Java: Difference between an Interface & an Abstract Class

Ever since when I appeared for my first interview during campus recruitment in my College (That was the year 2007 when I was in my 3rd year of Engineering!) to this day, when I am over 3 years experienced professional, this topic has always been indispensable & by far the most favorites among the interviewers taking a technical interview for the position of Software Engineers.

So I thought to share the way I handled this concept:

Difference Between Interface and Abstract Class

1. Main difference is methods of a Java interface are implicitly abstract and cannot have implementations. A Java abstract class can have instance methods that implements a default behavior.
2. Variables declared in a Java interface is by default static final. An  abstract class may contain non-final variables.
3. Members of a Java interface are public by default. A Java abstract class can have the usual flavors of class members like private, protected, etc..
4. Java interface should be implemented using keyword “implements”; A Java abstract class should be extended using keyword “extends”.
5. An interface can extend another Java interface only, an abstract class can extend another Java class and implement multiple Java interfaces.
6. A Java class can implement multiple interfaces but it can extend only one abstract class.
7. Interface is absolutely abstract and cannot be instantiated; A Java abstract class also cannot be instantiated, but can be invoked if a main() exists.
8. In comparison with java abstract classes, java interfaces are slow as it requires extra indirection.

Source - Books.