Reading notes -- Singleton pattern

Jedi — Sun, 09 Jul 2006 04:42:00 GMT

public class Singleton {

     private volatile static Singleton uniqueInstance;
     private Singleton(){

    }

     public static Singleton getInstance(){
         if (uniqueInstance == null ){
             synchronized (Singleton. class ){
                 if (uniqueInstance == null ){
                    uniqueInstance = new Singleton();
                }
            }
        }
         return uniqueInstance;
    }

}

1. private constructor
2. static getInstance
3. syncronized..waste a lot of time
4. double check..modified syncronize, so time-waste might occurs only when first time the instance construct

Jedi 2006-07-09 12:42 发表评论

Reading note of "Head First Design Patterns": Strategy Pattern

Jedi — Tue, 13 Jun 2006 04:42:00 GMT

Problem:   Started with a simple SimUDuck App..
   Joe's company makes a duck pond simulation game, SimUDuck, The game can show a large variety of duck species swimming and making quacking sounds.

Initial Design:

But now some new functionality should be added, for example: we need some of the ducks to FLY.

First Design:
   We add a method fly() into the Duck class. It seems worked, but something went horribly wrong because not all ducks can fly. so....

Second Design: Using inheritance and polymorphism
   Always override the fly() mehtod in the subclass where needed.

   Drawbacks: Everytime a new duck is added, you will be forced to look at and possibly override fly() and quack(). so is there a cleaner way of having only some of the duck types fly or quack?

Third Design: Using interface!

Drawbacks: It completely destroy code reuse for those behaviors.

1. Design Principles: Identify the aspects of your application that vary and separate them form what stays the same!
                               which means Encapsulate the parts that vary!
2. Design Principles: Program to an interface, not an implementation! (interface here means supertype! including interface
                               and abstract class!.. making use of the polymorphism functionality).
3. Design Principles:  Favor composition over interface!

Strategy Pattern: Using Composition!

Code implement:
FlyBehavior.java

public interface FlyBehavior{
public void fly();
}

FlyWithWings.java

public class FlyWithWings implements FlyBehavior{
   public void fly(){
      System.out.println("I'm flying!!");
   }
}

FlyNoWay.java

public class FlyNoWay implements FlyBehavior{
   public void fly(){
      System.out.println("I can't fly");
   }
}

Duck.java

public abstract class Duck{
   FlyBehavior flyBehavior;
   public Duck(){
   }

   public abstract void display();

   public void performFly(){
      flyBehavior.fly();
   }

   public void swim(){
      System.out.println("All ducks float, even decoys!");
   }
}

MallardDuck.java

public class MallardDuck{
   public MallardDuck(){
      flyBehavior=new FlyWithWings();
   }

   public void display(){
      System.out.println("I'm a real mallard duck");
   }
}

The Definition of Strategy Pattern: The Strategy Pattern defines a family of algorithms, encapsulates each one, and makes them intercahgeable. Strategy lets the algorithm vary indepanedtl from client that use it

Problems:
1. It's weird to have a class that's jast a behavior: classes represent things both have state and methods. a flying behavior might have instance variables representing the attributes for the flying behavior.
2. Be care of Over-Design: implement your code first, then refractoring!

Jedi 2006-06-13 12:42 发表评论

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Reading notes -- Singleton pattern

Reading note of "Head First Design Patterns": Strategy Pattern