Re: how do I merg two collections?

Lew <>
Tue, 6 Jan 2015 15:38:18 -0800 (PST)
Eric Sosman wrote:

BOB wrote:


It's been quite a while since i programmed in Java...
I'm a bit confused by the differences between a collection and a set.


It looks to me like they are just subclasses of a common baseclass and provide a different interface to it.


  Collection<String> c = new ArrayList<String>();

This is how I instantiated the collection.. but I assume I could have said
Set<String> c = new ArrayList<String>();

Nope. You cannot assign incompatible types.

An 'ArrayList' is not a 'Set', it's a 'List', as the name implies.


and then I would be able handle the array of strings as a set rather than a collection

Am I getting this?


     Sort of -- but only sort of, and not completely.

He's being generous.

     First, Collection is not a class at all, but an interface.
Remember interfaces? An interface specifies a bunch of methods, and
every class that "implements" the interface promises to implement all
those methods. (Interfaces can do a few other things, too, but their
main feature is that they specify methods that their implementing
classes must provide.) The Collection interface specifies methods like
contains() and iterator() and size(), and every class that wants to be
a Collection must provide all those methods. ArrayList and Vector and
HashSet and LinkedList and many others implement Collection.

Interfaces and classes are fundamental to understanding Java and it's type-based programming. It's quite close to the beginning of the Tutorials:

     Second, Set is also an interface. It specifies methods like
removeAll() and retainAll(), and all classes that implement Set must
provide these methods. HashSet and EnumSet and TreeSet and various
other classes implement the Set interface.

Note that 'ArrayList' does not implement 'Set'.

     Third, Set is a sub-interface of Collection, meaning that every
class that implements Set must also implement everything Collection
requires. A class cannot be a Set without also being a Collection:
HashSet provides all the methods of Set *and* all those of Collection.
The reverse, however, is not true: Plenty of classes implement
Collection without implementing Set: ArrayList is a Collection but
not a Set, Stack is a Collection but not a Set, and so on.


     Set<String> c = new ArrayList<String>();

The compiler will scold you for this. Why? Because the ArrayList
class does *not* implement the Set interface; it is a Collection,
but it is not a Set.

     It seems to me that in the "quite a while" since you last used
Java you've forgotten large chunks of what you must have once known.
Perhaps it's time to re-open an old textbook (or a new one) and
start afresh at Page 1, not at Page 100. This stuff is pretty basic,
and if you've forgotten the basics, well, ...


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