Java generics and array initialization

Question

What\'s the explanation for the following:

public class GenericsTest {
    //statement 1
    public ArrayList[] lists;

    public GenericsTes         


        

Answer 1

There seems to be obscure cases where you could inadvertently cause a ClassCastException as explained here http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf (section 7.3)

an intersting discussion on this topic could be found here http://courses.csail.mit.edu/6.170/old-www/2006-Spring/forum/index.php%3Ftopic=324.msg1131.html

Answer 2

In this case, I would avoid using arrays for just this reason. The declaration of "lists" in your original code could be

List<List<Integer>> lists = new ArrayList<List<Integer>>(4);
for(int i = 0; i < 4; i++) lists.add(null); // or add an empty ArrayList<Integer>

(you should use the interface rather than the implementation in variable declarations)

Instead of array [] syntax, you would use get() or set(). Other than that, it's equivalent.

Answer 3

It's because you can't create, but you can use them:

public class GenericsTest {
    //statement 1
    public ArrayList<Integer>[] lists;

    public GenericsTest()
    {
        //statement 2
        lists = new ArrayList[4];
        //statement 3
        lists[0].add(new Integer(0));
        //statement 4
        lists[0].add(new String(""));
    }
}

Statement 3 is possible, statement 4 will lead to a compiler error.

Answer 4

So the actual question is: Why is there no error for declaring a generic array? ?

You will always get an error at the point you do something erroneous. Adding an error where there isn't technically a problem just adds to a clutter (although an editor might want to point that out to you).

In some circumstances you may want to bend the rules a bit with an unchecked cast. There's no need to force the code to be littered with more warning suppressions than necessary (other than to point out the folly).