4

Is new Stack<?>[N] equivalent to new Stack[N] for a generic data type Stack<Item>?

EDIT: While I understand that mixing generic types and arrays should best be avoided and that more robust solutions exist, my query still stands: widely recognized textbooks such as Algorithms, 4th Edition by Kevin Wayne and Robert Sedgewick (pg. 158) suggest using constructs like the following:

Stack<String>[] a = (Stack<String>[]) new Stack[N];
Stuart Marks
  • 127,867
  • 37
  • 205
  • 259
Fabian
  • 191
  • 1
  • 9
  • 3
    Fast non-answer: don't do it. Generics and arrays should not be used together. See https://docs.oracle.com/javase/tutorial/java/generics/restrictions.html#createArrays – GhostCat Mar 23 '15 at 11:39
  • @EddyG, I was hoping to create a stack of strings with the following code: Stack[] a = (Stack[]) new Stack[N]; It produces a warning but is this always considered bad practice? – Fabian Mar 23 '15 at 11:49
  • 1
    Again; all you need to know is in the documentation from Oracle. Instead of using arrays, just go for collections types, like ArrayList. Then it is straight forward to create an "ArrayList>" – GhostCat Mar 23 '15 at 11:51

2 Answers2

2

[ Turning my comment into a real answer ]

1) Mixing arrays and generic types should be avoided.

2) Instead, one can use Java Collection types, for example ArrayList when the need comes up to handle multiple "generic" objects, like in

ArrayList<Stack<String>> theStacks = new ArrayList<>();
GhostCat
  • 137,827
  • 25
  • 176
  • 248
2

Yes, they are equivalent, except for the return type (one is Stack[], the other is Stack<?>[]).

Note that Stack[] can be assigned to Stack<String>[] without an explicit cast (you will just get an unchecked conversion warning), whereas it is an error to assign a Stack<?>[] to Stack<String>[] without a cast.

new Stack[N] uses a raw type which is not recommended in new code.

newacct
  • 119,665
  • 29
  • 163
  • 224