java编程思想中的设计模式

1命令设计模式

命令设计模式首先需要一个只有单一方法的接口，然后从该接口实现具有各自不同的行为的多个子类。接下来，程序员就可以构造命令对象，并在需要的时候使用它们:

package com;
import java.util.*;
import static com.AlarmPoints.*;
enum AlarmPoints {
  STAIR1, STAIR2, LOBBY, OFFICE1, OFFICE2, OFFICE3,
  OFFICE4, BATHROOM, UTILITY, KITCHEN
}

interface Command { void action(); }
public class EnumMaps {

  public static void main(String[] args) {
    EnumMap<AlarmPoints,Command> em =
      new EnumMap<AlarmPoints,Command>(AlarmPoints.class);
    em.put(KITCHEN, new Command() {
      public void action() { print("Kitchen fire!"); }
    });
    em.put(BATHROOM, new Command() {
      public void action() { print("Bathroom alert!"); }
    });
    for(Map.Entry<AlarmPoints,Command> e : em.entrySet()) {
      printnb(e.getKey() + ": ");
      e.getValue().action();
    }
    try { // If there's no value for a particular key:
      em.get(UTILITY).action();
    } catch(Exception e) {
      print(e);
    }
  }

  public static void print(Object obj) {
    System.out.println(obj);
  }
  public static void printnb(Object obj) {
    System.out.print(obj);
  }
} /* Output:
BATHROOM: Bathroom alert!
KITCHEN: Kitchen fire!
java.lang.NullPointerException
*///:~

2职责链设计模式

程序员以多种不同的方式来解决一个问题，然后将它们链接起来。当一个请求到来时，它遍历这个链，直到链中的某个解决方案能够处理该请求。

下面的代码中，每一次尝试可以看作一个策略。

package com;//: enumerated/PostOffice.java
// Modeling a post office.
import java.util.*;
class Enums {
  private static Random rand = new Random(47);
  public static <T extends Enum<T>> T random(Class<T> ec) {
    return random(ec.getEnumConstants());
  }
  public static <T> T random(T[] values) {
    return values[rand.nextInt(values.length)];
  }
}
class Mail {
  // The NO's lower the probability of random selection:
  enum GeneralDelivery {YES,NO1,NO2,NO3,NO4,NO5}
  enum Scannability {UNSCANNABLE,YES1,YES2,YES3,YES4}
  enum Readability {ILLEGIBLE,YES1,YES2,YES3,YES4}
  enum Address {INCORRECT,OK1,OK2,OK3,OK4,OK5,OK6}
  enum ReturnAddress {MISSING,OK1,OK2,OK3,OK4,OK5}
  GeneralDelivery generalDelivery;
  Scannability scannability;
  Readability readability;
  Address address;
  ReturnAddress returnAddress;
  static long counter = 0;
  long id = counter++;
  public String toString() { return "Mail " + id; }
  public String details() {
    return toString() +
      ", General Delivery: " + generalDelivery +
      ", Address Scanability: " + scannability +
      ", Address Readability: " + readability +
      ", Address Address: " + address +
      ", Return address: " + returnAddress;
  }
  // Generate test Mail:
  public static Mail randomMail() {
    Mail m = new Mail();
    m.generalDelivery= Enums.random(GeneralDelivery.class);
    m.scannability = Enums.random(Scannability.class);
    m.readability = Enums.random(Readability.class);
    m.address = Enums.random(Address.class);
    m.returnAddress = Enums.random(ReturnAddress.class);
    return m;
  }
  public static Iterable<Mail> generator(final int count) {
    return new Iterable<Mail>() {
      int n = count;
      public Iterator<Mail> iterator() {
        return new Iterator<Mail>() {
          public boolean hasNext() { return n-- > 0; }
          public Mail next() { return randomMail(); }
          public void remove() { // Not implemented
            throw new UnsupportedOperationException();
          }
        };
      }
    };
  }
}

