Using threads and recursion in Java to calculate Fibonacci numbers

Question

I'm relatively new in the Java world and I have a problem which I don't understand.

I have a Class (to get the fibonacci row):

class Fib {
    public static int f(int x){
        if ( x < 2 )
            return 1;       
        else 
            return f(x-1)+ f(x-2);      
    }
}

The task now is to start f(x-1) and f(x-2) each in a separate Thread. One time with implementing the Thread class and the other with implementing Runnable. As you probably know, it's an exercise from my prof.

I know how to start a Thread in Java and I know how this whole Thread thing theoretically works, but I can't find a solution for starting separate Threads in this recursive function.

What has to be done in the run function?

Probably

public void run(){
//int foo=start f(this.x-1)
    //int bar=start f(this.x-2)  
    //return foo+bar?
}

And how can I paste x in my runnable function? Is x passed into the object at creation?

Class Fib ...{
  int x;
  public ... run ... 
  public ... f(x)....

}

in the main method

(new Fib(x)).start();

Or am I on a totally wrong path?

Answer 1

For this to work, you need 1) a way to pass the number into the new thread, 2) to start the thread, 3) to wait for the thread to finish, and 4) a way to get the result back from the thread.

You can pass in the number through the constructor. You can have a public data member called "answer" to contain the result of the computation. Starting the thread can be done with the start() method, and the join() method waits for the thread to complete.

The following example demonstrates this. That should be a good starting point; from here you can abstract away some of the messiness to get a better API as desired.

public class Fib extends Thread
{
    private int x;
    public int answer;

    public Fib(int x) {
        this.x = x;
    }

    public void run() {
        if( x <= 2 )
            answer = 1;
        else {
            try {
                Fib f1 = new Fib(x-1);
                Fib f2 = new Fib(x-2);
                f1.start();
                f2.start();
                f1.join();
                f2.join();
                answer = f1.answer + f2.answer;
            }
            catch(InterruptedException ex) { }
        }
    }

    public static void main(String[] args)
        throws Exception
    {
        try {
            Fib f = new Fib( Integer.parseInt(args[0]) );
            f.start();
            f.join();
            System.out.println(f.answer);
        }
        catch(Exception e) {
            System.out.println("usage: java Fib NUMBER");
        }
    }
}