Difference between Collection and Arraylist in Java?

Question

Possible Duplicate:
What is the benefit of polymorphism using Collection interface to create ArrayList object?

 ArrayList al=new ArrayList();
 Collection c=new ArrayList();

What is difference between object al and c? Are both of them are same or what?

Answer 1

The Collections API is a set of classes and interfaces that support operations on collections of objects.

Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap. Example of interfaces: Collection, Set, List and Map.

Whereas, ArrayList: It is re-sizable array implementation. Belongs to 'List' group in collection. It permits all elements, including null. It is not thread -safe.

Collections: It implements Polymorphic algorithms which operate on collections.

Collection: It is the root interface in the collection hierarchy.

The following interfaces (collection types) extends the Collection interface:

• List
• Set
• SortedSet
• NavigableSet
• Queue
• Deque

Java does not come with a usable implementation of the Collection interface, so you will have to use one of the listed subtypes. The Collection interface just defines a set of methods (behaviour) that each of these Collection subtypes share. This makes it possible ignore what specific type of Collection you are using, and just treat it as a Collection. This is standard inheritance, so there is nothing magical about, but it can still be a nice feature from time to time.

Answer 2

The second is coding to interfaces. It allows the ArrayList to be swapped for another Collection (e.g. Vector or TreeSet) without any side effects.

Answer 3

Same object is created, but reference is different.

So in second case you can work with your ArrayList only as if it is just Collection, unless casting.

Answer 4

Collection is an Interface. Look at the methods.

List interface has methods to access by index. List also extends Collection interface.

ArrayList is a concrete implementation that implements List interface. ArrayList

What you are doing is some abstraction.

If you do :

Collection foo = new ArrayList();

you wont have access to List interface methods. such as accessing with index.

Answer 5

In al you are blocked to use only arraylists. You can't convert/cast anything but for arraylist.

In c you can convert/cast any class which implements the Collection interface.