Abstract class

• Abstract classes can have constants, members, method stubs and defined methods.
• Methods and members of an abstract class can be defined with any visibility.
• A child class can only extend a single abstract (or any other) class.
• A child class can define abstract methods with the same or less restrictive visibility.
• Abstract  class  can  have  both  abstract and concrete methods.
• In abstract class keyword ‘abstract’ is mandatory to declare a method as an abstract.
• Abstract class can have  static, final  or static final  variable with any access specifier.

Interface

• Interfaces can only have consts and methods stubs.
• All methods of an interface must be defined as public.
• An interface can extend or a class can implement multiple other interfaces.
• A class implementing an interface must define the methods with the exact same visibility.
• Interface can  have only abstract methods.
• In an interface keyword ‘abstract’ is optional to declare a method as an abstract
• Interface  can  have only static final (constant) variable i.e. by default.