List can be instantiated by any class implementing the interface.By this way,Java provides us polymorphic behaviour.See the example below:
List<String> list = new ArrayList<String>();
Instead of instantiating an ArrayList directly,I am using a List to refer to ArrayList object so that we are using only the List interface methods and do not care about its actual implementation.
Examples of classes implementing List are ArrayList,LinkedList,Vector.You probably want to create a List depending upon your requirements.
Example:- a LinkedList is more useful when you hve to do a number of inertion or deletions .Arraylist is more performance intensive as it is backed by a fixed size array and array contents have to be changed by moving or regrowing the array.
Again,using a List we can simply change our object instantiation without changing any code further in your programs.
Suppose we are using ArrayList<String> value = new ArrayList<String>();
we may use a specific method of ArrrayList and out code will not be robust
By using List<String> value = new ArrayList<String>();
we are making sure we are using only List interface methods..and if we want to change it to a LinkedList we simply have to change the code :
List<String> value = new ArrayList<String>();
------ your code uses List interface methods.....
value = new LinkedList<String>();
-----your code still uses List interface methods and we do not have to change anything---- and we dont have to change anything in our code further
By the way a LinkedList also works a Deque which obviously also you cannot instantiate as it is also an interface