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Exploring Java

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Creating GUI Components
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14.2 Text Components

AWT gives us two basic text components: TextArea is a multiline text editor with vertical and horizontal scrollbars; TextField is a simple, single line text editor. Both TextField and TextArea derive from the TextComponent class, which provides the functionality they have in common. This includes methods for setting and retrieving the displayed text, specifying whether the text is "editable" or read-only, manipulating the cursor position in the display, and manipulating the selected text.

Both TextAreas and TextFields send TextEvents to listeners when their text is modified. In order to receive these events, you must implement the java.awt.TextListener interface and register by calling the component's addTextListener() method. In addition, TextField components generate an ActionEvent whenever the user presses the Return key within the field. To get these events, implement the ActionListener interface, and call addActionListener() to register.

The next sections contain a couple of simple applets that show you how to work with text areas and fields.

14.2.1 A TextEntryBox

Our first example, TextEntryBox, creates a TextArea and ties it to a TextField, as you can see in Figure 14.1. When the user hits Return in the TextField, we receive an ActionEvent and add the line to the TextArea's display. Try it out. You may have to click your mouse in the TextField to give it focus before typing in it. If you fill up the display with lines, you can test drive the scrollbar:

Figure 14.1: The TextEntryBox applet

Figure 14.1
import java.awt.*;
import java.awt.event.*;
public class TextEntryBox extends java.applet.Applet 
implements ActionListener { 
    TextArea area;
    TextField field;
    
    public void init() {
        setLayout( new BorderLayout() );
        add( "Center", area = new TextArea() );
        area.setFont( new Font("TimesRoman",Font.BOLD,18) );
        area.setText("Howdy!\n");
        add( "South", field = new TextField() );
        field.addActionListener ( this );
    }
    public void actionPerformed(ActionEvent e) {
        area.append( field.getText() + '\n' );
        
        field.setText("");
    }
}

TextEntryBox is exceedingly simple; we've done a few things to make it more interesting. First, we set the applet's layout manager to BorderLayout. We use BorderLayout to position the TextArea above the TextField; the text area goes in the "North" region of the layout, and the text field is in the "South" region. We give the text area a bigger font using Component's setFont() method; fonts are discussed in Chapter 16. Finally, we want to be notified whenever the user presses Return in the text field, so we register our applet (this) as a listener for action events by calling field.addActionListener(this).

Pressing Return in the TextField generates an action event, and that's where the fun begins. We handle the event in the actionPerformed() method of our container, the TextEntryBox applet. Then we use the getText() and setText() methods to manipulate the text the user has typed. These methods can be used for both TextField and TextArea, because these components are derived from the TextComponent class, and therefore have some common functionality.

Our event handler is called actionPerformed(), as required by the ActionListener interface. First, actionPerformed() calls field.getText() to read the text that the user typed into our TextField. It then adds this text to the TextArea by calling area.append(). Finally, we clear the text field by calling the method field.setText(""), preparing it for more input.

By default, TextField and TextArea are editable; you can type and edit in both text components. They can be changed to output-only areas with the setEditable() method. Both text components also support selections. A selection is a subset of text that is highlighted for copying and pasting in your windowing system. You select text by dragging the mouse over it; you can then copy and paste it into other text windows. You can get the selected text explicitly with getSelectedText().

14.2.2 TextWatcher Applet

Our next example is a TextWatcher that consists of two linked text areas; Figure 14.2 shows what the applet looks like. Anything the user types into either area is reflected in both. It demonstrates how to handle a TextEvent, which is generated whenever the text changes in a TextComponent. It also demonstrates how to use an adapter class, which is a more realistic way of setting up event listeners. Registering the applet as a listener is okay for simple programs, but the technique shown here will serve you better in more complex situations:

import java.awt.*;
import java.awt.event.*;
public class TextWatcher extends java.applet.Applet { 
    TextArea area1, area2;
    
    public void init() {
        setLayout( new GridLayout(2,1) );
        add( area1 = new TextArea() );
        add( area2 = new TextArea() );
        area1.addTextListener ( new TextSyncAdapter( area2 ));
        area2.addTextListener ( new TextSyncAdapter( area1 ));
    }
    class TextSyncAdapter implements TextListener {
        TextArea targetArea;
        TextSyncAdapter( TextArea targetArea ) {
            this.targetArea = targetArea;
        }
        public void textValueChanged(TextEvent e) {
            TextArea sourceArea = (TextArea)e.getSource();
            if ( ! targetArea.getText().equals( sourceArea.getText() ) )
                targetArea.setText( sourceArea.getText() );
        }
    }
}

Figure 14.2: The TextWatcher applet

Figure 14.2

Setting up the display is simple. We use a GridLayout and add two text areas to the layout. Then we add our listeners for text events, which are generated whenever the text in a TextComponent is changed. There is one listener for each text area; both are TextSyncAdapter objects. One listens to events from area1 and modifies the text in area2; the other listens to events from area2 and modifies the text in area1.

