array of stacks

Question

Is it possible to create an array of stacks without having to cast the stacks as they come out? Eclipse gives me a warning about not being able to make a generic array of Stack when I do something like this:

Stack<Card>[] cards = new Stack<Card>[52];

if you indent your code snippets with four spaces it'll format correctly — Cogsy, May 08 '09 at 06:17
Similar question with some good answers: http://stackoverflow.com/questions/217065/cannot-create-an-array-of-linkedlists-in-java — Andy White, May 08 '09 at 06:24
I think it's an error and not a warning, since "new Stack[52]" is not allowed in Java - a bit hidden here: http://java.sun.com/docs/books/jls/third_edition/html/arrays.html#10.3 — user85421, May 08 '09 at 07:46

score 9 · Answer 1 · answered May 08 '09 at 06:31

Joshua Bloch does an excellent job of describing this problem in Effective Java, Second Edition. Check out the relevant section on Google Book Search.

The advice he offers is to prefer lists to arrays. Your code might then look something like:

List<Stack<Card>> cards = new ArrayList<Stack<Card>>();

score 2 · Answer 2 · answered May 08 '09 at 06:17

2

You can do the following, though this gives you a compiler "unchecked" warning.

Stack<Card>[] cards = (Stack<Card>[]) new Stack[52];

answered May 08 '09 at 06:17

Peter Lawrey

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Wow, thanks for remembering me that I hate generics :) To be honest making code ugly only that your stupid IDE does not complain is not a good reason to do. – Norbert Hartl May 08 '09 at 06:24
Thanks for the answer, but I would like to not have to cast them as they come out. Have to follow a tight spec that doesn't allow them to be casted out. – May 08 '09 at 06:29
@Nobert Hartl: it's not the IDE, it's the Java Language Specification! – user85421 May 08 '09 at 07:49

score 2 · Answer 3 · 2009-05-08T09:25:58.600

2

Stack<Card>[] decks = new Stack[9];       // Declare
Card c = decks[5].pop();                  // This compiles - java 'knows' the type
Integer i = decks[4].pop();               // This will not compile

edited May 08 '09 at 09:25

answered May 08 '09 at 09:09

score 1 · Answer 4 · answered May 08 '09 at 06:17

1

Why do you use arrays anyway ?

It is a low level programming structure.

Using List or Set instead (eg org.apache.commons.collections.list.LazyList) if you don't want to bother with innitialization.

Or at least

Arrays.asList(new Stack[52]) to wrap an array into a list.

I couldnt reproduce jour error anywany .. :( perchaps it's because a different warning/errorlevel set.

answered May 08 '09 at 06:17

Łukasz Bownik

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Using an array I can limit the number of stacks within it, as the list is an interface, all implementations of the list are not exactly what I need it for. – May 08 '09 at 06:23
see org.apache.commons.collections.list.FixedSizeList – KitsuneYMG May 08 '09 at 21:59

score 0 · Answer 5 · answered May 08 '09 at 06:13

0

Well, the array does not need to be a generic because he is always defined as this. Why do you think you have to cast? I think that eclipse is somewhat confused here.

answered May 08 '09 at 06:13

Norbert Hartl

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eclipse is not confused: "...It is a compile-time error if the element type is not a reifiable type..." see http://java.sun.com/docs/books/jls/third_edition/html/arrays.html#10.3 – user85421 May 08 '09 at 07:48

score 0 · Answer 6 · answered May 08 '09 at 06:22

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It's to do with type erasure. Basically the generic types only exist at compile time and have no presence at run time

Have at look at this forum post for a better explanation.

answered May 08 '09 at 06:22

Cogsy

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array of stacks

6 Answers6