What does <?> sign mean in ArrayList

Question

In an ArrayList for android such as the simplelistadapter I commonly see ArrayList<?> and in some tutorials I've reviewed the <?> is replaced by some value. But I'm still not quite sure what variable or qualifier this stands for.

Answer 1

It's called Generics.

For example, you can do this:

ArrayList<Integer> list = new ArrayList();
list.add(new Integer(1));
list.add(new Integer(2));
.....

Integer i = (Integer)list.get(1);

Or you can do:

ArrayList<Integer> list = new ArrayList<Integer>();
list.add(new Integer(1));
list.add(new Integer(2));
.....

Integer i = list.get(1);

As you can see, no need for casting.