Thread: How to use Generics (1 generic anyway)

Generic classes like

Vector<> , ArrayLIst<>, Set<> , HashSet<>, Collection<>, LinkedList<>, TreeSet<>, and Tree<>

all are generic. (Ones like Map<><>, HashMap<><> , and TreeMap<><> I don't know much about other than that they use tables like Hash Tables, which I really don't know much about yet. Sorry.)

These store Object and its subclasses. They don't store primitives but do store

Integer, Character, Double, Float, Long, Short, Byte, Boolean, String, and other types like that.

Unlike arrays, these guys can be expanded or decreased in size.

Kind of like arrays, these classes can also store other Generic, or even of the same type, inside of them.

ArrayLists, LinkedLists, and Vectors have methods to get the data at them at a specific index.

Like arrays, data for all these Generics start with index 0.

Classes like Set and HashSet and TreeSet don't have a way of directly accessing the data in them, except maybe printing out all the data in a toString(). However, using an Iterator, you may be able to do this somehow...though I'm not fully sure how exactly as I haven't used them much myself.

TreeSets and Trees like binary search trees require the generic parameter (Usually I've use a T) to implement comparable.

This is done using

ClassName<T extends Comparable>

For some reason you have to use extends for both extending an implementing.

Also,

ClassName<T> extends Comparable is the class in general, not the parameter being made to have to implement Comparable..

You can extend more than one as long as only 1 at most is a class.

There's also the notoation

<?T?> but I don't know much about that.

These generics in general are usually set up in some format like this

GenericClass<ClassName> gc = new GenericClass<ClassName>(params);

Originally Posted by helloworld922

Is there a question associated with this?

My thoughts exactly. I would hope this isn't supposed to be some sort of how to...because not only is it difficult to follow, but some of this information is incorrect.

What does the <?T?> do?

It was originally going to be a how to(What's incorrect? If I was wrong anywhere...I'd like to know.)

Sets don't have a get() method.

1.) How would you get the stuff at each index then?

2.) What are double Generics used for and how do they work?

3.) What info was incorrect? (Maybe the stuff about Sets...but by directly access...I meant something like a get method, i.e., not having to use an Iterator...whatever that is. Learned a bit about it but can't really recall much.