another thread On same lock object Synchronized(lock) { lock.notify(); }