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How to Use Formatted Text Fields - Java Tutorial 5.0 英文版

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Trail: Creating a GUI with JFC/Swing
Lesson: Using Swing Components

How to Use Formatted Text Fields

Release 1.4 introduced a subclass of JTextField called JFormattedTextField. Formatted text fields provide a way for developers to specify the legal set of characters that can be entered into a text field. Specifically, JFormattedTextField adds a formatter and an object value to the features inherited from JTextField. The formatter performs the translation from the field's value into the text it displays, and vice versa.

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

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Version note: Before 1.4, text field formatting required more effort. You could check the field's value when the user pressed Enter by putting format-checking code in your action listener, but you couldn't do any checking before the action event was generated unless you implemented a custom model (Document) for the text field. 1.3 introduced input verification, but that isn't specialized for text fields and is tied to focus changes.

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If the possible values of a formatted text field have an obvious order, consider using a spinner instead. A spinner uses a formatted text field, by default, but adds two buttons that let the user step through a sequence of values.

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 is about to lose the keyboard focus. It lets you check whether the value of the component is legal and optionally change it or stop focus from being transferred.

Here is a picture of a GUI that uses formatted text fields to display numbers in four different formats.

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Try this: 

1. Run FormattedTextFieldDemo using Java^TM Web Start. Or, to compile and run the example yourself, consult the example index.
2. Experiment with different loan amounts, annual percentage rates (APR), and loan lengths.
   Note that the Month Payment field is updated when you either press Enter or move the focus out of the field you're editing, as long as the text you type is valid.
3. Enter invalid text such as "abcd" in the Loan Amount field and then press Enter.
   Nothing happens. When you move the focus from the field, the text reverts to the field's last valid value.
4. Enter marginally valid text such as "2000abcd" in the Loan Amount field and press Enter.
   The Monthly Payment field is updated, though the Loan Amount field still looks strange. When you move the focus from the Loan Amount field, the text it displays is updated to be a nicely formatted version of its value — for example, "2,000".

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You can find the full code in FormattedTextFieldDemo.java. Here is the code that 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 amountField takes a java.text.Format argument. The Format object is used by the field's formatter to translate between the field's text and value.

The rest of the code sets up amountField. The setValue method sets the field's value property to a floating point number represented as a Double object. The setColumns method, inherited from JTextField, provides a hint as to the preferred size of the field. Finally, the call to addPropertyChangeListener 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 doesn't explain the API inherited from JTextField. 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 FormattedTextFieldDemo.

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 numPeriodsField is almost identical to the code you saw before. The only difference is that the format is slightly different, thanks to the code percentFormat.setMinimumFractionDigits(2).

The code that creates numPeriodsField doesn't explicitly set a format or formatter. Instead, it sets the value to an Integer and lets the field use the default formatter for Integers. We couldn't do this in the previous two fields because we didn't want to use the default formatter for Doubles; the result didn't look exactly like we wanted it to. We'll discuss how to specify formats and formatters a little later.

The payment field is different from the other fields because it's uneditable, uses a different color for its text, and doesn't happen to have a property change listener. However, it's otherwise the same as the other fields. We could have chosen to use a text field or label instead. Whatever the component, we could still use paymentFormat to parse the payment amount into the text to be displayed.

Setting and Getting the Field's Value

Keep this in mind when using a formatted text field:

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

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The text property is defined by JTextField; it always reflects what the field displays. The value property, defined by JFormattedTextField, might not reflect the latest text displayed in the field. While the user is typing, the text property changes, but the value property doesn't until the changes are committed.

To be more precise, the value of a formatted text field can be set using either the setValue method or the commitEdit method. The setValue method sets the value to the specified argument. Although the argument can technically be any Object, it needs to be something that the formatter can convert into a string. Otherwise, the text field won't display anything useful.

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, the user presses Tab to change the focus to another component. You can use the setFocusLostBehavior method to specify that something different should happen when the field loses the focus.

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Note: Some formatters might update the value constantly, making the focus-lost behavior moot since the value will always be the same as what the text specifies.

