[Chapter 8] 8.2 TextField

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| Chapter 8 Input Fields |

## 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
       }
    }

#### Figure 8.2: Using the TextField constructors

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)

  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)
public Dimension preferredSize (int columns)

  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 ()
public Dimension preferredSize ()

  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)
public Dimension minimumSize (int columns)

  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 ()
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)
public void setEchoCharacter (char c)

  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 `TextField`s, 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)

  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)

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

protected void processEvent(AWTEvent e)

  The `processEvent()` method receives all `AWTEvent`s 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)

  The `processActionEvent()` method receives all `ActionEvent`s 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);
       }
    }

Text Component |  | TextArea