The SwingUtilities.invokeLater() method in a Java Swing program

Question

I am studying Java Swing and I have some questions related to this simple code tutorial that I am reading:

package com.andrea.execute;

import javax.swing.JFrame;
import javax.swing.SwingUtilities;

/* An istance of a SimpleExample class is a JFrame object: a top level container */
public class SimpleExample extends JFrame {

    /* Create a new SimpleExample object (that is a JFrame object) and set some property */
    public SimpleExample() {
        setTitle("Simple example");
        setSize(300, 200);
        setLocationRelativeTo(null);                // Center the window on the screen. 
        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }

    public static void main(String[] args) {

        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                SimpleExample ex = new SimpleExample();
                ex.setVisible(true);
            }
        });
    }
}

The logic is pretty simple: I have a SimpleExample class that inherits from JFrame Swing class. So a SimpleExample will be a toplevel container.

This class contain also the main() method and now I have 2 doubts:

1) Why in the main() method is execute this code:

SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                SimpleExample ex = new SimpleExample();
                ex.setVisible(true);
            }
        });

It call the invokeLater() method on the SwingUtilities class and pass to it a new Runnable oject.

Reading the documentation I know that is a way to places the application on the Swing Event Queue. It is used to ensure that all UI updates are concurrency-safe.

The thing that I have some problem to understand is how it is implemented.

Inside the input parameter of invokeLater() method it pass this "stuff":

new Runnable() {
        @Override
        public void run() {
            SimpleExample ex = new SimpleExample();
            ex.setVisible(true);
        }
    });

What is it this stuff? What does it represent? and How does it work?

Answer 1

It is an anonymous class that implements Runnable interface.

You can use it without anonymous class like:

class SimpleExample extends JFrame {

    public SimpleExample() {
        setTitle("Simple example");
        setSize(300, 200);
        setLocationRelativeTo(null);                
        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }

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

class App implements Runnable {

    public void run() {
        SimpleExample ex = new SimpleExample();
        ex.setVisible(true);
    }
}

But anonymous class is more convenient.