Why are interfaces preferred to abstract classes?

Solution 1:

That interview question reflects a certain belief of the person asking the question. I believe that the person is wrong, and therefore you can go one of two directions.

1. Give them the answer they want.
2. Respectfully disagree.

The answer that they want, well, the other posters have highlighted those incredibly well. Multiple interface inheritance, the inheritance forces the class to make implementation choices, interfaces can be changed easier.

However, if you create a compelling (and correct) argument in your disagreement, then the interviewer might take note. First, highlight the positive things about interfaces, this is a MUST. Secondly, I would say that interfaces are better in many scenarios, but they also lead to code duplication which is a negative thing. If you have a wide array of subclasses which will be doing largely the same implementation, plus extra functionality, then you might want an abstract class. It allows you to have many similar objects with fine grained detail, whereas with only interfaces, you must have many distinct objects with almost duplicate code.

Interfaces have many uses, and there is a compelling reason to believe they are 'better'. However you should always be using the correct tool for the job, and that means that you can't write off abstract classes.

Solution 2:

In general, and this is by no means a "rule" that should be blindly followed, the most flexible arrangement is:

interface
   abstract class
       concrete class 1       
       concrete class 2

The interface is there for a couple of reasons:

• an existing class that already extends something can implement the interface (assuming you have control over the code for the existing class)
• an existing class can be subclasses and the subclass can implement the interface (assuming the existing class is subclassable)

This means that you can take pre-existing classes (or just classes that MUST extend from something else) and have them work with your code.

The abstract class is there to provide all of the common bits for the concrete classes. The abstract class is extended from when you are writing new classes or modifying classes that you want to extend it (assuming they extend from java.lang.Object).

You should always (unless you have a really good reason not to) declare variables (instance, class, local, and method parameters) as the interface.

Solution 3:

You only get one shot at inheritance. If you make an abstract class rather than an interface, someone who inherits your class can't also inherit a different abstract class.