public class PostOffice {
  enum MailHandler {
    GENERAL_DELIVERY {
      boolean handle(Mail m) {
        switch(m.generalDelivery) {
          case YES:
            print("Using general delivery for " + m);
            return true;
          default: return false;
        }
      }
    },
    MACHINE_SCAN {
      boolean handle(Mail m) {
        switch(m.scannability) {
          case UNSCANNABLE: return false;
          default:
            switch(m.address) {
              case INCORRECT: return false;
              default:
                print("Delivering "+ m + " automatically");
                return true;
            }
        }
      }
    },
    VISUAL_INSPECTION {
      boolean handle(Mail m) {
        switch(m.readability) {
          case ILLEGIBLE: return false;
          default:
            switch(m.address) {
              case INCORRECT: return false;
              default:
                print("Delivering " + m + " normally");
                return true;
            }
        }
      }
    },
    RETURN_TO_SENDER {
      boolean handle(Mail m) {
        switch(m.returnAddress) {
          case MISSING: return false;
          default:
            print("Returning " + m + " to sender");
            return true;
        }
      }
    };
    abstract boolean handle(Mail m);
  }
  static void handle(Mail m) {
    for(MailHandler handler : MailHandler.values())
      if(handler.handle(m))
        return;
    print(m + " is a dead letter");
  }
  public static void main(String[] args) {
    for(Mail mail : Mail.generator(10)) {
      print(mail.details());
      handle(mail);
      print("*****");
    }
  }
  public static void print(Object obj) {
    System.out.println(obj);
  }
} /* Output:
Mail 0, General Delivery: NO2, Address Scanability: UNSCANNABLE, Address Readability: YES3, Address Address: OK1, Return address: OK1
Delivering Mail 0 normally
*****
Mail 1, General Delivery: NO5, Address Scanability: YES3, Address Readability: ILLEGIBLE, Address Address: OK5, Return address: OK1
Delivering Mail 1 automatically
*****
Mail 2, General Delivery: YES, Address Scanability: YES3, Address Readability: YES1, Address Address: OK1, Return address: OK5
Using general delivery for Mail 2
*****
Mail 3, General Delivery: NO4, Address Scanability: YES3, Address Readability: YES1, Address Address: INCORRECT, Return address: OK4
Returning Mail 3 to sender
*****
Mail 4, General Delivery: NO4, Address Scanability: UNSCANNABLE, Address Readability: YES1, Address Address: INCORRECT, Return address: OK2
Returning Mail 4 to sender
*****
Mail 5, General Delivery: NO3, Address Scanability: YES1, Address Readability: ILLEGIBLE, Address Address: OK4, Return address: OK2
Delivering Mail 5 automatically
*****
Mail 6, General Delivery: YES, Address Scanability: YES4, Address Readability: ILLEGIBLE, Address Address: OK4, Return address: OK4
Using general delivery for Mail 6
*****
Mail 7, General Delivery: YES, Address Scanability: YES3, Address Readability: YES4, Address Address: OK2, Return address: MISSING
Using general delivery for Mail 7
*****
Mail 8, General Delivery: NO3, Address Scanability: YES1, Address Readability: YES3, Address Address: INCORRECT, Return address: MISSING
Mail 8 is a dead letter
*****
Mail 9, General Delivery: NO1, Address Scanability: UNSCANNABLE, Address Readability: YES2, Address Address: OK1, Return address: OK4
Delivering Mail 9 normally
*****
*///:~

3模板方法

一些功能在基类中实现，并且其一个或多个抽象方法在派生类中定义，这种结构在设计模式中称为模板方法。

例如下面的PairManager类:

//: concurrency/CriticalSection.java
// Synchronizing blocks instead of entire methods. Also
// demonstrates protection of a non-thread-safe class
// with a thread-safe one.
package com;
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;
import java.util.*;

class Pair { // Not thread-safe
  private int x, y;
  public Pair(int x, int y) {
    this.x = x;
    this.y = y;
  }
  public Pair() { this(0, 0); }
  public int getX() { return x; }
  public int getY() { return y; }
  public void incrementX() { x++; }
  public void incrementY() { y++; }
  public String toString() {
    return "x: " + x + ", y: " + y;
  }
  public class PairValuesNotEqualException
  extends RuntimeException {
    public PairValuesNotEqualException() {
      super("Pair values not equal: " + Pair.this);
    }
  }
  // Arbitrary invariant -- both variables must be equal:
  public void checkState() {
    if(x != y)
      throw new PairValuesNotEqualException();
  }
}

