Question

I’m using a NumberFormatter and JFormattedTextField, but the .getValue() doesn’t return the same value as the user is seeing.

I think the input-string is parsed using NumberFormats parse-method, and I get the Numberformat from NumberFormat.getNumberInstance(); with the actual Locale. So I don’t think I easily can extend it and write my own parse-method?

In example, if the user types 1234.487
the getValue() will return:
1234.487 but the user will be displayed 1,234.49

Another example, using NumberFormat.getCurrencyInstance();. The user types 1234.487 and the getValue() will return 1234.487 but the user will be displayed $1,234.49

Instead, I want an ParseException be generated if the Formatter can’t format the value without rounding. Same thing if the user types 4.35b6, by default the formatter will display 4.35 and the value will be 4.35, but I want a ParseException, because the user typed in an invalid value.

Here is the code that I have tried with:

NumberFormat nf = NumberFormat.getNumberInstance();
nf.setMaximumFractionDigits(2);
nf.setMinimumFractionDigits(2);
final JFormattedTextField ftf = new JFormattedTextField(nf);
ftf.setValue(new BigDecimal("1234.50"));

// Print the value from ftf
final JTextField txt = new JTextField(12);
txt.addFocusListener(new FocusAdapter() {
    public void focusGained(FocusEvent e) {
        txt.setText(ftf.getValue().toString());
    }
});

How to get the same value as the user is seeing?

Answer 1

You should extend NumberFormatter rather than NumberFormat and override stringToValue so that it verifies that when the string is parsed you get back the original value:

class StrictNumberFormatter extends javax.swing.text.NumberFormatter {
    @Override
    public Object stringToValue(String text) throws ParseException {
        Object parsedValue = super.stringToValue(text);
        String expectedText = super.valueToString(parsedValue);
        if (! super.stringToValue(expectedText).equals(parsedValue)) {
            throw new ParseException("Rounding occurred", 0);
        }
        return parsedValue;
    }

    public StrictNumberFormatter(NumberFormat nf) {
        super(nf);
    }
}

NumberFormat nf = NumberFormat.getNumberInstance();
nf.setMaximumFractionDigits(2);
nf.setMinimumFractionDigits(2);
JFormattedTextField.AbstractFormatter formatter = new StrictNumberFormatter(nf);
final JFormattedTextField ftf = new JFormattedTextField(formatter);
ftf.setValue(new BigDecimal("1234.50"));

This formatter will reject the new text if the values before and after rounding do not match.

Note: this compares the values, not the strings. It will tolerate changes like removing grouping characters ("$1,000" => "$1000") and trailing zeros ("$1.20" => "$1.2"). Basically if the NumberFormat returns the same value then it is acceptable. But any restrictions imposed by the NumberFormat still apply, e.g. you must not remove the currency symbol or insert leading spaces etc.