Centralising code with inheritance

Introduction to Object-Oriented Programming in Java

Sani Yusuf

Lead Software Engineering Content Developer

Working with multiple classes

  // Toyota class
  class Toyota {
      public String model;
      public String color;
      public String licensePlate;  
  }

  // Honda class
  class Honda {
      public String model;
      public String color;
      public String licensePlate;       
  }

  // Mercedes class
  class Mercedes {
      public String model;
      public String color;
      public String licensePlate;        
  }

Duplication is common when working with classes

The power of inheritance

Inheritance solved the problem of duplicate code
All shared code can be in one place
Classes can then inherit using the extends keyword

  // Car class
  class Car {
     // All public properties 
     // are inherited
     public String model;
     public String color;
     public String licensePlate;  
 }

  // Toyota inheriting from Car class
  class Toyota extends Car {

  }

  // Honda inheriting from Car class
  class Honda extends Car {

  }

  // Mercedes inheriting from Car class
  class Mercedes extends Car {

  }

Introducing base class

The Base class is the class that contains shared code
The Car class is an example of a base class


  class Car {
     public String model;
     public String color;
     public String licensePlate;  
 }

Inheriting with subclasses

The subclass is the class that inherits the base class
The Toyota class is an example of a subclass
A subclass inherits all public properties & methods of the base class
The super() method calls the constructor if the base class

  // Car Base Class
  class Car {
     public String model;
     public String color;
     public String licensePlate;  
 }

 // Toyota subclass
  class Toyota extends Car {


     // Toyota constructor
     public Toyota(){ 
       // super() calls 
       // baseclass constructor (Car)
       super();
     }
  }

Creating inheritance instances

Properties from the base class can be used by object instances


  // Car class  
  class Car {
    public String color;   

    // Car constructor
    public Car(String color){
      this.color = color;
    }    
  }


  // Toyota subclass
  class Toyota extends Car {
     public Toyota(){
       super();
     }
  }

  // Main class 
  public class Main {
    public static void main(String[] args) {
        Toyota myToyota = new Toyota("Brown");
        // "color" property inherited from "Car"
        System.out.println(
          myCar.color); // Brown
    }
  }

Let's practice!

Introduction to Object-Oriented Programming in Java