Language Support

• Intrinsic types
  byte, char, short, int, long, float, double, Object, String, and arrays are supported. However, there is no 64-bit integral type in JavaScript, so variables of type long are mapped onto JavaScript double-precision floating point values. To ensure maximum consistency between hosted mode and web mode, we recommend that you use int variables.
• Exceptions
  try, catch, finally and user-defined exceptions are supported as normal, although Throwable.getStackTrace() is not supported for web mode. See Throwable for additional details.
• Assertions
  The GWT compiler parses Java assert statements, but it does not emit code JavaScript code for them.
• Multithreading and Synchronization
  JavaScript interpreters are single-threaded, so while GWT silently accepts the synchronized keyword, it has no real effect. Synchronization-related library methods are not available, including Object.wait(), Object.notify(), and Object.notifyAll()
• Reflection
  For maximum efficiency, GWT compiles your Java source into a monolithic script, and does not support subsequent dynamic loading of classes. This and other optimizations preclude general support for reflection. It is possible to query an object for its class name using GWT.getTypeName(Object).
• Finalization
  JavaScript does not support object finalization during garbage collection, so GWT isn't able to be honor Java finalizers in web mode.
• Strict Floating-Point
  The Java language specification precisely defines floating-point support, including single-precision and double-precision numbers as well as the strictfp keyword. GWT does not support the strictfp keyword and can't ensure any particular degree of floating-point precision in translated code, so you may want to avoid calculations in client-side code that require a guaranteed level of floating-point precision.