53 lines
1.9 KiB
Markdown
53 lines
1.9 KiB
Markdown
---
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title: Abstract Classes in Java
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---
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Let's discuss abstract classes. Before diving into this tutorial it is better that you have understood concepts of classes
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and inheritance.
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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.
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Consider the following example to understand abstract classes:
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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.
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``` java
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abstract class Vehicle
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{
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//variable that is used to declare the no. of wheels in a vehicle
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private int wheels;
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//Variable to define the type of motor used
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private Motor motor;
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//an abstract method that only declares, but does not define the start
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//functionality because each vehicle uses a different starting mechanism
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abstract void start();
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}
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public class Car extends Vehicle
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{
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...
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}
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public class Motorcycle extends Vehicle
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{
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...
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}
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```
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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;
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``` java
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Vehicle newVehicle = new Vehicle(); // Invalid
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Vehicle car = new Car(); // valid
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Vehicle mBike = new Motorcycle(); // valid
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Car carObj = new Car(); // valid
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Motorcycle mBikeObj = new Motorcycle(); // valid
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```
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If the child class doesn't implement the abstract methods of the father, it becomes an abstract class.
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#### More Information
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- [Oracle Java Docs - Abstract Classes](https://docs.oracle.com/javase/specs/jls/se7/html/jls-8.html#jls-8.1.1.1)
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