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Java AWT

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Input Fields
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8.2 TextField

TextField is the TextComponent for single-line input. Some constructors permit you to set the width of the TextField on the screen, but the current LayoutManager may change it. The text in the TextField is left justified, and the justification is not customizable. To change the font and size of text within the TextField, call setFont() as shown in Chapter 3, Fonts and Colors.

The width of the field does not limit the number of characters that the user can type into the field. It merely suggests how wide the field should be. To limit the number of characters, it is necessary to override the keyDown() method for the Component. Extending TextField contains an example showing how to do this.

TextField Methods

Constructors

public TextField ()

This constructor creates an empty TextField. The width of the TextField is zero columns, but it will be made wide enough to display just about one character, depending on the current font and size.

public TextField (int columns)

This constructor creates an empty TextField. The TextField width is columns. The TextField will try to be wide enough to display columns characters in the current font and size. As I mentioned previously, the layout manager may change the size.

public TextField (String text)

This constructor creates a TextField with text as its content. In Java 1.0 systems, the TextField is 0 columns wide (the getColumns() result), but the system will size it to fit the length of text. With Java 1.1, getColumns() actually returns text.length.

public TextField (String text, int columns)

This constructor creates a TextField with text as its content and a width of columns.

The following example uses all four constructors; the results are shown in Figure 8.2. With the third constructor, you see that the TextField is not quite wide enough for our text. The system uses an average width per character to try to determine how wide the field should be. If you want to be on the safe side, specify the field's length explicitly, and add a few extra characters to ensure that there is enough room on the screen for the entire text.

import java.awt.TextField;
public class texts extends java.applet.Applet {
   public void init () {
        add (new TextField ());                   // A
        add (new TextField (15));                 // B
        add (new TextField ("Empty String"));     // C
        add (new TextField ("Empty String", 20)); // D
   }
}
Sizing

public int getColumns ()

The getColumns() method returns the number of columns set with the constructor or a later call to setColumns(). This could be different from the displayed width of the TextField, depending upon the current LayoutManager.

public void setColumns (int columns) (New)

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

public Dimension getPreferredSize (int columns) (New)
public Dimension preferredSize (int columns) (Deprecated)

The getPreferredSize() method returns the Dimension (width and height) for the preferred size of a TextField with a width of columns. The columns specified may be different from the number of columns designated in the constructor.

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

public Dimension getPreferredSize () (New)
public Dimension preferredSize () (Deprecated)

The getPreferredSize() method returns the Dimension (width and height) for the preferred size of the TextField. Without the columns parameter, this getPreferredSize() uses the constructor's number of columns (or the value from a subsequent call to setColumns()) to calculate the TextField's preferred size.

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

public Dimension getMinimumSize (int columns) (New)
public Dimension minimumSize (int columns) (Deprecated)

The getMinimumSize() method returns the minimum Dimension (width and height) for the size of a TextField with a width of columns. The columns specified may be different from the columns designated in the constructor.

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

public Dimension getMinimumSize () (New)
public Dimension minimumSize ()

The getMinimumSize() method returns the minimum Dimension (width and height) for the size of the TextField. Without the columns parameter, this getMinimumSize() uses the constructor's number of columns (or the value from a subsequent call to setColumns()) to calculate the TextField's minimum size.

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

Echoing character

It is possible to change the character echoed back to the user when he or she types. This is extremely useful for implementing password entry fields.

public char getEchoChar ()

The getEchoChar() method returns the currently echoed character. If the TextField is echoing normally, getEchoChar() returns zero.

public void setEchoChar (char c) (New)
public void setEchoCharacter (char c) (Deprecated)

The setEchoChar() method changes the character that is displayed to the user to c for every character in the TextField. It is possible to change the echo character on the fly so that existing characters will be replaced. A c of zero, (char)0, effectively turns off any change and makes the TextField behave normally.

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

public boolean echoCharIsSet ()

The echoCharIsSet() method returns true if the echo character is set to a nonzero value. If the TextField is displaying input normally, this method returns false.

Miscellaneous methods

public synchronized void addNotify ()

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

protected String paramString ()

When you call the toString() method of TextField, the default toString() method of Component is called. This in turn calls paramString(), which builds up the string to display. The TextField level can add only one item. If the echo character is nonzero, the current echo character is added (the method getEchoChar()). Using new TextField (`Empty String`, 20), the results displayed could be:

java.awt.TextField[0,0,0x0,invalid,text="Empty String",editable,selection=0-0]

TextField Events

With the 1.0 event model, TextField components can generate KEY_PRESS and KEY_ACTION (which calls keyDown()), KEY_RELEASE and KEY_ACTION_RELEASE (which calls keyUp()), and ACTION_EVENT (which calls action()).

