Question

What are abstract classes? What are interfaces? What is the difference?

Answer 1

Abstract Class:

• Abstract classes are classes that contain one or more abstract methods. An abstract method is a method that is declared, but contains no implementation.
• Abstract classes may not be instantiated, and require subclasses to provide implementations for the abstract methods.
• Abstract Class: abstract class A{}
• Abstract Method: abstract void printStatus();

Interface:

• An interface is a reference type in Java. It is similar to class. It is a collection of abstract methods. A class implements an interface,
• thereby inheriting the abstract methods of the interface.
• Along with abstract methods, an interface may also contain constants, default methods, static methods, and nested types.
• Method bodies exist only for default methods and static methods.
• Writing an interface is similar to writing a class. But a class describes the attributes and behaviors of an object.
• And an interface contains behaviors that a class implements.
• Unless the class that implements the interface is abstract, all the methods of the interface need to be defined in the class.
• You cannot instantiate an interface.An interface does not contain any constructors.All of the methods in an interface are abstract.

Difference between Abstract Class and Interface:

• Abstract class can have abstract and non-abstract methods.
• Interface can have only abstract methods. Since Java 8, it can have default and static methods also.

• Abstract class can have final, non-final, static and non-static variables.
• Interface has only static and final variables.

• Abstract class can provide the implementation of interface.
• Interface can't provide the implementation of abstract class.

• The abstract keyword is used to declare abstract class.
• The interface keyword is used to declare interface.

• Abstract class doesn't support multiple inheritance.
• Interface supports multiple inheritance.

• Abstract class can be extended using keyword “extends”.
• A Java interface can be implemented using keyword “implements”.

• Abstract class can extend only one class or one abstract class at a time.
• Interface can extend any number of interfaces at a time.