软件设计原则(第二部分).ppt

1、Software Design Principle An Introduction Part Two Dr. ZHOU Xiao-cong Department of Computer Science SUN YAT-SEN UNIVERSITY, GZ 510275,Introduction Single Responsibility Principle (SRP) Open Closed Principle (OCP) Liskov Substitution Principle (LSP) Dependency Inversion Principle (DIP) Interface Seg

2、regation Principle (ISP) Least Knowledge Principle (LKP),Software Design Principle,Introduction Single Responsibility Principle (SRP) Open Closed Principle (OCP) Liskov Substitution Principle (LSP) Dependency Inversion Principle (DIP) Interface Segregation Principle (ISP) Least Knowledge Principle (

3、LKP),Software Design Principle,Barbara Liskov,The first woman in the United States to be awarded a Ph.D. from a computer science department In 1968, Stanford University She received 2008 Turing Award from the ACM for her work in the design of programming languages and software methodology that led t

4、o the development of object-oriented programming ACM: Association for Computing Machinery The Turing Award The highest distinction in Computer Science The Nobel Prize of computing,Definition,Design by contract Behavioral subtyping Let q(x) be a property provable about objects x of type T. Then q(y)

5、should be true for objects y of type S where S is a subtype of T.,Subtype,A subtype is a datatype that is related to another datatype (the supertype) by some notion of substitutability Program constructs written to operate on elements of the supertype can also operate on elements of the subtype Typi

6、cal program construct: subroutines, functions, methods Polymorphism in object-oriented programming language Subtyping relation is a relation on types On specification level More abstractly, on concepts,Subclass,A subclass is a class that inherits some properties from its superclass A subclass is an

7、extension of its superclass A subclass reuses the properties or services of its superclass Subclass relation is a relation on implementation level superclass, base class, or parent class child classes, derived classes, or subclasses,The principles (1),standard requirements on signatures Contravarian

8、ce of method arguments in the subtype Covariance of return types in the subtype No new exceptions should be thrown by methods of the subtype except where those exceptions are themselves subtypes of exceptions thrown by the methods of the supertype The members of derived classes must expect no more t

9、han the corresponding members of the base class; and should promise no less,The principles (2),Behavioral conditions that subtype must meet Preconditions cannot be strengthened in a subtype Postconditions cannot be weakened in a subtype Invariants of the supertype must be preserved in a subtype Hist

10、ory constraint: the history properties of the supertype must be preseved in a subtype History properties are properties true of all sequences of states e.g. the bound of an array does not change,A typical violation,The square-rectangle problem Also circle-ellipse problem /wiki/

11、Circle-ellipse_problem A square is a rectangle ? The square class inherits the rectangle class will lead to many problems!,Benefits and the risk of fallout from the change increases dramatically.,Separation through delegation (1),Separation through delegation (2),Benefits dont talk to strangers. Onl

12、y talk to your immediate friends.,An example (1),An example (2),An example (3),What is this bad?,The Paperboy class now knows that the customer has a wallet, and can manipulate it “the paperboy is being exposed to more information than he needs to be” It is not corresponding to the real-word These t

13、hree classes are now tightly coupled,Improving the original code (1),Improving the original code (2),What is this better?,it better models the real world scenario The paperboy does not have direct access to the wallet the Wallet class can now change, and the paperboy is completely isolated from that

14、 change Code will be more maintainable, because changes will not ripple through a large project we are now free to change the implementation of getPayment(),The Law of Demeter for functions,A method M of an object O may only invoke the methods of the following kinds of objects O itself Ms parameters any objects created/instantiated within M Os direct component objects,When and how to apply the LoD,Chained get Statements value = object.getX().getY().getTheValue(); Lots of temporary objects Importing Many Cl