用繼承集中管理程式碼

Java 物件導向程式設計入門

Sani Yusuf

Lead Software Engineering Content Developer

操作多個類別

  // 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;        
  }

使用類別時，重複很常見

繼承的威力

繼承可解決重複程式碼問題
共享程式碼可集中在同一處
之後類別可用 extends 關鍵字繼承

  // 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 {

  }

認識基底類別

基底類別是放共用程式碼的類別
Car 類別就是基底類別的例子


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

用子類別來繼承

子類別是繼承基底類別的類別
Toyota 類別是子類別的例子
子類別會繼承基底類別的所有 public 屬性與方法
super() 會呼叫基底類別的建構式

  // 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();
     }
  }

建立繼承的實例

基底類別的屬性可由物件實例使用


  // 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(
          myToyota.color); // Brown
    }
  }

一起來練習吧！

Java 物件導向程式設計入門