Re: collection of a class

Lew <>
Sat, 31 Mar 2007 11:43:54 -0400
"ros" <> wrote:

I am working on this exercise where I have to add a method to a class
that returns 'collections' of another class.
Can anybody explain how I can do it?

Since you admit that it's homework, I am happy to provide hints.

First, let's be very precise about terminology. Did you want collections (no
quotes needed) of classes, or instances of classes (i.e., objects)? The first
cannot be done, but you can make collections of Class objects. All Java
collections are collections of objects.

Second, you should know that the collections classes and just about everything
else in the Java world is heavily documented, and where to find the
documentation. The Sun tutorials


and API documentation


are two of your best resources, and of course, GIYF.

For collections, both the tutorials and the API docs give a good introduction:


For how to return something from a method, see


Now let us try something - start with a collection of some type of object:

   Collection <SomeType> coll = new HashSet <SomeType> ();

This line or one similar to it might appear somewhere inside the method you
are writing.

Somewhere further down in that method you will have a line something like

   coll.add( someTypeThing );

and further down still,

   return coll;

You will have written the method signature to signal that the return type is

   Collection <SomeType>
(it's in the tutorial how to do that)

and Bob's your uncle.

-- Lew

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