java null cast

摘自 Java Language Specification
The null reference can always be cast to any reference type.In practice, the programmer can ignore the null type and just pretend that null is merely a special literal that can be of any reference type.

Object obj = null;
String str = (String) obj;
Integer a = (Integer) obj;
//int b = a;
//NullPointerException

还有一种用法

doSomething( (MyClass) null );

If doSomething is overloaded, you need to cast the null explicitly to MyClass so the right overload is chosen:
详细见:
http://stackoverflow.com/questions/315846/why-null-cast
但是并没有所说的warning