Printer Simulation using Java and Java Threads

Question

I have this assignment which is to simulate the behaviour of a printer where print jobs may be submitted to a printer with varying time intervals or simultaneously. So, thread.start() must be used here, and the printer will take a job from the printer queue and print it, while the rest of the other jobs wait for their turn.

In total I only need to write 3 classes, the Admin class, the PrintRequest class and the Printer class.

Here is my codes:

public class Admin{
    public static void main(String[] args){
        Printer printer = new Printer();
        for(int x=0; x<10;x++){
            PrintRequest printRequest = new PrintRequest(x,printer);
            printRequest.start();
        }
    }
}

public class PrintRequest extends Thread {
    private static int duration; 
    private static int id;
    private static Printer printer;

    public PrintRequest(int id, Printer printer){
        duration = (int)Math.ceil(Math.random()*1000);
        this.id = id;
        this.printer = printer;
    }
    public void run(){
        printer.printJob(id, duration);
    }
}

public class Printer{
    public static synchronized void printJob(int id, int duration){
        System.out.println("Printing " + id);
        try{Thread.sleep(duration);}
        catch(InterruptedException ex){System.out.println(ex.getMessage());}
        System.out.println("Completed printing " + id);
    }
}

There is some problems with the output, it is not the desired output that I want -- printing from ID 0-10. Instead the output is as such:

So what is the problem in my code? I would think it is the problem with the Admin class loop but how should I improve on it such that it can function correctly?

Answer 1

There are several problems with your code.

First, I think you should lookup what a static variable is. As it is, all your PrintRequests have the same id, I don't think that's what you want.

Also, your line that generates the sleep duration only generates number between 0 and 1000. This means that a job lasts less than a second, and your output may not look like what you expect.

Change your line to

duration = (int)(1000 + (Math.random()*5000));

This gave me the following output

Printing 0
Printing 2
Printing 1
Printing 4
Printing 5
Printing 6
Printing 7
Printing 8
Printing 3
Printing 9
Completed printing 6
Completed printing 4
Completed printing 8
Completed printing 2
Completed printing 9
Completed printing 3
Completed printing 1
Completed printing 0
Completed printing 7
Completed printing 5

You may ask : why isn't printing in order ?

That's because you cannot control sequentially the order in which Java will launch your threads in a loop like you've made. If you really want to, you will have to add some more mechanisms to enhance that.