The presence of this annotation on a type indicates that the type may be used with the Google Web Toolkit (GWT). When applied to a method, the return type of the method is GWT compatible. It's useful to indicate that an instance created by factory methods has a GWT serializable type. In the following example,

@GwtCompatible
class Lists {
  ...
  @GwtCompatible(serializable = true)
  static <E> List<E> newArrayList(E... elements) {
    ...
  }
}

The return value of Lists.newArrayList(E[]) has GWT serializable type. It is also useful in specifying contracts of interface methods. In the following example,

@GwtCompatible
interface ListFactory {
  ...
  @GwtCompatible(serializable = true)
  <E> List<E> newArrayList(E... elements);
}

The newArrayList(E[]) method of all implementations of ListFactory is expected to return a value with a GWT serializable type.

Note that a GwtCompatible type may have some GwtIncompatible methods.