Collection vs ArrayList

Question

Collection<string> collection = new ArrayList<string>();

I noticed the above code is in fact the same as ArrayList<string> collection = new ArrayList<string>();

Would it mean I can have any name to replace the first Collection/ArrayList like

anyname<string> collection = new ArrayList<string>();

Thanks!

Answer 1

You are able to do that because ArrayList implements Collection

Class ArrayList<E>

java.lang.Object
  java.util.AbstractCollection<E>
    java.util.AbstractList<E>
      java.util.ArrayList<E>

All Implemented Interfaces:
Serializable, Cloneable, Iterable<E>, Collection<E>, List<E>, RandomAccess

----> http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html

Answer 2

ArrayList implements the interface Collection so collection can be cast to a Collection

see http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html

and this tutorial on using Interfaces http://docs.oracle.com/javase/tutorial/java/IandI/interfaceAsType.html

Answer 3

Collection is a interface, while ArrayList is one of the implementations of Collection.

Because ArrayList --> List --> Collection, so you can declare an ArrayList instance as Collection, List and ArrayList. Other types won't work.

Answer 4

why-define-a-java-object-using-interface-e-g-map-rather-than-implementation I hope this will help you understand , why use an interface type rather than concrete implementation while initializing an object and storing it's reference to a type variable.

Answer 5

With an ArrayList (that implements Collection) you are allowed to do this .