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protected IConverterLocator newConverterLocator() {
    ConverterLocator converterLocator = new ConverterLocator();
    converterLocator.set(Money.class, new MoneyConverter());
    return converterLocator;
}

In Wicket 1.4

This is the same as in 1.3.

My customers do not know what a "Double" is, so the message "'value' is not a valid Double" doesn't make sense. I wanted to provide a simpler message.

Custom message for class Double

First, as with other converters, you override newConverterLocator in your application.

public class Application extends WebApplication {
...
    @Override
    protected IConverterLocator newConverterLocator() {
        ConverterLocator locator = (ConverterLocator) super.newConverterLocator();
        locator.set(Double.class, new MyDoubleConverter());
        return locator;
    }
    ....

This tells Wicket for the Double class, use MyDoubleConverter for converting, which also takes care of reporting the error when not parsable. It seems the only reason I need to create MyDoubleConverter is for variable substitution in my message. (There doesn't seem to be a default key for the value. For the class, it looks like you can use "type" although I didn't try it.)

// This is an inner class within my Application
private static final class MyDoubleConverter extends DoubleConverter {
    private static final long serialVersionUID = 1L;

    @Override
    protected ConversionException newConversionException(String message, Object value,
            Locale locale) {
        final ConversionException newConversionException = super.newConversionException(message, value, locale);
        newConversionException.setVariable("value", value);
        return newConversionException;
    }
}

I tried implementing it as an anonymous class, but got a java.io.NotSerializableException. Once I moved this to an inner class it worked. If you know why, let me know!

The final piece of the puzzle is to put the message in your properties file. I put it in my BasePage.properties file.

IConverter.Double='${value}' is not a valid number

Providing a custom converter for specific components

...

Note that we used the converter of the most generic type, wicket.util.convert.IConverter. If you use this type, you have to look at the requested target type (thus you code like: if(c.equals(URL.class))) yourself. While using wicket.util.convert.Converter is easier, as in that case you can couple {{wicket.util.convert.ITypeConverter}} s, using IConverter directly is transparent (easier to debug) and it forces to do only one type of conversion.