Java Generics Type Casting Necessary?

Question

This is the code from my textbook:

Stack<String>[] a = (Stack<String>[]) new Stack[N];

My questions are:

1. Why is it "new Stack[N]"?

2. Why do you have to do type conversion on the new stack array created? I tried it with just

   Stack<String>[] a = new Stack[N];
   

and it compiled and ran fine. Even after pushing Strings into a and printing the pop method. Also, pushing a int in would immediately give me a compiler error so why is it necessary to type cast it to

Stack<String>[] 

specifically?

Answer 1

You can not create arrays with parameterized type (see Restrictions on Generics). So you can not use ... = new Stack<String>[N]. Using ... = new Stack[N] will work but you will have a warning for unchecked convertion (use -Xlint in javac to see the warning).

So the only proper way is to create the array with a raw parameter and then to apply a checked cast to the wanted type.

Answer 2

1. Not sure what you mean by that question, but if you mean why not new Stack<String>[N] it is because java arrays does not support parameterized array creation.

2. Because of my answer in (1) the created array is not parameterized, but you are assigning it to a parameterized variable. And therefor the optional, but preferred checked cast.