Java – Why Generic type can not instantiated

Your example is in some way a special case. In a non trivial program you can typically not determine the class of the object a certain reference points to better than "it is of the type the reference was declared for or a subtype of it".

The whole concept of polymorphism is based on the fact that the concrete class is only known at runtime but not at compile time. This of course means that the compiler must ensure that the methods that are called on a reference will be available on the referenced object at runtime. The only methods for which this holds true are the methods of the class for which the variable was declared (including all inherited methods from the super classes).

EDIT

As @David said in his comment the reason behind this is that typing is done statically by the programmer in java. If java had type inference the typing would be done by the compiler (but still statically) and your example would be absolutely valid. But it could be invalidated by another line doing this ob = new A().

If java were a dynamic typed language the compiler wouldn't care at all (at compile time) what methods you call on an object.

In good software design what you want, or better yet in an idealized world you must, have an interface which communicates intent. Taking away the type parameter of the List expressed that intent clearly. You're telling the caller "I'm only interested in acting on a List and nothing more." This is why the second is better than the first.

As an aside, the reason that the second method requires two function calls is that Java does not have type inference so you are forced to declare an E to capture one of the swapped values.