With the 1.1 event model, you register an ActionListener with the method addActionListener(). Then when the user presses Return within the TextField the ActionListener.actionPerformed() method is called through the protected TextField.processActionEvent() method. Key, mouse, and focus listeners are registered through the three Component methods of addKeyListener(), addMouseListener(), and addFocusListener(), respectively. Action

public boolean action (Event e, Object o)

The action() method for a TextField is called when the input focus is in the TextField and the user presses the Return key. e is the Event instance for the specific event, while o is a String representing the current contents (the getText() method).

Keyboard

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 TextField peer, which receives the event after the TextField itself. Therefore, a TextField 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 input to uppercase.

public boolean keyDown (Event e, int 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). Among other things, keyUp() may be used to determine how long the key has been pressed.

Mouse

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

NOTE:

Mouse events are not generated for TextField with JDK 1.0.2. Your run-time environment may behave differently. See Appendix C for more information about platform dependencies.

Focus

The TextField component does not reliably generate focus events.

NOTE:

The GOT_FOCUS and LOST_FOCUS events can be generated by TextFields, but these events are not reliable across platforms. With Java 1.0, they are generated on most UNIX platforms but not on Windows NT/95 platforms. They are generated on all platforms under Java 1.1. See Appendix C for more information about platform dependencies.

public boolean gotFocus (Event e, Object o)

The gotFocus() method is triggered when the TextField gets the input focus. e is the Event instance for the specific event, while o is a String representation of the current contents (getText()).

public boolean lostFocus (Event e, Object o)

The lostFocus() method is triggered when the input focus leaves the TextField. e is the Event instance for the specific event, while o is a String representation of the current contents (getText()).

Listeners and 1.1 event handling

With the 1.1 event model, you register event listeners that are told when an event occurs. You can register text event listeners by calling the method TextComponent.addTextListener().

public void addActionListener(ActionListener listener) (New)

The addActionListener() method registers listener as an object interested in receiving notifications when an ActionEvent passes through the EventQueue with this TextField as its target. The listener.actionPerformed() method is called when these events occur. Multiple listeners can be registered. The following code demonstrates how to use an ActionListener to reverse the text in the TextField.

// Java 1.1 only 
import java.applet.*;
import java.awt.*;
import java.awt.event.*;
class MyAL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
        System.out.println ("The current text is: " +
            e.getActionCommand());
        if (e.getSource() instanceof TextField) {
            TextField tf = (TextField)e.getSource();
            StringBuffer sb = new StringBuffer (e.getActionCommand());
            tf.setText (sb.reverse().toString());
        }
    }
}
public class text11 extends Applet {
    public void init () {
        TextField tf = new TextField ("Help Text", 20);
        add (tf);
        tf.addActionListener (new MyAL());
   }
}

public void removeActionListener(ActionListener listener) (New)

The removeActionListener() method removes listener as a interested listener. If listener is not registered, nothing happens.

protected void processEvent(AWTEvent e) (New)

The processEvent() method receives all AWTEvents with this TextField as its target. processEvent() then passes them along to any listeners for processing. When you subclass TextField, overriding processEvent() allows you to process all events yourself, before sending them to any listeners. In a way, overriding processEvent() is like overriding handleEvent() using the 1.0 event model.

If you override processEvent(), remember to call super.processEvent(e) last to ensure that regular event processing can occur. If you want to process your own events, it's a good idea to call enableEvents() (inherited from Component) to ensure that events are delivered even in the absence of registered listeners.

protected void processActionEvent(ActionEvent e) (New)

The processActionEvent() method receives all ActionEvents with this TextField as its target. processActionEvent() then passes them along to any listeners for processing. When you subclass TextField, overriding the method processActionEvent() allows you to process all action events yourself, before sending them to any listeners. In a way, overriding processActionEvent() is like overriding action() using the 1.0 event model.

If you override the processActionEvent() method, remember to call super.processActionEvent(e) last to ensure that regular event processing can occur. If you want to process your own events, it's a good idea to call enableEvents() (inherited from Component) to ensure that events are delivered even in the absence of registered listeners.

The following applet is equivalent to the previous example, except that it overrides processActionEvent() to receive events, eliminating the need for an ActionListener. The constructor calls enableEvents() to make sure that events are delivered, even if no listeners are registered.

// Java 1.1 only
import java.applet.*;
import java.awt.*;
import java.awt.event.*;
class MyTextField extends TextField {
    public MyTextField (String s, int len) {
        super (s, len);
        enableEvents (AWTEvent.ACTION_EVENT_MASK);
    }
    protected void processActionEvent(ActionEvent e) {
        System.out.println ("The current text is: " +
            e.getActionCommand());
        TextField tf = (TextField)e.getSource();
        StringBuffer sb = new StringBuffer (e.getActionCommand());
        tf.setText (sb.reverse().toString());
        super.processActionEvent(e)
    }
}
public class text12 extends Applet {
   public void init () {
        TextField tf = new MyTextField ("Help Text", 20);
        add (tf);
   }
}


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