Abstract keyword

A. Abstract classes

1. Abstract classes cannot be instantiated.

2. The class extending the abstract class can create an instance of that abstract class.

3. If a class has even one abstract method it should be declared as an abstract class.

4. A abstract class may or may not have any abstract method.

5. Abstract class defines what an abstract method does not how it is implemented.

6. It is necessary to include all the method of abstract class in the first concrete class failing to do so

the incomplete class should also be declared as an abstract class.

7. We don't create a static abstract method in an abstract class as static methods cannot be overridden by subclasses.

B. Abstract Method

1. Abstract method does not have the input argument or the implementation part, just the prototype.

2. If a abstract method is present the class must also be declared as abstract.

Interface Keyword

1. All variables in interface are public static final.

2. All methods are public abstract.

3. The next concrete class implementing the interface should implement all its abstract method (simply all method)

or it itself should be declared as an abstract class.

4. An Interface is a 100% abstract class as each of its methods are abstract.

5. An Interface can implement a class or extend an Interface.

Abstract class vs Interface

1. Abstract class can have method with implementations while Interface cannot.

2. Abstract class can have different access modifies while Interface cannot it can only public.

3. A class can extend only one abstract class but implement many Interfaces

From the Oracle documentation

Consider using abstract classes if :

You want to share code among several closely related classes.

You expect that classes that extend your abstract class have many common methods or fields, or require access modifiers other than public (such as protected and private).

You want to declare non-static or non-final fields.

Consider using interfaces if :

You expect that unrelated classes would implement your interface. For example,many unrelated objects can implement Serializable interface.

You want to specify the behaviour of a particular data type, but not concerned about who implements its behaviour.

You want to take advantage of multiple inheritance of type.

abstract class establishes "is a" relation with concrete classes. interface provides "has a" capability for classes.

Java 8 indroduced Default methods in Interface which allow the interfaces to have methods with implementation without affecting

the classes that implement the interface.