Methods for Adding UI Components - Java Tutorial 5.0 英文版

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Trail: Deployment
Lesson: Applets

Methods for Adding UI Components

The Simple applet's display code (implemented in its paint method) is flawed: It doesn't support scrolling. Once the text it displays reaches the end of the display rectangle, you can't see any new text. Here's an example of the problem:

This figure has been reduced to fit on the page.
Click the image to view it at its natural size.

The simplest cure for this problem is to use a pre-made user interface (UI) component that has the right behavior.

Note: This page glosses over many details. To really learn about using UI components, go to Creating a User Interface (AWT Only).

Pre-Made UI Components

Swing supplies the following UI components (the class that implements each component is listed in parentheses):

Buttons (javax.swing.JButton)
Checkboxes (javax.swing.JCheckBox)
Single-line text fields (javax.swing.JTextField)
Larger text display and editing areas (javax.swing.JTextArea)
Labels (javax.swing.JLabel)
Lists (javax.swing.JList)
Pop-ups (javax.swing.Popup)
Scrollbars (javax.swing.JScrollBar)
Sliders (javax.swing.JSlider)
Drawing areas (java.awt.Canvas)
Menus (javax.swing.JMenu,javax.swing.JMenuBar javax.swing.JMenuItem, javax.swing.JCheckBoxMenuItem)
Containers (javax.swing.JPanel, javax.swing.JWindow and its subclasses)

Methods for Using UI Components in Applets

Because the JApplet class inherits from the AWT Container class, it's easy to add components to applets and to use layout managers to control the components' onscreen positions. Here are some of the Container methods an applet can use:

add
Adds the specified Component.
remove
Removes the specified Component.
setLayout
Sets the layout manager.

Adding a Non-Editable Text Field to the `Simple` Applet

To make the Simple applet use a scrolling, non-editable text field, we can use the JTextField class. Here is the revised ScrollingSimple class

/*
 * Java 5 version.
 */

import javax.swing.JApplet;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;

//Since we're adding a Swing component, we now need to
//extend JApplet. We need to be careful to access
//components only on the event-dispatching thread.
public class ScrollingSimple extends JApplet {

    JTextField field;

    public void init() {
        //Execute a job on the event-dispatching thread:
        //creating this applet's GUI.
        try {
            SwingUtilities.invokeAndWait(new Runnable() {
                public void run() {
                    createGUI();
                }
            });
        } catch (Exception e) {
            System.err.println("createGUI didn't successfully complete");
        }

        addItem(false, "initializing... ");
    }

    private void createGUI() {        
        //Create the text field and make it uneditable.
        field = new JTextField();
        field.setEditable(false);

        //Set the layout manager so that the text field will be
        //as wide as possible.
        setLayout(new java.awt.GridLayout(1,0));

        //Add the text field to the applet.
        add(field);
    }

    public void start() {
        addItem(false, "starting... ");
    }

    public void stop() {
        addItem(false, "stopping... ");
    }

    public void destroy() {
        addItem(false, "preparing for unloading...");
        cleanUp();
    }
    
    private void cleanUp() {
        //Execute a job on the event-dispatching thread:
        //taking the text field out of this applet.
        try {
            SwingUtilities.invokeAndWait(new Runnable() {
                public void run() {
                    remove(field);
                }
            });
        } catch (Exception e) {
            System.err.println("cleanUp didn't successfully complete");
        }
        field = null;
    }

    private void addItem(boolean alreadyInEDT, String newWord) {
        if (alreadyInEDT) {
            addItem(newWord);
        } else {
            final String word = newWord;
            //Execute a job on the event-dispatching thread:
            //invoking addItem(newWord).
            try {
                SwingUtilities.invokeAndWait(new Runnable() {
                    public void run() {
                        addItem(word);
                    }
                });
            } catch (Exception e) {
                System.err.println("addItem didn't successfully complete");
            }
        }
    }
        
    //Invoke this method ONLY from the event-dispatching thread.
    private void addItem(String newWord) {
        String t = field.getText();
        System.out.println(newWord);
        field.setText(t + newWord);
    }
}

The createGUI method creates an uneditable text field (a JTextField instance). It sets the applet's layout manager to one that makes the text field as wide as possible (you'll learn about layout managers in Laying Out Components within a Container ) and then adds the text field to the applet.

Below is the resulting applet.

Note: If you don't see the applet running above, you need to install Java Plug-in, which happens automatically when you install the J2SE JRE or JDK. This applet requires JDK 5.0 or later. You can find more information in the Java Plug-in home page.