Java

ลำดับตอนที่ #18 : Java V - Associating Vehicle and Person

Class type Fields

Defining Vehicle Owners

          Now let's learn a bit about associations. Using the Person class from Java IV, let's make a program in which a person can own a vehicle.

Class Type Fields

          We can create a class instance as a field by declaring a variable of class type. Let's create an instance field called owner of type Person like in the example below.

Getter and Setter

          Next let's define getter and setter methods. We'll define them in the same way that we learned previously, but note that the getter's return value and the setter's parameter are class types.

Return Value of Instance

          When we call the getOwner method with an instance of the Car/ Bicycle class, the return value will be the value of the owner field, that is, an instance of the Person class. As shown below on the left, we can call the printData method (an instance method of the Person class) right after the getOwner method.

     

The buy Method

Defining the buy Method

          Let's define the buy method so that a Person class instance can purchase a vehicle. Each vehicle is an instance of the Car class or the Bicycle class. The argument of the buy method should accept an instance of either type, so we need to overload the method (see below).

buy Method

          Inside the buy method, change the owner using an instance taken as an argument to the setter method.

     

Polymorphism

Duplicate Code in the buy Method

          Let's take another look at the buy method. Here we find a problem: each time we make a subclass of the Vehicle superclass, we need to define another method with the same name that takes a different argument type in each subclass.

Improving the buy Method

          There is a way to solve this problem. As shown in the examples below, by accepting an instance of the Vehicle type as a parameter, the buy method is able to take an instance of the Car class as well as the Bicycle class.

     

Assigning Subclass Instances to Superclass Variables

          Since the Car class inherits the Vehicle class, an instance of the Car class is also type Vehicle. When this relationship is found, we can assign an instance of a subclass to the class type variable of a superclass. This is why we can change the class type of the buy method's argument to Vehicle (this is called polymorphism).