public int getValue()

This week I committed my first change as a member of the GWT Incubator project. That change was to introduce an interface, HasValue. This by itself is no monumental achievement. The interface is very simple:

public interface HasValue<T> {
 T getValue();
 void setValue(T value);
}

But this simple interface will facilitate the creation of more complex features such as libraries for data binding or validation.

HasValue provides a layer of abstraction around Widgets, allowing access to the underlying data model through a common API. HasValue works with simple Widgets and data types as well as complex data types and composite Widgets.

Consider two Widgets (both from the Incubator) for selecting a Date value:

DatePicker picker = new DatePicker();
DropDownListBox<Date> dropDown = new DropDownListBox<Date>();

Both implement HasValue<Date> so you could, for example, write a BirthdayValidator that ensures the selected Date is before today. This validator would have a HasValue<Date> parameter and could validate either of the two Widgets above, or any other Widget implementing the HasValue<Date> interface, without additional coding to support those Widgets.

Right now HasValue exists only in the Incubator, but there is support for including it in the next GWT release, version 1.6. Hopefully that release will see HasValue implemented by most of the standard GWT Widgets. In the mean time, I would love to see how GWT application developers use HasValue in their own projects. If you haven't worked with the Incubator before, you'll want to setup the project locally so you can build the latest features. Here's where to start:

Setting up the GWT Incubator project