Java Abstract Classes

Let’s discuss abstract classes. Before diving into this tutorial it is better that you have understood concepts of classes
and inheritance.

Abstract classes are classes that can be subclassed (i.e. extended) but cannot be instantiated. You can think of them as a class version of interfaces, or as an interface with actual code attached to the methods.

Consider the following example to understand abstract classes:

You have a class Vehicle which defines certain basic functionality (methods) and certain components (object variables) that a machinery should have, to be classified as a vehicle. You cannot create an object of Vehicle because a vehicle in itself is an abstract concept. You can however extend the functionality of the vehicle class to create a Car or a Motorcycle.

abstract class Vehicle { //variable that is used to declare the no. of wheels in a vehicle private int wheels; //Variable to define the type of motor used private Motor motor; //an abstract method that only declares, but does not define the start //functionality because each vehicle uses a different starting mechanism abstract void start(); } public class Car extends Vehicle { ... } public class Motorcycle extends Vehicle { ... }

You cannot create an object of Vehicle class anywhere in your program. You can however, extend the abstract vehicle class and create objects of the child classes;

Vehicle newVehicle = new Vehicle(); // Invalid Vehicle car = new Car(); // valid Vehicle mBike = new Motorcycle(); // valid Car carObj = new Car(); // valid Motorcycle mBikeObj = new Motorcycle(); // valid

If the child class doesn’t implement the abstract methods of the parent, it becomes an abstract class. An abstract class can also contain constructors. A constructor of abstract class is called when an instance of a inherited class is created.

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