passing objects to another thread

Question

i am creating a GUI application and in the background I want to run an additional task. I will paste some code, to prevent pasting a mess of code that was generated by Swing, I will leave some parts out, assume that the window.java is working as intended.

window.java:

public class window {
frame = new JFrame();
JLabel lbl1 = new JLabel("Start Counter");
frame.add(lbl1);

Thread counter = new Thread(new counter());
    counter.start();
}

counter.java

public class regCheck extends window implements Runnable
{

public void run()
{
    int i = 0;
    while (true)
    {
        window.lbl1.setText(i);
        try {Thread.sleep(1000);}
            catch (InterruptedException e) {e.printStackTrace();}
        i++;
    }
}
}

what I want this example to do is create a label within a window and count upwards until the program is closed. The easy answer here is to say "pass in the Jlabel" however in reality I have multiple things that I need to change not just a label.

the line "window.lbl1.setText(i);" does not work here, it is just to illustrate what I want to achieve.

Kevin Condon · Accepted Answer · 2015-03-18 18:14:59Z

Use the MVC pattern. Create a model that has the counter with a setValue() method that fires a listener notification. You can extend java.util.Observable to make that easier to do. Add a getValue() method to retrieve the new count. Make the setter and getter synchronized for thread safety.

Now your thread can be passed an instance of the model and call setValue() to update the value in its run() method.

Finally, your view can be passed the same instance of the model and add a listener to it. To make it easier your view can implement java.util.Observer. In the listener update() callback within the view, call the model's getValue() and use the return as the argument to setText(). Since the listener update is not being invoked from the AWT event dispatcher thread, you have to call setText() using javax.swing.SwingUtilities.invokeLater() in order to meet the thread safety requirements of Swing.

Iłya Bursov · Accepted Answer · 2015-03-18 18:03:53Z

1

it is actually architectural question, you can pass any arguments to other thread and this thread of course could modify different labels, but I prefer another variant:

you have window, which has its objects/controls and can manipulate them
you have separate thread which increase counter

this separate thread should notify main window object about changes and window object should change it's controls accordingly, for example change text in one or several controls

one simple variant is to have interface, like ICounterHandler with one method

void onCounterChanged(int newCounterValue);

counter thread should accept ICounterHandler in constructor, save it and call this method when needed, preferable asynchronous

of course there are many other variants, but you can start with this one

answered Mar 18, 2015 at 18:03

Iłya Bursov

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2 Comments

penfold1992 Over a year ago

I apologize that my knowledge is very basic in Java at the moment. so instead of accessing through counter.java, I create a method within window.java which will update that value via a argument and then call the method from counter.java I am creating the window in a method called "initialize" and therefore all my declarations of the items are within initialize so i am stuck with a similar problem of getting a objects from one method into this new method that I shall call with counter.java

penfold1992 Over a year ago

the problem is that my window's initialize method contains all the code for the window... so from the counter class, the following code causes an issue window a = new window; this starts up several instances of the main window which is not what I want especially seeing as this currently serves no purpose when I am unable to pass the taskbar from initialize to to new method

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