java 闭包与回调

Posted 2021-02-01 飞龙dragon

闭包(closure)是一个可调用的对象,它记录了一些信息,这些信息来自于创建它的作用域.

内部类是面向对象的闭包,因为它不仅包含外围类对象(创建内部类的作用域)的信息,还自动拥有一个指向此外围类对象的引用,在此作用域内,内部类有权操作所有的成员,包括private成员

回调的价值在于它的灵活性--可以运行时动态决定调用什么方法

//: innerclasses/Callbacks.java
// Using inner classes for callbacks
package object;
import static net.util.Print.*;

interface Incrementable {
  void increment();
}

// Very simple to just implement the interface:
class Callee1 implements Incrementable {
  private int i = 0;
  public void increment() {
    i++;
    print(i);
  }
}    

class MyIncrement {
  public void increment() { print("Other operation"); }
  static void f(MyIncrement mi) { mi.increment(); }
}    

// If your class must implement increment() in
// some other way, you must use an inner class:
class Callee2 extends MyIncrement {
  private int i = 0;
  public void increment() {
    super.increment();
    i++;
    print(i);
  }
  private class Closure implements Incrementable {
    public void increment() {
      // Specify outer-class method, otherwise
      // you‘d get an infinite recursion:
      Callee2.this.increment();
    }
  }
  Incrementable getCallbackReference() {
    return new Closure();
  }
}    

class Caller {  
  private Incrementable callbackReference;
  Caller(Incrementable cbh) { callbackReference = cbh; }//Claller对象可以使用此引用回调Caller类
  void go() { callbackReference.increment(); }
}

public class Callbacks {
  public static void main(String[] args) {
    Callee1 c1 = new Callee1();
    Callee2 c2 = new Callee2();
    MyIncrement.f(c2);
    Caller caller1 = new Caller(c1);
    Caller caller2 = new Caller(c2.getCallbackReference());
    caller1.go();
    caller1.go();
    caller2.go();
    caller2.go();
  }    
} /* Output:
Other operation
1
1
2
Other operation
2
Other operation
3
*///:~