封裝與存取修飾詞

Java 物件導向程式設計入門

Sani Yusuf

Lead Software Engineering Content Developer

認識封裝

可用行動裝置來說明封裝
你只能使用對外公開的功能，如螢幕、喇叭等
內部運作對使用者完全隱藏

Public 屬性

標記為 public 的屬性可由物件實例存取


  class Car {
    // Public property
    public String color;


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


    public class Main {

      // main method
      public static void main(String[] args) {
          Car myCar = new Car("brown");
          // Can access public members 
          // from object instance
          System.out.println(
            myCar.color); // Brown
        }
      }

Private 屬性

標記為 private 的屬性不可由物件實例存取
標記為 private 的屬性僅能在類別內使用


  // Car class
  class Car {
     public String color;

     // Private properties 
     private String model;


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


    // Main class
    public class Main {
      // main method
      public static void main(String[] args) {
          Car myCar = new Car("brown");
          // Calling private properties causes errors        
          System.out.println(
            myCar.model); // Java compilation error
        }
      }

Public 方法


  // Car class
  class Car {
      public String color;
      private String model;

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

      // Public method
      public String getModel(){
          return this.model;
      }    
  }

Private 方法


  class Car {


      // Private method, "calculateSpeed" can only be used within "Car" class
      private calculateSpeed(){
        // Trademarked formula code
      }

      public int getSpeed(){
        // "calculateSpeed" can be used anywhere within "Car" class
        return this.calculateSpeed();
      }
    }

Static 方法

標記為 static 的屬性可不需物件實例即可存取
常用來放共用程式碼／函式庫


   // Formula class 
   static class Formula {

     // Method for calculating square
     static int getSquare(int number) {
        return number * number;
      }
   }


  // Main class 
  public class Main {

    public static void 
      main(String[] args) {
          // We can use "getSquare" 
          // without an object instance
        System.out.println(
          Formula.getSquare(5)); // 25
      }
  }

重點回顧

存取修飾詞：public、private、static
存取修飾詞有助於強化應用程式安全
static 讓你可不建立實例就使用類別

一起來練習吧！

Java 物件導向程式設計入門