Java的回调机制实例详解

interface CallBack // 回调接口
{
  void methodToCallBack( );  //回调方法
}
class CallBackImpl implements CallBack   // 实现回调接口和回调方法
{
  public void methodToCallBack( )
  {
    System.out.println("I've been called back");
  }
}
public class Caller
{
  public void register(CallBack callback)   // 注册回调接口
  {
    callback.methodToCallBack( );      // 通过回调接口调用回调方法
  }
  public static void main(String[] args)
  {
    Caller caller = new Caller( );         //B处
    CallBack callBack = new CallBackImpl( );    //A处
    caller.register(callBack);           //C处
  }
}

import java.awt.Rectangle;
interface Measurer
{
  // 这个anObject，并不明确。不明确的用意是，先不绑定任何特定的对象
  double measure( Object anObject );
}
// 把抽象的measure方法具体化
class AreaMeasurer implements Measurer
{
  public double measure(Object anObject)
  {
    // 把抽象的对象明确化，由于Object对象是所有对象的鼻祖，所以，它可以化身任何对象
    Rectangle aRectangle = (Rectangle) anObject;
    double area = aRectangle.getWidth() * aRectangle.getHeight();
    return area;
  }
}
class Car
{ 
  private double price;
  private double taxRate;
  
  Car(double price, double taxRate)
  {
    this.price = price;
    this.taxRate = taxRate;
  }
  public double getPrice()
  {
    return price;
  }
  public double getRate()
  {
    return taxRate;
  }
}
class CarMeasurer implements Measurer
{
 
  public double measure(Object anObject)
  {
    Car aCar = (Car) anObject;
    double totalPrice = aCar.getPrice() * (1 + aCar.getRate());
    return totalPrice;
  }
}
class Data
{
  public static double average(Object[] objects, Measurer meas)  // 前者提供数据，后者提供方法
  {
    double sum = 0.0;
    for (Object obj : objects)
    {
      sum = sum + meas.measure(obj);
    }
    if (objects.length > 0)
    {
      return sum / objects.length;
    } else {
      return 0;
    }
  }
}
public class callBackDemo
{
  public static void main(String[] args)
  {
    Measurer areaMeas = new AreaMeasurer();
    Rectangle[] rects = new Rectangle[]
    {
      new Rectangle(5, 10, 20, 30),
      new Rectangle(10, 20, 30, 40),
      new Rectangle(20, 30, 5, 15)
    };
    double averageArea = Data.average(rects, areaMeas);
    System.out.println("average area is: " + averageArea);
        
    Measurer carMeas = new CarMeasurer();
    Car[] cars = new Car[]
    {
      new Car(20000, 0.12),
      new Car(30000, 0.16),
      new Car(90000, 0.18),
    };
    double averagePrice = Data.average(cars, carMeas);
    System.out.println("average price is: " + averagePrice);
  }
}