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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 JFormattedTextField inherits the setText method from JTextField, you don't usually invoke setText on a formatted text field. If you do, the field's display will change accordingly but the value will not be updated (unless the field's formatter updates it constantly).

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

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. Here is the property change listener from FormattedTextFieldDemo:

//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 InternationalFormatter, such as DateFormatter and NumberFormatter, use Format objects to translate between the field's text and value. You can get a Format object by invoking one of the factory methods in DateFormat or NumberFormat, or by using one of the SimpleDateFormat constructors.

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Note: A third commonly used formatter class, MaskFormatter, does not descend from InternationalFormatter and does not use formats. It's discussed in Using MaskFormatter.

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You can customize certain format aspects when you create the Format, and others through format-specific API. For example, DecimalFormat objects, which inherit from NumberFormat and are often returned by its factory methods, can be customized using the setMaximumFractionDigits and setNegativePrefix methods. For information about using Formats, 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 using the JFormattedTextField constructor that takes a Format as an argument. You can see this in the previous code snippets that create amountField and rateField.

Using MaskFormatter

The MaskFormatter class implements a formatter that specifies exactly which characters are legal in each position of the field's text. For example, the following code creates a MaskFormatter that lets the user enter a 5-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 using Java Web Start. Or, to compile and run the example yourself, consult the example index. Here is a picture of the program's GUI:

[PENDING: This snapshot needs to be retaken, since the GUI has changed slightly.]

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

Character	Description
#	Any valid number (Character.isDigit).
' (single quote)	Escape character, used to escape any of the special formatting characters.
U	Any character (Character.isLetter). All lowercase letters are mapped to uppercase.
L	Any character (Character.isLetter). All uppercase letters are mapped to lowercase.
A	Any character or number (Character.isLetter or Character.isDigit).
?	Any character (Character.isLetter).
*	Anything.
H	Any hex character (0-9, a-f or A-F).

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's 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. As an example of the former use, you might have a field that should interpret user typing a certain way but display the value (when the user isn't typing) another way. As an example of the latter use, you might have several fields that have values of a custom class — say, PhoneNumber. You could set up the fields to use a formatter factory that can return 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 invoking the setFormatterFactory method on the field. To create a formatter factory, you can often use an instance of DefaultFormatterFactory. A DefaultFormatterFactory object lets you specify the formatters returned when a value is being edited, not being edited, or has a null value.

The following pictures show an application based on FormattedTextFieldDemo 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 isn't forced to enter it. Similarly, while the user is editing the APR field, the % is not required. You can run the example using Java Web Start. Or, to compile and run the example yourself, consult the example index.

Here is the code that creates the formatters and sets them up, using instances of DefaultFormatterFactory:

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 specifies the display formatter, which is used when the field doesn't have the focus. The third specifies the edit formatter, used when the field has the focus. The code doesn't use a fourth argument, but if it did, it 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 NumberFormatter. 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 vice versa. 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 do so as well. FormattedTextFieldDemo doesn't need to worry about this conversion because it uses only one format for both display and editing.

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

The Formatted Text Field API

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

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

Classes Related to Formatted Text Fields

Class or Interface	Purpose
JFormattedTextField	Subclass of JTextField that supports formatting arbitrary values.
JFormattedTextField.AbstractFormatter	The superclass of all formatters for JFormattedTextField. A formatter enforces editing policies and navigation policies, handles string-to-object conversions, and manipulates the JFormattedTextField as necessary to enforce the desired policy.
JFormattedTextField.AbstractFormatterFactory	The superclass of all formatter factories. Each JFormattedTextField uses a formatter factory to obtain the formatter that best corresponds to the text field's state.
DefaultFormatterFactory	The formatter factory normally used. Dishes out formatters based on details such as the passed-in parameters and focus state.
DefaultFormatter	Subclass of JFormattedTextField.AbstractFormatter that formats arbitrary objects using the toString method.
MaskFormatter	Subclass of DefaultFormatter that formats and edits strings using a specified character mask. (For example, 7-digit phone numbers can be specified using "###-####".)
InternationalFormatter	Subclass of DefaultFormatter that uses an instance of java.text.Format to handle conversion to and from a String.
NumberFormatter	Subclass of InternationalFormatter that supports number formats using an instance of NumberFormat.
DateFormatter	Subclass of InternationalFormatter that supports date formats using an instance of DateFormat.

