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Java: Better Interfaces via JFormattedTextField


Custom Formatters

If you have a data type that is variable in length and does not fit well with one of the existing formatter classes, or requires special rules for conversion, then you can create your own formatter class based on the DefaultFormatter class or any of the other Formatter classes (e.g. NumberFormatter). You will need to override the stringToValue and valueToString methods to provide the behavior you desire. stringToValue accepts a String as an input, parses it, and returns an Object. If the parsing is fails, then a ParseException should be thrown. The type of Object returned should match the results of getValueClass. The code necessary to parse the String will depend on the data you are parsing. If you are parsing a proper name, then your method may just check to see that the String is not empty and force the first letter to be capitalized. If you are parsing a URL or email address, then you'll want to employ a Pattern object to test the validity and sequence of the characters.

valueToString accepts an Object and converts it to a String. If the conversion fails, then a ParseException is thrown. The conversion may be as simple as calling the toString() method of the Object or the conversion may adjust the data before returning it. For example, a field that holds a percentage may want to store 5.3% as .053, but display it as 5.3%. In this case the valueToString method would multiply the value by 100, format it with the desired number of decimal places, then tag on the percent sign.

The EmailFormatter class extends the DefaultFormatter class and uses a precompiled pattern to check an email address.For more information on the email format and pattern matcher, see "How to Validate Email, SSN, Phone Numbers in Java Using Regular Expressions by Zaheer Paracha.

The code below creates a JFormattedTextField for entering an email address using EmailFormatter.java class which you can download here.)


 JFormattedTextField emailField = new JFormattedTextField();
 EmailFormatter emailFormatter = new EmailFormatter();
 DefaultFormatterFactory emailFormatterFactory = 
                new DefaultFormatterFactory(emailFormatter);
 emailField.setFormatterFactory(emailFormatterFactory);

If you want a custom formatter that handles numbers, then you should create a class that extends NumberFormatter. The PercentFormatter class extends NumberFormatter to create a class to handle percentage data. The constructor accepts a NumberFormat object to handle formatting and parsing. The valueToString method first multiplies the value by 100, then calls super class's valueToString method, finally a percent is added and the String returned. The stringToValue first strips off any extra white space or percent signs, then converts to a double using the super class's stringToValue method, divides by 100, then returns a Double object. In this way the value is stored in a form amenable to using in calculations, but displayed in a manner the user expects.

The code below creates a JFormattedTextField for entering an percent using PercentFormatter.java which you can download here.


    JFormattedTextField percentField = new JFormattedTextField();
    PercentFormatter percentFormatter = 
               new PercentFormatter(new DecimalFormat("#.##"));
    DefaultFormatterFactory percentFormatterFactory = 
               new DefaultFormatterFactory(percentFormatter);
    percentField.setFormatterFactory(percentFormatterFactory);


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