[Chapter 8] 8.3 TextArea - e-Reading Library

8.3 TextArea

TextArea is the TextComponent for multiline input. Some constructors permit you to set the rows and columns of the TextArea on the screen. However, the LayoutManager may change your settings. As with TextField, the only way to limit the number of characters that a user can enter is to override the keyDown() method. The text in a TextArea appears left justified, and the justification is not customizable.

In Java 1.1, you can control the appearance of a TextArea scrollbar; earlier versions gave you no control over the scrollbars. When visible, the vertical scrollbar is on the right of the TextArea, and the horizontal scrollbar is on the bottom. You can remove either scrollbar with the help of several new TextArea constants; you can't move them to another side. When the horizontal scrollbar is not present, the text wraps automatically when the user reaches the right side of the TextArea. Prior to Java 1.1, there was no way to enable word wrap.

TextArea Variables

Constants

The constants for TextArea are new to Java 1.1; they allow you to control the visibility and word wrap policy of a TextArea scrollbar. There is no way to listen for the events when a user scrolls a TextArea.

public static final int SCROLLBARS_BOTH

The SCROLLBARS_BOTH mode is the default for TextArea. It shows both scrollbars all the time and does no word wrap.

public static final int SCROLLBARS_HORIZONTAL_ONLY

The SCROLLBARS_HORIZONTAL_ONLY mode displays a scrollbar along the bottom of the TextArea. When this scrollbar is present, word wrap is disabled.

public static final int SCROLLBARS_NONE

The SCROLLBARS_NONE mode displays no scrollbars around the TextArea and enables word wrap. If the text is too long, the TextArea displays the lines surrounding the cursor. You can use the cursor to move up and down within the TextArea, but you cannot use a scrollbar to navigate. Because this mode has no horizontal scrollbar, word wrap is enabled.

public static final int SCROLLBARS_VERTICAL_ONLY

The SCROLLBARS_VERTICAL_ONLY mode displays a scrollbar along the right edge of the TextArea. If the text is too long to display, you can scroll within the area. Because this mode has no horizontal scrollbar, word wrap is enabled.

TextArea Methods

Constructors

public TextArea ()

This constructor creates an empty TextArea with both scrollbars. The TextArea is 0 rows high and 0 columns wide. Depending upon the platform, the TextArea could be really small (and useless) or rather large. It is a good idea to use one of the other constructors to control the size of the TextArea.

public TextArea (int rows, int columns)

This constructor creates an empty TextArea with both scrollbars. The TextArea is rows high and columns wide.

public TextArea (String text)

This constructor creates a TextArea with an initial content of text and both scrollbars. The TextArea is 0 rows high and 0 columns wide. Depending upon the platform, the TextArea could be really small (and useless) or rather large. It is a good idea to use one of the other constructors to control the size of the TextArea.

public TextArea (String text, int rows, int columns)

This constructor creates a TextArea with an initial content of text. The TextArea is rows high and columns wide and has both scrollbars.

The following example uses the first four constructors. The results are shown in Figure 8.3. With the size-less constructors, notice that Windows 95 creates a rather large TextArea. UNIX systems create a much smaller area. Depending upon the LayoutManager, the TextAreas could be resized automatically.

import java.awt.TextArea;
public class textas extends java.applet.Applet {
    public void init () {
        add (new TextArea ());                     // A
        add (new TextArea (3, 10));                // B
        add (new TextArea ("Empty Area"));         // C
        add (new TextArea ("Empty Area", 3, 10));  // D
    }
}

Figure 8.3: TextArea constructor

[Graphic: Figure 8-3]

public TextArea (String text, int rows, int columns, int scrollbarPolicy): The final constructor creates a TextArea with an initial content of text. The TextArea is rows high and columns wide. The initial scrollbar display policy is designated by the scrollbarPolicy parameter and is one of the TextArea constants in the previous example. This constructor is the only way provided to change the scrollbar visibility; there is no setScrollbarVisibility() method. Figure 8.4 displays the different settings.

Figure 8.4: TextArea policies

[Graphic: Figure 8-4]

Setting text

The text-setting methods are usually called in response to an external event. When you handle the insertion position, you must translate it from the visual row and column to a one-dimensional position. It is easier to position the insertion point based upon the beginning, end, or current selection (getSelectionStart() and getSelectionEnd()).

public void insert (String string, int position) public void insertText (String string, int position)

The insert() method inserts string at position into the TextArea. If position is beyond the end of the TextArea, string is appended to the end of the TextArea.

insertText() is the Java 1.0 name for this method.

public void append (String string) public void appendText (String string)

The append() method inserts string at the end of the TextArea.

appendText() is the Java 1.0 name for this method.

public void replaceRange (String string, int startPosition, int endPosition) public void replaceText (String string, int startPosition, int endPosition)

The replaceRange() method replaces the text in the current TextArea from startPosition to endPosition with string. If endPosition is before startPosition, it may or may not work as expected. (For instance, on a Windows 95 platform, it works fine when the TextArea is displayed on the screen. However, when the TextArea is not showing, unexpected results happen. Other platforms may vary.) If startPosition is 0 and endPosition is the length of the contents, this method functions the same as TextComponent.setText().

replaceText() is the Java 1.0 name for this method.

