如何使用格式化文本框

Formatted text fields provide a way for developers to specify the valid set of characters that can be typed in a text field. Specifically, the JFormattedTextField class adds a formatter and an object value to the features inherited from the JTextField class. The formatter translates the field's value into the text it displays, and the text into the field's value.

Using the formatters that Swing provides, you can set up formatted text fields to type dates and numbers in localized formats. Another kind of formatter enables you to use a character mask to specify the set of characters that can be typed at each position in the field. For example, you can specify a mask for typing phone numbers in a particular format, such as (XX) X-XX-XX-XX-XX.

If the possible values of a formatted text field have an obvious order, use a spinner instead. A spinner uses a formatted text field by default, but adds two buttons that enable the user to choose a value in a sequence.

Another alternative or adjunct to using a formatted text field is installing an input verifier on the field. A component's input verifier is called when the component nearly loses the keyboard focus. The input verifier enables you to check whether the value of the component is valid and optionally change it or stop the focus from being transferred.

This GUI uses formatted text fields to display numbers in four different formats.

You can find the entire code for this program in FormattedTextFieldDemo.java. This code creates the first field.

amountField = new JFormattedTextField(amountFormat);
amountField.setValue(new Double(amount));
amountField.setColumns(10);
amountField.addPropertyChangeListener("value", this);
...
amountFormat = NumberFormat.getNumberInstance();

The constructor used to create the amountField object takes a java.text.Format argument. The Format object is used by the field's formatter to translate the field's value to text and the text to the field's value.

The remaining code sets up the amountField object. The setValue method sets the field's value property to a floating-point number represented as a Double object. The setColumns method, inherited from the JTextField class, hints about the preferred size of the field. The call to the addPropertyChangeListener method registers a listener for the value property of the field, so the program can update the Monthly Payment field whenever the user changes the loan amount.

The rest of this section covers the following topics:

• Creating and Initializing Formatted Text Fields
• Setting and Getting the Field's Value
• Specifying Formats
• Using MaskFormatter
• Specifying Formatters and Using Formatter Factories

This section does not explain the API inherited from the JTextField class. That API is described in How to Use Text Fields.

Creating and Initializing Formatted Text Fields

The following code creates and initializes the remaining three fields in the FormattedTextFieldDemo example.

rateField = new JFormattedTextField(percentFormat);
rateField.setValue(new Double(rate));
rateField.setColumns(10);
rateField.addPropertyChangeListener("value", this);

numPeriodsField = new JFormattedTextField();
numPeriodsField.setValue(new Integer(numPeriods));
numPeriodsField.setColumns(10);
numPeriodsField.addPropertyChangeListener("value", this);

paymentField = new JFormattedTextField(paymentFormat);
paymentField.setValue(new Double(payment));
paymentField.setColumns(10);
paymentField.setEditable(false);
paymentField.setForeground(Color.red);

...
percentFormat = NumberFormat.getNumberInstance();
percentFormat.setMinimumFractionDigits(2);

paymentFormat = NumberFormat.getCurrencyInstance();

The code for setting up the rateField object is almost identical to the code listed previously for other fields. The only difference is that the format is slightly different, thanks to the code percentFormat.setMinimumFractionDigits(2).

The code that creates the numPeriodsField object does not explicitly set a format or formatter. Instead, it sets the value to an Integer and enables the field to use the default formatter for Integer objects. The code did not do this in the previous two fields because the default formatter is not being used for Double objects. The result was not what was needed. How to specify formats and formatters is covered later in this section.

The payment field is different from the other fields because it is uneditable, uses a different color for its text, and does not have a property change listener. Otherwise, it is identical to the other fields. We could have chosen to use a text field or label instead. Whatever the component, we could still use the paymentFormat method to parse the payment amount into the text to be displayed.

Setting and Getting the Field's Value

Keep the following in mind when using a formatted text field:

A formatted text field's text and its value are two different properties, and the value often lags behind the text.

The text property is defined by the JTextField class. This property always reflects what the field displays. The value property, defined by the JFormattedTextField class, might not reflect the latest text displayed in the field. While the user is typing, the text property changes, but the value property does not change until the changes are committed.

To be more precise, the value of a formatted text field can be set by using either the setValue method or the commitEdit method. The setValue method sets the value to the specified argument. The argument can technically be any Object, but the formatter needs to be able to convert it into a string. Otherwise, the text field does not display any substantive information.

