Swing application

Swing application

Swing is a graphical user interface library for the Java SE platform. This example Swing application creates a single window with "Hello, world!" inside:

import java.awt.BorderLayout;
import javax.swing.*;

public class Hello extends JFrame {
  public Hello() {
     super("hello");
     setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
     getContentPane().setLayout(new BorderLayout());
     getContentPane().add(new JLabel("Hello, world!"));
     pack();
  }

  public static void main(String[] args) {
     new Hello().setVisible(true);
  }
}

The first import statement directs the Java compiler to include the BorderLayout class from the java.awt package in the compilation; the second import includes all of the public classes and interfaces from the javax.swing package.

The Hello class extends the JFrame class; the JFrame class implements a window with a title bar with a close control.

The Hello() constructor initializes the frame by first calling the superclass constructor, passing the parameter "hello", which is used as the window's title. It then calls the setDefaultCloseOperation(int) method inherited from JFrame to set the default operation when the close control on the title bar is selected to JFrame.EXIT_ON_CLOSE — this causes the JFrame to be disposed of when the frame is closed (as opposed to merely hidden), which allows the JVM to exit and the program to terminate. Next, the layout of the frame is set to a BorderLayout; this tells Swing how to arrange the components that will be added to the frame. A JLabel is created for the string "Hello, world!" and the add(Component) method inherited from the Container superclass is called to add the label to the frame. The pack() method inherited from the Window superclass is called to size the window and lay out its contents, in the manner indicated by the BorderLayout.

The main() method is called by the JVM when the program starts. It instantiates a new Hello frame and causes it to be displayed by calling the setVisible(boolean) method inherited from the Component superclass with the boolean parameter true. Note that once the frame is displayed, exiting the main method does not cause the program to terminate because the AWT event dispatching thread remains active until all of the Swing top-level windows have been disposed.