封装与访问修饰符

Java 面向对象编程入门

Sani Yusuf

Lead Software Engineering Content Developer

理解封装

可用手机来解释封装
你只能访问公开功能，如屏幕、扬声器等
内部实现对用户完全隐藏

公共属性

标记为 public 的属性可由对象实例访问


  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
        }
      }

私有属性

标记为 private 的属性可由对象实例访问
标记为 private 的属性仅能在类内使用


  // 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
        }
      }

公共方法


  // 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;
      }    
  }

私有方法


  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();
      }
    }

静态方法

标记为 static 的属性可在无实例时访问
常用于存放共享代码/库


   // 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) {
          // 可在无对象实例时使用 "getSquare"
        System.out.println(
          Formula.getSquare(5)); // 25
      }
  }

小结

访问修饰符：public、private、static
访问修饰符有助于保护应用
static 允许无需创建实例即可使用类

Passons à la pratique !

Java 面向对象编程入门