Graphics Input - Events

Introduction

Input in a GUI program is processed with an Event Handler, which is code that is executed when an event occurs. An event can be one of the following.

• A mouse click.
• A mouse press down.
• A mouse press release.
• A key click.

A GUI program can have several buttons, popup menus, and text areas all of which can generate an input. For example in MS-Word, you can type text in your file (key clicks), you can search for a string (Control-F, followed by typing text, followed by clicking on Search), you can select a specific heading (Selecting a popup menu), and you can print (Selecting File > Print and using the Print Panel). The corresponding GUI program has an event handler that is called when each corresponding event occurs.

Hello World Example

This section develops the Hello World examples from The Basic GUI Application of David Eck’s book.

This section shows two figures and corresponding code. The first figure is a screen shot of the code running. The second figure shows an ActionEvent object flowing from the OK button to the event handler. The figures and code show a JFrame with is content pane (red border in figure) set to a JPanel. The JPanel contains two components.

• JButton that is labeled OK.
  • The JButton has an ActionListener added to it.
    • The ActionListener is called when the JButton is selected.
    • An ActionListener is an object.
  • We create a class ButtonHandler that implements the ActionListener interface.
    • The ActionListener interface has one method, actionPerformed.
    • The code for our implementation of the actionPerformed method is simply to exit the progrm.
  • When a user clicks on OK, an ActionEvent object is passed as a parameter to the actionPerformed method of our ButtonHandler.
• HelowWorldDisplay which is our class that extends a JPanel and overrides the paintComponent method in order to drawString Hello World!.
• The JButton and HelloWorldDisplay are added to the JPanel using BorderLayout, with Hello World! in the CENTER and the JButton in the SOUTH.

Hello World GUI Screen Shot

Hello World Event Flow

Hello World Code

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

public class HelloWorldGUI2 {

    private static class HelloWorldDisplay extends JPanel {
        public void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.drawString( "Hello World!", 20, 30 );
        }
    }

    private static class ButtonHandler implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            System.exit(0);
        }
    }

    public static void main(String[] args) {

        HelloWorldDisplay displayPanel = new HelloWorldDisplay();
        JButton okButton = new JButton("OK");
        ButtonHandler listener = new ButtonHandler();
        okButton.addActionListener(listener);

        JPanel content = new JPanel();
        content.setLayout(new BorderLayout());
        content.add(displayPanel, BorderLayout.CENTER);
        content.add(okButton, BorderLayout.SOUTH);

        JFrame window = new JFrame("GUI Test");
        window.setContentPane(content);
        window.setSize(250,100);
        window.setLocation(100,100);
        window.setVisible(true);

    }

}

Annonymous Class - ButtonHandler

If you study sample graphics code from the Internet, you will see annonymous classes.

In the previous we created a ButtonHandler class that implemented the ActionListener interface. ActionListener is an interface and you cannot apply the new operator as

ActionListener actListener = new ActionListener();

However, you can define an anonymous class (a class without a name) that implements an ActionListener interface. When doing this, the anonymous class defines the method(s) of the interface (e.g., public void actionPerformed() which allows the interface to seem to be a class in which the new operator is applied. The following code creates an anonymous class that implements an ActionListener interface, constructs an object, and assigns it to a variable. The resulting object is used as an action listener on the OK button.

JButton okButton = new JButton("OK");
ActionListener listener = new ActionListener(){
      public void actionPerformed(...)   {System.exit(0);}
};
okButton.addActionListener(listener);

Abstract Classes

Graphics programming often use abstract classes. An abstract class is somewhat like a class and an interface combined

• Interface likeness: You define methods that must be implemented
• Interface likeness: You cannot create objects from them as they are defined
• Class likeness: You can create an object if you provide the real code for an abstract method
• Class likeness: You can extend them

The following code demonstrates an abstract class GraphicObject that has one real method (moveTo) and two abstract methods (draw and resize). When the class Circle extends GraphicObject, Circle has to provide the code for draw and resize, but reuses the moveTo code from the superclass.

abstract class GraphicObject {
    int x, y;
    ...
    void moveTo(int newX, int newY) {
        ...
    }
    abstract void draw();
    abstract void resize();
}
class Circle extends GraphicObject {
    void draw() {
        ...
    }
    void resize() {
        ...
    }
}

AbstractAction

java.swing.AbstractAction is an abstract class that has the actionPerformed method. The sample Capitalizer code in Graphics Events Code uses the AbstractAction class. The following code snippet demonstrates creating an anonymous AbstractAction object that implements the actionPerformed method

new AbstractAction("To Lower Case") {
    public void actionPerformed(ActionEvent e) {
        area.setText(area.getText().toLowerCase());
    }
}

You can actually do several actions in one big swoop

JPanel buttonPanel = new JPanel();

buttonPanel.add(new JButton(new AbstractAction("To Lower Case") {
            public void actionPerformed(ActionEvent e) {
                area.setText(area.getText().toLowerCase());
            }
        }));

Hello World JOptionPane Example

When a user makes an invalid selection on a GUI program, you often pop-up a dialog box that forces the user to acknowledge the error before returning to your program. The Java JOptionPane class is used for these pop-up dialog boxes. This section shows a JOptionPane Hello World example screen shot and code.

JOptionPane Screen Shot

JOptionPane Code

import javax.swing.JOptionPane;

/**
 * Displays a dialog box with "Hello World."  
 * The program terminates when the user selects OK
 */
public class HelloWorldGUI1 {
    public static void main(String[] args) {
        JOptionPane.showMessageDialog( null, "Hello World!" );
    }

}

JOptionPane with a JFrame

The JOptionPane is designed to show messages that must be acknowledged and then return to the main window. Suppose you have a GUI with a JTextField in which a user must type a number. You can use a JOptionPane for an error message. This section shows a screen shot and some code snippets to demonstrate how you would use a JOptionPane. The screen shot shows several input text fields. The code snippet demonstrates processing the Acres of Land input text field. The user has to enter a number in the Acres of Land input text field and then select Simulate One Year. The similateYearButtonActionPerformed method processes the button clicked event. The text is the acresOfLandTextField is parsed to an integer. If it is not an integer, an exception occurs and an JOptionPane is used to inform the user.

public class EndorUI extends JFrame {
   private JTextField acresOfLandTextField;

   public void optionPane(String s) {
      JOptionPane.showMessageDialog(rootPane, s, "Error", JOptionPane.ERROR_MESSAGE);
   }

   private void initComponents() {
      // just one of many components
      acresOfLandTextField = new javax.swing.JTextField();
   }

   private void similateYearButtonActionPerformed(ActionEvent evt) {

      try {
         acres = Integer.parseInt(acresOfLandTextField.getText());
      }
      catch (Exception e) {
         optionPane("Illegal value for Acres of Land");
         goodData = false;
      }
   }
}