Java

ลำดับตอนที่ #16 : Java V - Class Inheritance

Class Inheritance

Duplicate Code in Classes

          As shown in the examples below, the Car and Bicycle classes have duplicate code, which we should avoid in order to make it easier to edit and revise programs. Both of these classes are "vehicles". The duplicate part indicates properties and actions common to all vehicles in general, rather than properties unique to cars or bicycles.

     

Inheriting Class Content

          Object-oriented programing enables a class to inherit the content of another class. We can remove the duplicate code by combining the overlapping fields and methods to create a Vehicle class. Then the Car and Bicycle classes can inherit from that class. Let's learn how to implement this in the following section.

Inheritance

          When a class inherits the fields and methods of another class it's called inheritance. By using original fields and methods too, we can customize the new (child) class. A class from which content is inherited is called a superclass, and the child class that inherits content is called a subclass.

extends: Method Inheritance

          We use extends when defining a subclass using inheritance. A subclass is defined as follows: class SubclassName extends SuperclassName. extends means "to expand" — inheritance is used to create a subclass with expanded features based on its superclass.

Superclass Methods

Calling Superclass Methods

          The Car class (subclass) inherits the fields and methods of the Vehicle class (superclass). Therefore, although nothing is defined in the Car class, we can call an instance method of the Vehicle class for an instance of the Car class.

Customizing Subclasses

Subclass Fields and Methods

          We can customize a subclass by incorporating original fields and methods that aren't in the superclass. The fuel field is a unique instance field for the Car class. Let's declare the fuel field and define its getter method in the Car class. Here, we will also define the charge method created in Dojo II.

Note about Calling Method

          An instance of a subclass can call not only its own methods but those of its superclass as well. On the other hand, an instance of a superclass can't call methods of a subclass. Since class inheritance is unilateral, if we call a method of a subclass from a superclass, we get an error.

Structure of Calling Methods

          The figure below shows how a method gets called from an instance of a subclass. It will first look for the method in the subclass, and if it's defined in the subclass, it'll be called from the subclass. If it's not defined in the subclass, it'll look for the method in the superclass.

 

Method Overriding

Customizing Methods

          Let's consider a situation where we want to customize a superclass method inside the subclass that inherited it. For example, when calling the printData method for an instance of the Car class, we also want to display the amount of fuel, as shown in the example below.

Overriding

          We can override the content of a method inherited from a superclass by defining another method of the same name in the subclass. This is called overriding. If we define the printData method in the Car class (subclass), it overrides the printData inherited from the Vehicle class.

Structure of Overriding

          Although the superclass and the subclass have methods with the same name, the program first looks at the subclass, and as a result, the content of the method is overwritten inside the subclass. Here, the program calls the printData method defined in the Car class.

Access to Private Field

          Fields other than fuel such as name, color, and distance are declared as private fields in the Vehicle class, and external classes like Car can't directly access them (= encapsulation). When you want to allow external access to private fields, use a getter method.

     

super

Duplicate Code in Methods

          The printData method we overrode in the previous section contains duplicate code. Except for displaying the fuel amount, the code in the printData method in the Car class is the same as in the Vehicle class. Let's reduce redundancy by removing the duplicate code.

     

Calling Superclass Methods

          We can call an instance method of a superclass from an instance of a subclass as follows: super.methodName(). By calling the printData method of the Vehicle class from the Car class, we can remove all the code except for the part that displays the amount of fuel.