Sizing

public int getRows ()

The getRows() method returns the number of rows set by the constructor or a subsequent call to setRows(). This could be different from the displayed height of the TextArea.

public void setRows (int rows)

The setRows() method changes the preferred number of rows to display for the TextField to rows. Because the current LayoutManager will do what it wants, the new setting may be ignored. If rows < 0, setRows() throws the run-time exception IllegalArgumentException.

public int getColumns ()

The getColumns() method returns the number of columns set by the constructor or a subsequent call to setColumns(). This could be different from the displayed width of the TextArea.

public void setColumns (int columns)

The setColumns() method changes the preferred number of columns to display for the TextArea to columns. Because the current LayoutManager will do what it wants, the new setting may be ignored. If columns < 0, setColumns() throws the run-time exception IllegalArgumentException.

public Dimension getPreferredSize (int rows, int columns) public Dimension preferredSize (int rows, int columns)

The getPreferredSize() method returns the Dimension (width and height) for the preferred size of the TextArea with a preferred height of rows and width of columns. The rows and columns specified may be different from the current settings.

preferredSize() is the Java 1.0 name for this method.

public Dimension getPreferredSize (int rows, int columns) public Dimension preferredSize ()

The getPreferredSize() method returns the Dimension (width and height) for the preferred size of the TextArea. Without the rows and columns parameters, this getPreferredSize() uses the constructor's number of rows and columns to calculate the TextArea's preferred size.

preferredSize() is the Java 1.0 name for this method.

public Dimension getMinimumSize (int rows, int columns) public Dimension minimumSize (int rows, int columns)

The getMinimumSize() method returns the minimum Dimension (width and height) for the size of the TextArea with a height of rows and width of columns. The rows and columns specified may be different from the current settings.

minimumSize() is the Java 1.0 name for this method.

public Dimension getMinimumSize () public Dimension minimumSize ()

The getMinimumSize() method returns the minimum Dimension (width and height) for the size of the TextArea. Without the rows and columns parameters, this getMinimumSize() uses the current settings for rows and columns to calculate the TextArea's minimum size.

minimumSize() is the Java 1.0 name for this method.

Miscellaneous methods

public synchronized void addNotify ()

The addNotify() method creates the TextArea peer. If you override this method, call super.addNotify() first, then add your customizations for the new class. You will then be able to do everything you need with the information about the newly created peer.

public int getScrollbarVisibility()

The getScrollbarVisibility() method retrieves the scrollbar visibility setting, which is set by the constructor. There is no setScollbarVisibility() method to change the setting. The return value is one of the TextArea constants: SCROLLBARS_BOTH, SCROLLBARS_HORIZONTAL_ONLY, SCROLLBARS_NONE, or SCROLLBARS_VERTICAL_ONLY.

protected String paramString ()

When you call the toString() method of TextArea, the default toString() method of Component is called. This in turn calls paramString(), which builds up the string to display. The TextArea level adds the number of rows and columns for the TextArea, and Java 1.1 adds the scrollbar visibility policy. Using new TextArea(`Empty Area`, 3, 10), the results displayed could be:

java.awt.TextArea[text0,0,0,0x0,invalid,text="Empty Area",
editable,selection=0-0, rows=3,columns=10, scrollbarVisibility=both]

TextArea Events

With the 1.0 event model, the TextArea component can generate KEY_PRESS and KEY_ACTION (which calls keyDown()) along with KEY_RELEASE and KEY_ACTION_RELEASE (which called keyUp()). There is no ACTION_EVENT generated for TextArea.

NOTE:

The GOT_FOCUS and LOST_FOCUS events can be generated by this component but not reliably across platforms. Currently, they are generated on most UNIX platforms but not on Microsoft Windows NT/95 under Java 1.0. These events are generated under Java 1.1.

Similarly, the mouse events are not generated with JDK 1.0.2. See Appendix C for more information about platform dependencies.

