Transcript COP2212 Intro. to Programming in C

CS 4240
Principles of SW Design
More design principles
LSP, OCP, DIP, …
And another pattern
Decorator
© 2007 T. Horton
F-1
Well-known Design Principles
• Liskov Substitutability Principle
• Open-Closed Principle (OSP)
• Dependency Inversion Principle (ISP)
F-2
Substitutability
• Liskov Substitutability Principle:
– “Wherever we see a reference to an abstract object in
our code, we can legally replace that with a reference to
any subclass object.”
– Or another way: subclasses should always be
substitutable for their base classes.
• All about inheritance and “general” references
– Implies that we can “use” the subclass object in any way
that’s legal for the superclass
– Formal definition of IS-A (isn’t it?)
• Use it to evaluate your use of inheritance
F-3
Dependency Inversion Principle
• DIP says
– High level modules should NOT depend on lowlevel modules.
All modules should depend on abstractions
– Abstractions should not depend on details.
Details should depend on abstractions.
• Another way to say it:
– Depend on abstractions, not concrete things.
F-4
OO Principle: Open-Closed Principle
• The Open-Closed Principle (OCP)
– Classes should be open for extension, but
closed for modification.
• Don’t allow clients to alter your code.
• Allow clients to easily add things to your
classes.
– Provide means of extension
• Example of this: the Observer design pattern
• Note there’s a cost to making classes
extendable
F-5
Lots in Common Here
• First, these are formal “principles” for other
principles we talked about
– Or closely related ones
• Second, some of these are related
– OCP is a goal
– DIP is a way of helping you reach that goal
F-6
Another Design Problem, Another
Pattern
• You’re doing Beverages for a coffee shop
• Four types of coffee-drink:
– HouseBlend, DarkRoast, Decaf, Espresso
• Also can add (for a cost):
– SteamedMilk, Soy, Mocha, WhippedMilk
• Want a cost() method in each class to calculate costs
• Question: how to structure classes for this?
– Avoid class explosion. Same solution as for Customer
and Accounts? Need a Bridge?
F-7
One Solution
• Beverage abstract super-class
– Subclasses: HouseBlend, DarkRoast, Decaf,…
• Fields / Attributes:
– milk, soy, mocha, whip
• Methods / Operations:
– hasMilk(), setMilk(); hasSoy(), setSoy(); …
– cost()
• Issues?
F-8
Problems with This Approach
• Price for condiments? Alter existing code
• New condiments? Add methods; alter cost()
operation in superclass
• New beverage like ice tea that shouldn’t have
whipped milk?
• Want a double mocha?
F-9
Decorator Design Pattern
• “Decorate” an object
– Wrappers: a object defined so that it encloses
another object
• Key points:
– Decorators have the same supertype as the
object they wrap.
So you can use a decorated object in the same
place as the original object (a la Liskov)
– Can use more than one decorator on an object
– Can decorate objects at run-time
F-10
Decorators in Java I/O
• Used for input and output file streams
• Many stream types are wrappers
– Add extra functionality, e.g. push-back, line-numbering,
buffering, etc.
– Create by using “more basic” file-stream object in
constructor
– Can used a wrapped-stream where you’d use any stream
• See Java API:
http://java.sun.com/j2se/1.4.2/docs/api/java/io/FilterInp
utStream.html
• Also used in Java Swing
F-11
Decorators in Java I/O
FileInputStream istream = new FileInputStream(“foo.txt”);
BufferedInputStream bstream =
new BufferedInputStream(istream);
F-12
Issues with Decorators
• Disadvantages:
– May add many classes, makes package hard to
understand
• Like Java I/O streams
– Client code should not rely on knowing a
reference’s specific concrete class
• May get wrapped. Wrapping intended to
transparent to client code.
– Creating new objects could be more complex
• A factory class may help
F-13
Swing Examples of Decorators
• Example 1: Yikes -- dead link now -- never mind
Button with a diagonal line
– http://www.ideas2work.com/decorator-java.html
– Note: quick read, but not a lot of explanation of Swing and
how buttons are drawn or components
• Example 2 & 3: Component border, minimize
–
http://www.onjava.com/pub/a/onjava/2003/02/05/decorator.html?page=1
– Better explanation, especially of
• Decorator pattern
• Inheritance vs. decoration
• how components are
composed and painted in Swing
F-14