Can someone please explain what the difference between ArrayList<?>, ArrayList and ArrayList<Object> is, and when to use each? Are they all same or does each have some different meaning at the implementation level?
2 Answers
ArrayList<Object> is specifically a list of Objects whereas ArrayList<?> is a list whose concrete type we are unsure of (meaning we can't add anything to the list except null). You would use the latter when the list's type is irrelevant, e.g. when the operation you want to perform does not depend on the type of the list. For instance:
public static boolean isBigEnough(ArrayList<?> list) {
return list.size() > 42;
}
This is all covered in the generics tutorial (see the wildcards section).
Finally, ArrayList with no type parameter is the raw type: the only reason it's even allowed is for backwards compatibility with Java versions under 5, and you should refrain from using it whenever possible.
2 Comments
Prasad Kharkar
this is a new perspective :)
chrips
Very nice. I was using ArrayList in some test projects (as I saw it used in a few tutorials.) The takeaway is, Always use <Object> when you can
ArrayList<?> means "an ArrayList instance containing a type which is to be determined"
ArrayList is the class of an ArrayList
An ArrayList<Object> means an instance of ArrayList containing Object types.
This looks like it could be a good write-up on this (and more): http://docs.oracle.com/javase/tutorial/java/generics/types.html
1 Comment
Victor
I guess the OP is asking that since in JAVA everything extends from Object, what does it mean to have "a type which is to be determined"? Because the list does not accept primitives
Java GenericsObject. Bad things can happen when you use raw types.