All the real work is done by the TextSyncAdapter. This is an inner class; its definition is contained within TextWatcher and can't be referenced by classes outside of our TextWatcher. The adapter implements the TextListener interface, and therefore includes a textValueChanged() method.

textValueChanged() is the heart of the listener. First, it extracts the source area from the incoming event; this is the area that generated the event. The area whose text the listener is changing (the target area) was stored by the constructor. Then it tests whether the texts in both areas are the same. If they aren't, it calls the target area's setText() method to set its text equal to the source area's.

The one mysterious feature of this method is the test for equality. Why is it necessary? Why can't we just say "the text in one area changed, so set the text in the other"? A TextEvent is generated whenever a TextComponent is modified for any reason; this includes changes caused by software, not just changes that occur when a user types. So think about what happens when the user types into area1. Typing generates a TextEvent, which causes the adapter to change the text in area2. This generates another TextEvent, which, if we didn't do any testing, would cause us to change area1 again, which would generate another TextEvent, ad infinitum. By checking whether the texts in our two areas are equivalent, we prevent an infinite loop in which text events ping-pong back and forth between the two areas.

14.2.3 Managing Text Yourself

Text areas and text fields do the work of handling keystrokes for you, but they're certainly not your only options for dealing with keyboard input. Any Component can register KeyListeners to receive KeyEvents that occur when their component has focus. Here's an example that shows how you might use these to make your own text gadgets; Figure 14.3 shows what KeyWatcher looks like.

import java.awt.*;
import java.awt.event.*;
public class KeyWatcher extends java.applet.Applet {
    StringBuffer text = new StringBuffer();
    public void init () {
        setFont( new Font("TimesRoman",Font.BOLD,18) );
        addKeyListener ( new KeyAdapter() {
            public void keyPressed( KeyEvent e ) {
                System.out.println(e);
                type( e.getKeyCode(), e.getKeyChar() );
            }
        } );
        requestFocus();
    }
    public void type(int code, char ch ) {
        switch ( code ) {
            case ( KeyEvent.VK_BACK_SPACE ): 
                if (text.length() > 0)
                    text.setLength( text.length() - 1 );
                break;
            case ( KeyEvent.VK_ENTER ): 
                    System.out.println( text );  // Process line
                    text.setLength( 0 );
                break;
            default:
                if ( (ch >= ' ') && (ch <= '~') )
                    text.append( ch );
        }
        repaint();
    }
    
    public void paint(Graphics g) {
        g.drawString(text.toString() + "_", 20, 20);
    }
}

Figure 14.3: The KeyWatcher applet

Figure 14.3

Fundamentally, all we are doing is collecting text in a StringBuffer and using the drawString() method to display it on the screen. As you'd expect, paint() is responsible for managing the display.

In this applet, we're interested in receiving KeyEvents, which occur whenever the user presses any key. To get these events, the applet calls its own addKeyListener() method. The KeyListener itself is an anonymous class. It doesn't have a name and can't be used anywhere else. We create this class by getting a new KeyAdapter() and overriding its keyPressed() method. (Remember that KeyAdapter contains do-nothing implementations of the methods in the KeyListener interface.) All keyPressed() does is call our private method type() with two arguments: the key code of the key that was pressed, and the logical character represented by the keystroke.

type() shows you how to deal with keystrokes. Each key event is identified with a code, which identifies the actual key typed, and a character, which identifies what the user meant to type. This sounds confusing, but it isn't. Basically, there is a constant for each key on the keyboard: VK_ENTER for the Enter (Return) key, VK_A for the letter A, and so on. However, the physical keystroke isn't usually the same as what the user types: the uppercase character A is made up of two keystrokes (the A key and the Shift key), while lowercase a is made up of one.

Therefore, you can expect events for both physical keystrokes and typed characters. The int constant VK_UNDEFINED is used for the physical key code when the event doesn't correspond to a single keystroke. The char constant CHAR_UNDEFINED indicates that the event corresponds to a physical keystroke but not a typed character.

The type() method is called with both the key constant and the character as arguments. The way we use them is relatively simple and would need more work for an industrial strength program. Simply, if the physical key is VK_BACK_SPACE, we delete the last character from the StringBuffer we're building. If it's VK_ENTER, we clear the StringBuffer. If the physical key has any other value, we look at the character the user typed. If this is a printable character, we add it to the StringBuffer. Anything else we ignore. Once we've handled the event, we call repaint() to update the screen. Using key codes to handle operations like backspace or enter is easier and less bug-prone than working with odd Control key combinations.

A final note on our anonymous adapter class: in essence our adapter is letting us write a "callback," by calling type() whenever keyPressed() is called. That's one important use for adapters: to map methods in the various listener interfaces into the methods that make sense for your class. Unlike C or C++, Java won't let us pass a method pointer as an argument, but it will let us create an anonymous class that calls our method and passes an instance of that class.


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