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(); }
  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 = ;

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
string2  

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