Question

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Coding to interfaces?

I am reading the Collections tutorial from Java and it strongly recommends that codes referring to Collections be implemented using its interface type and not its actual implementation type. e.g:

Set<String> s = new HashSet<String>();

It says that it will give me flexibility to change implementations should I decide to change it later on.

Set<String> s = new TreeSet<String>();

Other than flexibility, are there any other benefits to implementing a collection using its interface type?

Answer 1

Yes, when using the interface class, you will only have access to the most default methods. These methods guarantee to be intuitive. When using the implementation class, you might see more methods that might confuse you or be abused.

For example: a collection interface will have a method that returns the number of elements, lets say: size(). But the implementation class might also provide a capacity() method that tells you how big the underlying array is.

But as the tutorial tells you, the most important reason is that you can change the implementation without any effort. Changing the implementation might be interesting for performance optimization is very specific cases.