chapter04_Java基础知识_4_反射,利用接口实现函数指针

• (1) 示例

    public class Solution {
  
        void display() {
            System.out.println("haha");
        }
  
        public static void main(String[] args) throws Exception {
  
            Class solutionClass = Class.forName("Solution");
    
            Solution solution = (Solution) solutionClass.newInstance();
     
            solution.display();
        }
    }
  

  但是，这种方法被废弃了

  原因：

    This method propagates any exception thrown by the nullary constructor, including a checked exception. 
    Use of this method effectively bypasses the compile-time exception checking that would otherwise be performed by the compiler. 
    The Constructor.newInstance method avoids this problem by wrapping any exception thrown by the constructor in a (checked) InvocationTargetException.
  

  所以，现在应该用这种方式

   public class Solution {
  
        void display() {
            System.out.println("haha");
        }
  
        public static void main(String[] args) throws Exception {
  
            Class solutionClass = Class.forName("Solution");
            Solution solution = (Solution) solutionClass.getDeclaredConstructor().newInstance();
            solution.display();
        }
   }
  

  (2) Class本身就是一个类，也可以实例化一个Class对象

  获取Class对象的三种方法

  1° 示例对象.getClass();

  2° 类.class;

  3° Class.forName(String className);静态方法

  (3) 创建新对象的四种方法

  1° new

  2° 反射

  3° clone()

  4° 反序列化

• Java实现函数指针

  (1) 函数指针

  目的是实现__回调函数__，将函数的指针作为参数传递给另一个函数。这样，这个函数不是被函数的实现方调用，而是在另一个地方被调用，这样可以动态实现功能（例如到底对两个数进行加减乘除中的哪一种操作，取决于传了哪个函数的指针）

  (2) Java实现函数指针的方法 – 利用接口

  在接口中声明要调用的方法，不同的实现类分别给出这个方法的不同实现，调用时传进不同的实现类实例

  示例

    interface PlayerButton {
        public void buttonPressed(Player player);
    }
  
    class StopButton implements PlayerButton {
        @Override
        public void buttonPressed(Player player) {
            player.stop();
        }
    }
  
    class StartButton implements PlayerButton {
        @Override
        public void buttonPressed(Player player) {
            player.start();
        }
    }
  
    class Player {
        public void start() {
            System.out.println("start");
        }
  
        public void stop() {
            System.out.println("stop");
        }
    }
  
    class FunctionPointerTest {
  
        public static void main(String... args) {
  
            Player player = new Player();
  
            pressButton(player, new StopButton());
            pressButton(player, new StartButton());
        }
  
        public static void pressButton(Player player, PlayerButton button) {
    
            button.buttonPressed(player);
        }
    }
  

  (3) 典型示例：

  排序传入自定义的Comparator
  public static void sort(List list, Comparator<? super T> c);