Effective Java

new E[DEFAULT_INITIAL_CAPACITY]
;
^
As explained in Item 28, you can’t create an array of a non-reifiable type, such
as 
E
. This problem arises every time you write a generic type that is backed by an
array. There are two reasonable ways to solve it. The first solution directly circum-
vents the prohibition on generic array creation: create an array of 
Object
and cast

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CHAPTER 5
GENERICS
132
it to the generic array type. Now in place of an error, the compiler will emit a
warning. This usage is legal, but it’s not (in general) typesafe:
Stack.java:8: warning: [unchecked] unchecked cast
found: Object[], required: E[]
elements =
 (E[]) new Object[DEFAULT_INITIAL_CAPACITY]
;
^
The compiler may not be able to prove that your program is typesafe, but you
can. You must convince yourself that the unchecked cast will not compromise the
type safety of the program. The array in question (
elements
) is stored in a private
field and never returned to the client or passed to any other method. The only
elements stored in the array are those passed to the 
push
method, which are of
type 
E
, so the unchecked cast can do no harm.
Once you’ve proved that an unchecked cast is safe, suppress the warning in as
narrow a scope as possible (Item 27). In this case, the constructor contains only the
unchecked array creation, so it’s appropriate to suppress the warning in the entire
constructor. With the addition of an annotation to do this, 
Stack
compiles cleanly,
and you can use it without explicit casts or fear of a 
ClassCastException
:
// The elements array will contain only E instances from push(E).
// This is sufficient to ensure type safety, but the runtime