The commitEdit method sets the value to whatever object the formatter determines is represented by the field's text. The commitEdit method is automatically called when either of the following happens:

• When the user presses Enter while the field has the focus.
• By default, when the field loses the focus, for example, when the user presses the Tab key to change the focus to another component. You can use the setFocusLostBehavior method to specify a different outcome when the field loses the focus.

When you set the value of a formatted text field, the field's text is updated to reflect the value. Exactly how the value is represented as text depends on the field's formatter.

Note that although the JFormattedTextField class inherits the setText method from the JTextField class, you do not usually call the setText method on a formatted text field. If you do, the field's display changes accordingly but the value is not updated (unless the field's formatter updates it constantly).

To obtain a formatted text field's current value, use the getValue method. If necessary, you can ensure that the value reflects the text by calling the commitEdit method before getValue. Because the getValue method returns an Object, you need to cast it to the type used for your field's value. 例如：

Date enteredDate = (Date)dateField.getValue();

To detect changes in a formatted text field's value, you can register a property change listener on the formatted text field to listen for changes to the "value" property. The property change listener is taken from the FormattedTextFieldDemo example:

//The property change listener is registered on each
//field using code like this:
//    someField.addPropertyChangeListener("value", this);

/** Called when a field's "value" property changes. */
public void propertyChange(PropertyChangeEvent e) {
    Object source = e.getSource();
    if (source == amountField) {
        amount = ((Number)amountField.getValue()).doubleValue();
    } else if (source == rateField) {
        rate = ((Number)rateField.getValue()).doubleValue();
    } else if (source == numPeriodsField) {
        numPeriods = ((Number)numPeriodsField.getValue()).intValue();
    }

    double payment = computePayment(amount, rate, numPeriods);
    paymentField.setValue(new Double(payment));
}

Specifying Formats

The Format class provides a way to format locale-sensitive information such as dates and numbers. Formatters that descend from the InternationalFormatter class, such as the DateFormatter and NumberFormatter classes, use Format objects to translate between the field's text and value. You can obtain a Format object by calling one of the factory methods in the DateFormat or NumberFormat classes, or by using one of the SimpleDateFormat constructors.

You can customize certain format aspects when you create the Format object, and others through a format-specific API. For example, DecimalFormat objects, which inherit from NumberFormat and are often returned by its factory methods, can be customized by using the setMaximumFractionDigits and setNegativePrefix methods. For information about using Format objects, see the Formatting lesson of the Internationalization trail.

The easiest way to associate a customized format with a formatted text field is to create the field by using the JFormattedTextField constructor that takes a Format as an argument. You can see this association in the previous code examples that create amountField and rateField objects.

Using MaskFormatter

The MaskFormatter class implements a formatter that specifies exactly which characters are valid in each position of the field's text. For example, the following code creates a MaskFormatter that lets the user to type a five-digit zip code:

zipField = new JFormattedTextField(
                    createFormatter("#####"));
...
protected MaskFormatter createFormatter(String s) {
    MaskFormatter formatter = null;
    try {
        formatter = new MaskFormatter(s);
    } catch (java.text.ParseException exc) {
        System.err.println("formatter is bad: " + exc.getMessage());
        System.exit(-1);
    }
    return formatter;
}

You can try out the results of the preceding code by running TextInputDemo. Click the Launch button to run TextInputDemo using Java™ Web Start (download JDK 7 or later). Alternatively, to compile and run the example yourself, consult the example index.

The program's GUI is displayed.

The following table shows the characters that you can use in the formatting mask:

Specifying Formatters and Using Formatter Factories

When specifying formatters, keep in mind that each formatter object can be used by at most one formatted text field at a time. Each field should have at least one formatter associated with it, of which exactly one is used at any time.

