Java & 混型

1.C++ & 混型

C++能够记住其模板参数的类型，对于混型来说，使用参数化类型更加优雅。

#include <string>
#include <ctime>
#include <iostream>
using namespace std;

template<class T> class TimeStamped : public T {
  long timeStamp;
public:
  TimeStamped() { timeStamp = time(0); }
  long getStamp() { return timeStamp; }
};

template<class T> class SerialNumbered : public T {
  long serialNumber;
  static long counter;
public:
  SerialNumbered() { serialNumber = counter++; }
  long getSerialNumber() { return serialNumber; }
};

// Define and initialize the static storage:
template<class T> long SerialNumbered<T>::counter = 1;

class Basic {
  string value;
public:
  void set(string val) { value = val; }
  string get() { return value; }
};    

int main() {
  TimeStamped<SerialNumbered<Basic> > mixin1, mixin2;    //这就是我想要的
  mixin1.set("string1");
  mixin2.set("string2");
  cout << mixin1.get() << " " << mixin1.getStamp() << " " << mixin1.getSerialNumber() << endl;
  cout << mixin2.get() << " " << mixin2.getStamp() << " " << mixin2.getSerialNumber() << endl;
}

输出：

string1 1129840250 1
string2 1129840250 2

二。Java & 混型

Java由于擦除会忘记基类类型。因此泛型类不能直接继承一个泛型参数。

1.使用接口产生混型

interface TimeStamped { 
  long getStamp(); 
}


interface SerialNumbered {
    long getSerialNumber(); 
}


interface Basic {
  public void set(String val);
  public String get();
}

实现接口

class TimeStampedImp implements TimeStamped {
  private final long timeStamp;
  public TimeStampedImp() {
    timeStamp = new Date().getTime();
  }
  public long getStamp() { return timeStamp; }
}


class SerialNumberedImp implements SerialNumbered {
  private static long counter = 1;
  private final long serialNumber = counter++;
  public long getSerialNumber() { return serialNumber; }
}



class BasicImp implements Basic {
  private String value;
  public void set(String val) { value = val; }
  public String get() { return value; }
}

使用

class Mixin extends BasicImp implements TimeStamped, SerialNumbered {   
  private TimeStamped timeStamp = new TimeStampedImp();
  private SerialNumbered serialNumber =  new SerialNumberedImp();

  public long getStamp() { return timeStamp.getStamp(); }     //代理
  public long getSerialNumber() {                             //代理
    return serialNumber.getSerialNumber();
  }
}

public class Mixins {
  public static void main(String[] args) {
    Mixin mixin1 = new Mixin(); 
    Mixin mixin2 = new Mixin();
    mixin1.set("string1");
    mixin2.set("string2");
    System.out.println(mixin1.get() + " " + mixin1.getStamp() +  " " + mixin1.getSerialNumber());
    System.out.println(mixin2.get() + " " + mixin2.getStamp() +  " " + mixin2.getSerialNumber());
  }
}

2.使用装饰器模式

class Basic {
  private String value;
  public void set(String val) { value = val; }
  public String get() { return value; }
}

class Decorator extends Basic {
  protected Basic basic;
  public Decorator(Basic basic) { this.basic = basic; }
  public void set(String val) { basic.set(val); }
  public String get() { return basic.get(); }
}    

class TimeStamped extends Decorator {
  private final long timeStamp;
  public TimeStamped(Basic basic) {
    super(basic);
    timeStamp = new Date().getTime();
  }
  public long getStamp() { return timeStamp; }
}

class SerialNumbered extends Decorator {
  private static long counter = 1;
  private final long serialNumber = counter++;
  public SerialNumbered(Basic basic) { super(basic); }
  public long getSerialNumber() { return serialNumber; }
}    

public class Decoration {
  public static void main(String[] args) {
    TimeStamped t = new TimeStamped( new Basic() );
    TimeStamped t2 = new TimeStamped( new SerialNumbered( new Basic() ));
    //! t2.getSerialNumber(); // Not available
    SerialNumbered s = new SerialNumbered(new Basic());
    SerialNumbered s2 = new SerialNumbered( new TimeStamped( new Basic() ));
    //! s2.getStamp(); // Not available
  }
}

局限性：

     可以添加多个修饰层，但最后一层才是有效的。

3.动态代理