http://mail.openjdk.java.net/pipermail/openjfx-dev/2012-December/004685.html

Mark Claassen proposed an API that seemed really simple and straightforward and allowed developers to add filtering capabilities very easily to TextInputControls. Here is the idea:

> Add the following sub-interface to the Content interface.  No methods are
> added to the Content interface.
>    interface Filter {
>         String filter(int index, String text, String currentContent);
>    }
> 
> Add the following to TextFieldContent and TextAreaContent:
>    private Content.Filter filter;
>    public void setContentFilter(Content.Filter filter) {
>         this.filter = filter;
>    }
> 
> Change the "insert" method of TextFieldContent and TextAreaContent to start
> with:
>    @Override public void insert(int index, String text, boolean
> notifyListeners) {
>         text = TextInputControl.filterInput(text, [boolean], [boolean]);
>         if (filter != null)
>              text = filter.filter(index, text, get());
> 
> Add a method in TextField:
>    public void setContentFilter(Content.Filter filter) {
>         ((TextFieldContent)getContent()).setContentFilter(filter);
>    }
> 
> Add a method in TextArea:
>    public void setContentFilter(Content.Filter filter) {
>         ((TextAreaContent)getContent()).setContentFilter(filter);
>    }
> 
> This would do a few things:
> 1) Use the current "TextInputControl.filterInput" mechanisms of TextField
> and TextArea (like to strip out special chars and newlines).
> 2) By specifying the Filter interface outside of TextField and TextArea, it
> allows the same filter to be used for TextField and TextArea
> 3) Makes it simple to do things like translate all input to upper case or
> limit the overall length
> 4) By not adding a method to Content directly, something like the
> InputMaskContent would not be forced to deal with the Filter interface at
> all.
> 
> This implementation would allow someone creating a filter to "filter" the
> text to have illegal characters.  I thought it was more validate this
> first, so that the implementer would only have to deal with proper input.
> However, TextInputControl.filterInput could be done both before and after
> the Filter.filter() call.
>         text = TextInputControl.filterInput(text, [boolean], [boolean]);
>         if (filter != null) {
>              text = filter.filter(index, text, get());
>              text = TextInputControl.filterInput(text, [boolean],
> [boolean]);
>         }
> 
> Another possible addition to this may be to allow the filter to return
> null, signifying an error input and causing the system to "beep".