Question

I’m just getting to grips with GUI programming in java. Here is a trivial program (from O’Reilly’s “Head First Java”) which on the face of it looks easy to understand, but there’s an aspect of it which I don’t follow.

import javax.swing.*;

public class Test {
    public static void main(String[] args) {

    JFrame frame=new JFrame();
    JButton button = new JButton("click me");

    frame.getContentPane().add(button); 
        frame.setSize(300,300);
    frame.setVisible(true);
    }
}

This simple program, when compiled and run, will open a window with a button on it.

What I don’t understand is what is happening with the flow of execution. When I run this program, the static main method of the Test class runs, all the commands in main() are executed — so why doesn’t the process terminate after the window appears? Why am I still sitting on what looks like an infinite loop? What is looping?

If I add the line

frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

then I find the result even more imcomprehensible. Now, of course, the program terminates
once I’ve closed the window. But again I don’t see why. The frame will be on the stack but I don’t see where the program flow is and just the existence of something on the stack is not enough to keep the program alive, surely? I’m missing something fundamental which as far as I can see is not covered in the book I’m reading. I am slightly surprised by this — “Head first Java” has been very good up until now at pointing out subtleties and explaining what is really going on, but doesn’t seem to address this point (at least not that I’ve spotted).

Answer 1

why doesn’t the process terminate after the window appears?

Because the Java Virtual Machine exits only after all non-daemon threads have finished. While not apparent, there’s in fact two threads in your program: the main thread, and the event dispatching thread, which does everything related to the Swing GUI components. The event dispatching thread keeps going as long as any GUI components are visible.

Actually the program, while it may work, is wrong, because you’re creating and accessing Swing components from the main thread. You ought to be doing all GUI work in the event dispatching thread. That is, it should be something like:

public static void main(String[] args) {

  EventQueue.invokeLater(new Runnable() {
    JFrame frame=new JFrame();
    JButton button = new JButton("click me");

    frame.getContentPane().add(button); 
    frame.setSize(300,300);
    frame.setVisible(true);
  });
}