You end up with a lot of problems when you buy a used car. Same can be said about reading something about computer programming. You read something and you figure out that you need to know many things that your read talks about. That is a lot of problems!
I was reading about design patterns and abstract classes came up. I thought that I should read more about abstract classes. And as I read about them, I thought why, then, do we need interfaces. So this article basically contains links that I found useful about abstract classes and interfaces.
A fair comparison between abstract classes and interfaces is in the following tutorial.
Abstract Methods and Classes (The Java™ Tutorials > Learning the Java Language > Interfaces and Inheritance)
This beginner Java tutorial describes fundamentals of programming in the Java programming language
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