JFormattedTextField Methods

Method or Constructor	Purpose
JFormattedTextField() JFormattedTextField(Object) JFormattedTextField(Format) JFormattedTextField(AbstractFormatter) JFormattedTextField(AbstractFormatterFactory) JFormattedTextField(AbstractFormatterFactory, Object)	Create a new formatted text field. The Object argument, if present, specifies the initial value of the field and causes an appropriate formatter factory to be created. The Format or AbstractFormatterargument specifies the format or formatter to be used for the field, causing an appropriate formatter factory to be created. The AbstractFormatterFactory argument specifies the formatter factory to be used, which determines which formatters are used for the field.
void setValue(Object) Object getValue()	Set or get the value of the formatted text field. You must cast the return type based on how the JFormattedTextField has been configured. If the formatter hasn't been set yet, invoking setValue sets the formatter to one returned by the field's formatter factory.
void setFormatterFactory(AbstractFormatterFactory)	Set the object that determines the formatters used for the formatted text field. It is often an instance of DefaultFormatterFactory.
AbstractFormatter getFormatter()	Get the formatter of the formatted text field. It is often an instance of DefaultFormatter.
void setFocusLostBehavior(int)	Specifies what should happen when the field loses the focus. Possible values are defined in JFormattedTextField as COMMIT_OR_REVERT (the default), COMMIT (commit if valid, otherwise leave everything the same), PERSIST (do nothing), and REVERT (change the text to reflect the value).
void commitEdit()	Sets the value to the object represented by the field's text (as determined by the field's formatter). If the text is invalid, the value remains the same and a ParseException is thrown.
boolean isEditValid()	Returns true if the formatter considers the current text to be valid (as determined by the field's formatter).

DefaultFormatter Options

Method	Purpose
void setCommitsOnValidEdit(boolean) boolean getCommitsOnValidEdit()	Set or get when edits are pushed back to the JFormattedTextField. If true, commitEdit is invoked after every valid edit. This property is false by default.
void setOverwriteMode(boolean) boolean getOverwriteMode()	Set or get the the behavior when inserting characters. If true, new characters overwrite existing characters in the model as they are inserted. The default value of this property is true in DefaultFormatter (and thus in MaskFormatter) and false in InternationalFormatter (and thus in DateFormatter and NumberFormatter).
void setAllowsInvalid(boolean) boolean getAllowsInvalid()	Set or get whether the value being edited is allowed to be invalid for a length of time. It is often convenient to allow the user to enter invalid values until a commit is attempted. DefaultFormatter initializes this property to true. Of the standard Swing formatters, only MaskFormatter sets it to false.

Examples that Use Formatted Text Fields

A few of our examples use formatted text fields.

Example	Where Described	Notes
FormattedTextFieldDemo	This section	Uses four formatted text fields.
SpinnerDemo	How to Use Spinners	Customizes the appearance of the formatted text fields used by two spinners.
SliderDemo3	How to Use Sliders	Pairs a formatted text field with a slider to allow editing of an integer value.
Converter	Using Models	Each ConversionPanel pairs a formatted text field with a slider.
CelsiusConverter2 [PENDING: When the 1.4 version of this example is added, there will be a link to it.]	Nowhere yet	Uses a formatted text field to let the user enter a decimal number.
TextInputDemo	This section	Shows how to use text fields, spinners, and formatted text fields together, and demonstrates how to use MaskFormatter. Includes code for selecting the text of the field that has just gotten the focus.
FormatterFactoryDemo	This section	A variation on FormattedTextFieldDemo that uses formatter factories to specify multiple formatters for two formatted text fields.
RegexFormatter	Regular Expression Based AbstractFormatter (in The Swing Connection	A regular expression formatter that you can use. Includes source code and information on how it was implemented.

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