Question

This has been bugging me for a while and have yet to find an acceptable answer. Assuming a class which is either a subclass or implements an interface why would I use the Parent class or Interface as the Type i.e.

List list = new ArrayList();
Vehicle car = new car();

In terms of the ArrayList this now only gives me access to the List methods. If I have a method that takes a List as a parameter then I can pass either a List or an ArrayList to it as the ArrayList IS A List. Obviously within the method I can only use the List methods but I can’t see a reason to declare it’s type as List. As far as I can see it just restricts me to the methods I’m allow to use elsewhere in the code.

A scenario where List list = new ArrayList() is better than ArrayList list = new ArrayList() would be much appreciated.

Answer 1

Using the interface or parent type is generally recommended if you only need the functionality of the parent type. The idea is to explicitly document that you don’t really care about the implementation, thus making it easier to swap out the concrete class for a different one later.

A good example are the Java collection classes:

If you always use List, Set etc. instead of e.g. ArrayList, you can later switch from ArrayList to LinkedList if you find that it gives e.g. better performance. To do that, just change the constructors (you don’t even have to change them all, you can mix). The rest of the code still sees an instance of List and continues working.

If you actually used ArrayList explicitly, you’d have to change it everywhere it’s used. If you don’t actually need an ArrayList specifically, there’s nothing to be gained from using it over the interface.

That’s why it’s generally recommended (e.g. in “Effective Java” (J.Bloch), Item 52: “Refer to Objects by their interfaces”.) to only use interfaces if possible.

Also see this related question: Why classes tend to be defined as interface nowadays?