Abstract Class vs Interface Demo Example

Interface and Abstract Class are used for abstraction.

Keyword:

An interface is preceded with keyword interface.
An Abstract class is preceded with keyword abstract.

Access modifier:

Data members in inteface are always public.
Data members in abstract class can be private, protected, etc.

Accessibility:

Data members in inteface are always static.
Data members in abstract class can be static and non-static.

Type of Variable:

Data members in inteface are always final.
Data members in abstract class can be final and non-final.

Methods:

Methods in interface are by default abstract and public, java 8 can have body to the methods.
Abstract class can have both abstract and non-abstract methods.

Inheritance vs Abstraction:

A class implements an interface.
A class extends abstract class.

Multiple implementations:

A class can implement more than one interface.
A class cannot extend more than one class.

#My Notes:

Interfaces (100% abstraction)

To be used needs to use impplements keyword.
All variables are public static final
Can only have abstract methods

Abstract Class

To be used needs to use extends keyword.
Can have abstract methdos (without body) and non-abstract mehtods (with body)
Abstract class can not be initialized.

Usage:

If we know the implementations we can use Abstract Class, if we don't know the implementation it's better to use and interface.