Use of interface concept

Question

Hi everyone !!!
Can anyone explain me what are the uses to the interface and why do we need to use interface.
I know one advantage is java doesn't support multiple inheritance so we need to use interface.
By using interface what can we do ? But i dont find advantages in that
can you explain me pls ....

Answer 1

One of the most popular uses of interfaces comes out of the Collections frame works.
For example, java.util.List, java.util.Map, and java.util.Set.
ArrayList is an implementation of the the interface List. By abstracting ArrayList into the interface, I can now change what type of List I am working with, and my code doesn't care....
For example, I have a method that creates an ArrayList that contains all the users on the server at a particular time. As the coder, I know the method is going to create an ArrayList. Now I have a choice, I can make the method's return type an ArrayList, or I can use the interface name and return a List:
public ArrayList getCurrentUsers() {
  ArrayList theList = new ArrayList();
  return theList;
-or-
public List getCurrentUsers() {
  List theList = new ArrayList();
  return theList;
} Both codes would be functionally equivalent. Any code that calls the first method may (but does not need to) do this:
ArrayList currentUsers = getCurrentUsers();Whereas any code that runs the second method would need to do this:
List currentUsers = getCurrentUsers();What's the difference? Well, let's say you decide to change your method to use a LinkedList instead of an ArrayList. If you employed method 1, you now have to make sure you change all the code that accessed the method is changed as well. But, since LinkedList is another implementation of List, if you employ method 2, you can be sure that none of the code that uses your method will break, because they only work through an interface wich both ArrayList and LinkedList share in comon.
So Interfaces are great for defining how a class or object looks, but not necessarily how it works. If you use the List interface to look at ArrayList or LinkedList then both of these classes will look the same, they are both sure to have the same methods defined in the List interface. However, they both will act differently, based on the underlying implementation.
In general terms, it is a level of abstraction that allows the implementer be more flexible with what he is working on, by ensuring a common means of accessing the object, or a set of rules that users must use when accessing the object. In the mean time, the coder is free to make changes, switch implementations, without affecting others.