With the Java 1.1 event model, there are no listeners specific to TextArea. You can register key, mouse, and focus listeners through the Component methods of addKeyListener(), addMouseListener(), and addFocusListener(), respectively. To register listeners for text events, call TextComponent.addTextListener(). Action

The TextArea component has no way to trigger the action event, since carriage return is a valid character. You would need to put something like a Button on the screen to cause an action for a TextArea. The following Rot13 program demonstrates this technique. The user enters text in the TextArea and selects the Rotate Me button to rotate the text. If the user selects Rotate Me again, it rotates again, back to the original position. Without the button, there would be no way to trigger the event. Figure 8.5 shows this example in action.

import java.awt.*;
public class Rot13 extends Frame {
    TextArea ta;
    Component rotate, done;
    public Rot13 () {
        super ("Rot-13 Example");
        add ("North", new Label ("Enter Text to Rotate:"));
        ta = (TextArea)(add ("Center", new TextArea (5, 40)));
        Panel p = new Panel ();
        rotate = p.add (new Button ("Rotate Me"));
        done = p.add (new Button ("Done"));
        add ("South", p);
    }
    public static void main (String args[]) {
        Rot13 rot = new Rot13();
        rot.pack();
        rot.show();
    }
    public boolean handleEvent (Event e) {
        if (e.id == Event.WINDOW_DESTROY) {
            hide();
            dispose();
            System.exit (0);
            return true;
        }
        return super.handleEvent (e);
    }
    public boolean action (Event e, Object o) {
        if (e.target == rotate) {
            ta.setText (rot13Text (ta.getText()));
            return true;
        } else if (e.target == done) {
            hide();
            dispose();
            System.exit (0);
        }
        return false;
    }
    String rot13Text (String s) {
        int len = s.length();
        StringBuffer returnString = new StringBuffer (len);
        char c;
        for (int i=0;i<len;i++) {
            c = s.charAt (i);
            if (((c >= 'A') && (c <= 'M')) ||
                ((c >= 'a') && (c <= 'm')))
                c += 13;
            else if (((c >= 'N') && (c <= 'Z')) ||
                ((c >= 'n') && (c <= 'z')))
                c -= 13;
            returnString.append (c);
        }
        return returnString.toString();
    }
}

Figure 8.5: TextArea with activator button

[Graphic: Figure 8-5]

Keyboard

Ordinarily, the TextArea component generates all the key events.

public boolean keyDown (Event e, int key)

The keyDown() method is called whenever the user presses a key. keyDown() may be called many times in succession if the key remains pressed. e is the Event instance for the specific event, while key is the integer representation of the character pressed. The identifier for the event (e.id) for keyDown() could be either Event.KEY_PRESS for a regular key or Event.KEY_ACTION for an action-oriented key (i.e., an arrow or function key). Some of the things you can do through this method are validate input, convert each character to uppercase, and limit the number or type of characters entered. The technique is simple: you just need to remember that the user's keystroke is actually displayed by the TextArea peer, which receives the event after the TextArea itself. Therefore, a TextArea subclass can modify the character displayed by modifying the key field (e.key) of the Event and returning false, which passes the Event on down the chain; remember that returning false indicates that the Event has not been completely processed. The following method uses this technique to convert all alphabetic characters to the opposite case:

public boolean keyDown (Event e, int key) {
    if (Character.isUpperCase ((char)key)) {
        e.key = Character.toLowerCase ((char)key);
    } else if (Character.isLowerCase ((char)key)) {
        e.key = Character.toUpperCase ((char)key);
    }
    return false;
}

If keyDown() returns true, it indicates that the Event has been completely processed. In this case, the Event never propagates to the peer, and the keystroke is never displayed.

public boolean keyUp (Event e, int key)

The keyUp() method is called whenever the user releases a key. e is the Event instance for the specific event, while key is the integer representation of the character pressed. The identifier for the event (e.id) for keyUp() could be either Event.KEY_RELEASE for a regular key, or Event.KEY_ACTION_RELEASE for an action-oriented key (i.e., an arrow or function key).

Mouse

Ordinarily, the TextArea component does not trigger any mouse events.

NOTE:

Mouse events are not generated for TextArea with JDK 1.0.2. See Appendix C for more information about platform dependencies.

The GOT_FOCUS and LOST_FOCUS events can be generated by this component but not reliably across platforms. With the JDK, they are generated on most UNIX platforms but not on Microsoft Windows NT/95 under JDK 1.0. These events are generated with JDK 1.1. See Appendix C for more information about platform dependencies.

There are no listeners specific to the TextArea class. You can register Key, mouse, and focus listeners through the Component methods of addKeyListener(), addMouseListener(), and addFocusListener(), respectively. Also, you register listeners for text events by calling TextComponent.addTextListener().


TextField		Extending TextField