In the last section, we saw what is abstract class and interface and now let's see the difference between interface and abstract class in Java.
1) First and the major difference between abstract class and an interface is that an abstract class is a class while the interface is an interface, means by extending the abstract class you can not extend another class because Java does not support multiple inheritances but you can implement multiple inheritance in Java.
2) The second difference between interface and abstract class in Java is that you can not create a non-abstract method in an interface, every method in an interface is by default abstract, but you can create a non-abstract method in abstract class. Even a class which doesn't contain any abstract method can be made abstract by using the abstract keyword.
3) The third difference between abstract class vs interface in Java is that interface is better suited for Type declaration and abstract class is more suited for code reuse and evolution perspective. The Effective Java has one item dedicated to explaining about why you should be using interface for type declaration. You should check that out as well.
4) The fourth difference between abstract class and interface in Java is that abstract class are slightly faster than interface because interface involves a search before calling any overridden method in Java. This is not a significant difference in most of the cases but if you are writing a time critical application then you may not want to leave any stone unturned.
5) Another notable difference between interface and abstract class is that when you add a new method in existing interface it breaks all its implementation and you need to provide an implementation in all clients which is not good. By using an abstract class you can provide a default implementation for a new method in the superclass without breaking existing clients.
That's all on the difference between abstract class and interface in Java, I will add more differences whenever I learn new things. As I said, in the first paragraph, after the introduction of default method in Java 8 (See Java 8 in Action) and the provision that you can have both static and default method inside an interface, the difference between abstract class and interface has become blur. Earlier, the interface only contains contract no implementation but now they can.
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