What is the difference between declaring List<Integer> vs ArrayList<Integer>?

Question

List<Integer> mylist = new ArrayList<Integer>();

ArrayList<Integer> mylist2 = new ArrayList<Integer>();

I am wondering what is the actual difference between the above two in java collections API. I am new to java collections API. I know that List is an interface that ArrayList class implements.

Answer 1

The List<Integer> version is the interface type - it only allows you to perform the methods declared by the interface, while the ArrayList<Interger> typed variable allows you to do anything that is declared in the ArrayList<Interger> and its supers. (including the List of course).

However, though it seems "useless" to chose the first - it actually allows you more flexibility - it will help you change your design much easier if you will later decide you want a LinkedList<Interger> (for example) and not an ArrayList<Interger> as the dynamic type.

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Addition:
Of course it doesn't mean you need to automatically chose the List<Integer> version. If you actually need to use the exact type of ArrayList - you should chose it. A good rule of thumb is to start with the interface version, and change it to the ArrayList<Integer> only if you find yourself straggling to get something you would have done very easily with an ArrayList type (or if you find yourself casting to an ArrayList...)

Answer 2

In your statement 1, since you are referring mylist as List<Integer> while it is still ArrayList<Integer>, hence you can use the methods available in the List interface ONLY. This is better statement, if you are using cross class.method functionality.

Also any method accepting List<Integer> can accept any of the implementation classes of List e.g. LinkedList<Integer> or your custom implementation class.

Your second statement, creates and references the object as ArrayList<Integer> ONLY. Some people perefer it when mylist is to be used locally in the method.

Answer 3

Always code to the interface.

(Car analogy alert)

You walk into Hertz Rent-a-Car. You say, I want an IVehicle. They say, "well, what kind of IVehicle?" You don't care. So they give you a DumpTruck. But you don't want a dump truck, you want something compact. You want an ICompact that extends IVehicle. So they give you a motorcycle. "No!" you exclaim, "I want an ICar!" "What kind of ICar?" they ask. "I don't care!" you exclaim. So they give you a FordCar, and you live happily ever after.

Does that clear it up?

Answer 4

First one is called coding to interfaces.

Using the List reference through out your code you can update the concrete implementation to something else like a LinkedList without breaking your client code.