GWT - did you forget to inherit a required module?

In my GWT application, on the client side Java file, I have written a line to print the Locale:

System.out.println(Locale.getDefault());

Here Locale is an instance of java.util.Locale.

I'm getting an exception: 09:31:53.656 [ERROR] [stockwatcher] Line 167 No source code is available for type java.util.Locale; did you forget to inherit a required module?

Surprisingly, in this class I've also used java.util.ArrayList and java.util.Date but I did not get any exception for those classes.

What am i doing wrong here?