// Protect a Pair inside a thread-safe class:
abstract class PairManager {
  AtomicInteger checkCounter = new AtomicInteger(0);
  protected Pair p = new Pair();
  private List<Pair> storage =
    Collections.synchronizedList(new ArrayList<Pair>());
  public synchronized Pair getPair() {
    // Make a copy to keep the original safe:
    return new Pair(p.getX(), p.getY());
  }
  // Assume this is a time consuming operation
  protected void store(Pair p) {
    storage.add(p);
    try {
      TimeUnit.MILLISECONDS.sleep(50);
    } catch(InterruptedException ignore) {}
  }
  public abstract void increment();
}

// Synchronize the entire method:
class PairManager1 extends PairManager {
  public synchronized void increment() {
    p.incrementX();
    p.incrementY();
    store(getPair());
  }
}

// Use a critical section:
class PairManager2 extends PairManager {
  public void increment() {
    Pair temp;
    synchronized(this) {
      p.incrementX();
      p.incrementY();
      temp = getPair();
    }
    store(temp);
  }
}

class PairManipulator implements Runnable {
  private PairManager pm;
  public PairManipulator(PairManager pm) {
    this.pm = pm;
  }
  public void run() {
    while(true)
      pm.increment();
  }
  public String toString() {
    return "Pair: " + pm.getPair() +
      " checkCounter = " + pm.checkCounter.get();
  }
}

class PairChecker implements Runnable {
  private PairManager pm;
  public PairChecker(PairManager pm) {
    this.pm = pm;
  }
  public void run() {
    while(true) {
      pm.checkCounter.incrementAndGet();
      pm.getPair().checkState();
    }
  }
}

public class CriticalSection {
  // Test the two different approaches:
  static void
  testApproaches(PairManager pman1, PairManager pman2) {
    ExecutorService exec = Executors.newCachedThreadPool();
    PairManipulator
      pm1 = new PairManipulator(pman1),
      pm2 = new PairManipulator(pman2);
    PairChecker
      pcheck1 = new PairChecker(pman1),
      pcheck2 = new PairChecker(pman2);
    exec.execute(pm1);
    exec.execute(pm2);
    exec.execute(pcheck1);
    exec.execute(pcheck2);
    try {
      TimeUnit.MILLISECONDS.sleep(500);
    } catch(InterruptedException e) {
      System.out.println("Sleep interrupted");
    }
    System.out.println("pm1: " + pm1 + "\npm2: " + pm2);
   System.exit(0);
  }
  public static void main(String[] args) {
    PairManager
      pman1 = new PairManager1(),
      pman2 = new PairManager2();
    testApproaches(pman1, pman2);
  }
} /* Output: (Sample)
pm1: Pair: x: 15, y: 15 checkCounter = 272565
pm2: Pair: x: 16, y: 16 checkCounter = 3956974
*///:~

4策略设计模式

Delayed接口有一个方法为getDelay(TimeUnit unit),这个方法强制我们去使用TimeUnit类，可以很容易地转换单位而无需作任何声明，这是策略设计模式的一个简单示例，在这种模式中，算法的一部分是作为参数传递进来的。

5代理

代理是为了提供额外的或不同的操作，而插入的用来代替“实际”对象的对象。

package com;//: typeinfo/SimpleProxyDemo.java


interface Interface {
  void doSomething();
  void somethingElse(String arg);
  
}

class RealObject implements Interface {
  public void doSomething() { System.out.println("doSomething"); }
  public void somethingElse(String arg) {
    System.out.println("somethingElse " + arg);
  }
}	

class SimpleProxy implements Interface {
  private Interface proxied;
  public SimpleProxy(Interface proxied) {
    this.proxied = proxied;
  }
  public void doSomething() {
    System.out.println("SimpleProxy doSomething");
    proxied.doSomething();
  }
  public void somethingElse(String arg) {
    System.out.println("SimpleProxy somethingElse " + arg);
    proxied.somethingElse(arg);
  }
  
}	

class SimpleProxyDemo {
  public static void consumer(Interface iface) {
    iface.doSomething();
    iface.somethingElse("bonobo");
  }
  public static void main(String[] args) {
    consumer(new RealObject());
    consumer(new SimpleProxy(new RealObject()));
  }
} /* Output:
doSomething
somethingElse bonobo
SimpleProxy doSomething
doSomething
SimpleProxy somethingElse bonobo
somethingElse bonobo
*///:~