Can someone explain what is an event-dispatching thread for GUIs & why use it over main thread?

[Benjamin Scabbia]

Hey guys,

I'm currently learning about Swing but I can't get my head round this piece of the code.
Here is a simplified gui (not interested in the gui part but the execution)

public class SwingDemo implements ActionListener {
	SwingDemo(){
		JFrame jfrm = new JFrame("Simple gui pro"); 
		//rest of code

public static void main(String[] args) {
		new SwingDemo();
	}

I get the above, create a new instance of SwingDemo in the main thread which starts up the gui through the constructor.
However, then the tutorial says that I should avoid doing the above but do this instead:

public class SwingDemo implements ActionListener {
	SwingDemo(){
		JFrame jfrm = new JFrame("Simple gui pro"); 
		//rest of code

public static void main(String[] args) {
		SwingUtilities.invokeLater(new Runnable() { //why do this instead?
			public void run(){
				new SwingDemo();
			}
		});
	}
}

Reading, it talks about an event-dispatching thread which has completely lost me... Can someone explain why not just instantiate the object directly instead of creating another thread?

Thanks!

[Darryl Burke]

Have you discovered the Oracle tutorials? This is the relevant trail: Concurrency in Swing

[Campbell Ritchie]

Benjamin Scabbia wrote: . . .

public class SwingDemo implements ActionListener {
	SwingDemo(){
		JFrame jfrm = new JFrame("Simple gui pro"); 
		//rest of code

public static void main(String[] args) {
		SwingUtilities.invokeLater(new Runnable() { //why do this instead?
			public void run(){
				new SwingDemo();
			}
		});
	}
}

. . .

That is exactly how you should start off a GUI app (though you will find slightly different versions in different sources). In Java8 you can dispense with the anonymous class, writing something like this

public class SwingDemo implements ActionListener {
	SwingDemo(){
		JFrame jfrm = new JFrame("Simple gui pro"); 
		//rest of code

public static void main(String[] args) {
		SwingUtilities.invokeLater(() -> new SwingDemo());
	}
}

The compiler knows that invokeLater only takes a Runnable and Runnable has only one method and that method takes no arguments, so you can reduce that part to (). Then you write the contents of that method: new SwingDemo() and separate that from the () by the arrow operator. If you explore the folder you find the SwingDemo$1.class file is not created by that technique.

[Benjamin Scabbia]

Darryl Burke wrote:Have you discovered the Oracle tutorials? This is the relevant trail: Concurrency in Swing

Thanks it's starting to make sense . I will definitely start using Oracle tutorials more!

[Benjamin Scabbia]

Campbell Ritchie wrote: That is exactly how you should start off a GUI app (though you will find slightly different versions in different sources). In Java8 you can dispense with the anonymous class, writing something like this

public class SwingDemo implements ActionListener {
	SwingDemo(){
		JFrame jfrm = new JFrame("Simple gui pro"); 
		//rest of code

public static void main(String[] args) {
		SwingUtilities.invokeLater(() -> new SwingDemo());
	}
}

The compiler knows that invokeLater only takes a Runnable and Runnable has only one method and that method takes no arguments, so you can reduce that part to (). Then you write the contents of that method: new SwingDemo() and separate that from the () by the arrow operator. If you explore the folder you find the SwingDemo$1.class file is not created by that technique.

That's very interesting, does this also help performance then? (for larger programs).
Is this a lamda expression? The syntax looks very similar

SwingUtilities.invokeLater(() -> new SwingDemo());

[Campbell Ritchie]

Benjamin Scabbia wrote: . . .
That's very interesting, does this also help performance then? (for larger programs).

No. The performance should be the same in both cases.

But if you allow 0.5 seconds to click a button and 0.01μs to run the actionPerformed method, are you going to notice the difference?

Is this a lamda expression? . . .

Yes. Or more precisely this is a lambda expression:-
() -> new SwingDemo()