You can specify the formatters to be used by a formatted text field in several ways:

• Use the JFormattedTextField constructor that takes a Format argument.
  A formatter for the field is automatically created that uses the specified format.
• Use the JFormattedTextField constructor that takes a JFormattedTextField.AbstractFormatter argument.
  The specified formatter is used for the field.
• Set the value of a formatted text field that has no format, formatter, or formatter factory specified.
  A formatter is assigned to the field by the default formatter factory, using the type of the field's value as a guide. If the value is a Date, the formatter is a DateFormatter. If the value is a Number, the formatter is a NumberFormatter. Other types result in an instance of DefaultFormatter.
• Make the formatted text field use a formatter factory that returns customized formatter objects.
  This is the most flexible approach. It is useful when you want to associate more than one formatter with a field or add a new kind of formatter to be used for multiple fields. An example of the former use is a field that interprets the user typing in a certain way but displays the value (when the user is not typing) in another way. An example of the latter use is several fields with custom class values, for example, PhoneNumber. You can set up the fields to use a formatter factory that returns specialized formatters for phone numbers.

You can set a field's formatter factory either by creating the field using a constructor that takes a formatter factory argument, or by calling the setFormatterFactory method on the field. To create a formatter factory, you can often use an instance of DefaultFormatterFactory class. A DefaultFormatterFactory object enables you to specify the formatters returned when a value is being edited, is not being edited, or has a null value.

The following figures show an application based on the FormattedTextFieldDemo example that uses formatter factories to set multiple editors for the Loan Amount and APR fields. While the user is editing the Loan Amount, the $ character is not used so that the user is not forced to type it. Similarly, while the user is editing the APR field, the % character is not required.

Click the Launch button to run FormatterFactoryDemo using Java™ Web Start (download JDK 7 or later). Alternatively, to compile and run the example yourself, consult the example index.

The following code that creates the formatters and sets them up by using instances of the DefaultFormatterFactory class:

private double rate = .075;  //7.5 %
...
amountField = new JFormattedTextField(
                    new DefaultFormatterFactory(
                        new NumberFormatter(amountDisplayFormat),
                        new NumberFormatter(amountDisplayFormat),
                        new NumberFormatter(amountEditFormat)));
...
NumberFormatter percentEditFormatter =
        new NumberFormatter(percentEditFormat) {
    public String valueToString(Object o)
          throws ParseException {
        Number number = (Number)o;
        if (number != null) {
            double d = number.doubleValue() * 100.0;
            number = new Double(d);
        }
        return super.valueToString(number);
    }
    public Object stringToValue(String s)
           throws ParseException {
        Number number = (Number)super.stringToValue(s);
        if (number != null) {
            double d = number.doubleValue() / 100.0;
            number = new Double(d);
        }
        return number;
    }
};
rateField = new JFormattedTextField(
                     new DefaultFormatterFactory(
                        new NumberFormatter(percentDisplayFormat),
                        new NumberFormatter(percentDisplayFormat),
                        percentEditFormatter));
...
amountDisplayFormat = NumberFormat.getCurrencyInstance();
amountDisplayFormat.setMinimumFractionDigits(0);
amountEditFormat = NumberFormat.getNumberInstance();

percentDisplayFormat = NumberFormat.getPercentInstance();
percentDisplayFormat.setMinimumFractionDigits(2);
percentEditFormat = NumberFormat.getNumberInstance();
percentEditFormat.setMinimumFractionDigits(2);

The boldface code highlights the calls to DefaultFormatterFactory constructors. The first argument to the constructor specifies the default formatter to use for the formatted text field. The second argument specifies the display formatter, which is used when the field does not have the focus. The third argument specifies the edit formatter, which is used when the field has the focus. The code does not use a fourth argument, but if it did, the fourth argument would specify the null formatter, which is used when the field's value is null. Because no null formatter is specified, the default formatter is used when the value is null.

The code customizes the formatter that uses percentEditFormat by creating a subclass of the NumberFormatter class. This subclass overrides the valueToString and stringToValue methods of NumberFormatter so that they convert the displayed number to the value actually used in calculations, and convert the value to a number. Specifically, the displayed number is 100 times the actual value. The reason is that the percent format used by the display formatter automatically displays the text as 100 times the value, so the corresponding editor formatter must display the text at the same value. The FormattedTextFieldDemo example does not need to take care of this conversion because this demo uses only one format for both display and editing.

You can find the code for the entire program in FormatterFactoryDemo.java.

Formatted Text Field API

The following tables list some of the commonly used APIs for using formatted text fields.

• Classes Related to Formatted Text Fields
• JFormattedTextField Methods
• DefaultFormatter Options

Examples That Use Formatted Text Fields

This table lists examples that use formatted text fields and points to where those examples are described.