What is the difference following statement between?

List myList = new ArrayList(); and 

ArrayList myList = new ArrayList();

Above declarations look exactly the same in a short program, but if you go on to use 

myList 100 times in your program you can start to see a difference. The first 

declaration ensures that you only call methods on myList that are defined by the 

List interface (so no ArrayList specific methods).

If you've programmed to the interface this way,

later on you can decide that you really need

List myList = new TreeList();
and you only have to change your code in that one spot. 

You already know that the rest of your code doesn't do anything 

that will be broken by changing the implementation because 

you programmed to the interface.