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!