CS 160: Software Engineering October 22 Class Meeting Department of Computer Science San Jose State University Fall 2014 Instructor: Ron Mak mak

CS 160: Software EngineeringOctober 22 Class Meeting

Department of Computer ScienceSan Jose State University

Fall 2014Instructor: Ron Mak

www.cs.sjsu.edu/~mak

http://www.cs.sjsu.edu/~mak

http://www.cs.sjsu.edu/~mak

Computer Science Dept.Fall 2014: October 22

CS 160: Software Engineering© R. Mak

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Project Team Design Reviews

Monday, October 27

Section 3 Code Monsters 160 Zaibatsu Tin Bullet

Section 4 Quiet Coders Surprise Error Dream Team

Wednesday, October 29

Section 3 List Ninja Merge Monkeys Noisy Coders

Section 4 Activate League of Gentlemen Not Applicable



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Project Team Design Reviews, cont’d

Present and defend key aspects of your design. 20 minutes per team.

Use PowerPoint slides Turn in your slides after your presentation.

Briefly discuss your overall design.

Show and explain the design of your most important or core classes. UML class diagrams and/or sequence diagrams

_



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Project Team Design Reviews, cont’d

Briefly discuss your implementation plan.

Questions and answers at the end.

Audience: Very technical

Other development engineers. Technical managers, stakeholders, financial backers,

etc. Expect “deep” questions from the audience!

_



5

A Useful Combination of Techniques

Interfaces

Coding to the interface

Polymorphism

Design patterns strategy design pattern factory method design pattern

Dynamic class loading



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Interfaces and Classes

A Java class defines:

an interface a set of operations (methods)

an implementation statements that specify how to carry out the operations statements that specify how to represent object state

It can be useful to separate an interface from its implementation.



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Java Interface

A Java interface type Defines a set of operations (methods). Does not define their implementations.

A Java class can implement an interface. The class must implement (supply the statements

for) each of the interface’s methods.

A Java class can implement multiple interfaces. The class must implement all the methods of the

interfaces that it implements.



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The Interface as a Contract

A Java interface can be implemented by multiple classes.

Each class can implement an interface method in a different but related way.

An interface is a contract.

Any class that implements the interface is guaranteed to implement each and every one of the interface methods.



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Interface Example

Interface Shape defines the draw() method.

Classes Rectangle, Triangle, and Circle each implements interface Shape. Each class must therefore implement

the draw() method.

Each class implements draw() in a different but related way. Each class’s draw() method draws the shape. But each shape is drawn differently.



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A Class Hierarchy Puzzle

Animal

Lion Dog Piranha Goldfish Parrot Hummingbird

Mammal Fish Bird

We want to add the category HouseholdPet.

Do we make it a superclass? Where does it belong in this class hierarchy?

How do we also add the category Biter?



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A Class Hierarchy Puzzle

Make HouseholdPet and Biter Java interfaces. A Java class can implement multiple interfaces.

Animal

Mammal Fish Bird


«interface»HouseholdPet

«interface»Biter



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Java Subclass

If a class C is a subclass of superclass S, then C “is a” S:

Animal

Mammal Fish Bird


Dog is an Animal.Dog is a Mammal.



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Java Interface

If a class C implements interface N, then C “is a” N:

Animal

Mammal Fish Bird


«interface»HouseholdPet

«interface»Biter

Dog is a HouseholdPet.

Dog is a Biter.



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Java instanceof Operator

If the value of variable x is of type Dog, then the following conditionals are all true:

x instanceof Dog x instanceof Mammal x instanceof Animal x instanceof HouseholdPet x instanceof Biter

_



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Polymorphism

A variable can have an interface type. Example:

At run time, variable sh can have any object as a value that was instantiated from a class that implements the Shape interface.

A call to sh.draw() will call the rectangle, triangle, or circle draw method, depending on the value of sh.

Polymorphism: The ability to determine automatically at run time which method to call.

Shape sh;



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Polymorphism, cont’d

A variable can have an interface type. A value cannot have an interface type.

A value can be an object instantiated from a class that implements an interface. Or a value can be a scalar.



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Coding to the Interface

Compare:

Are there advantages of one vs. the other? The first declaration allows variable sh to be

assigned only a Rectangle object. The second declaration allows sh to be assigned

a Rectangle, Triangle, or Circle object. Or any object instantiated from a class that

implements interface Shape. This more flexible style is coding to the interface.

Shape sh;

Rectangle sh;



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Design Patterns

A design pattern is

A description of a problem. A solution that you can apply

to many programming situations.

Design patterns show you how to build good software with good object-oriented design qualities.

Design patterns are proven object-oriented experience.



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Design Patterns, cont’d

Design patterns are not code, but are general solutions to design problems. You apply them to your specific application.

Design patterns are not invented – they’re discovered.

Design patterns address how to manage change._



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Design patterns give programmers a very high-level, short-cut vocabulary to discuss design issues.

Independent of specific implementations or programming languages.

“We should use the factory method design pattern here.”

“The decorator pattern will simplify this code.”



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Each design pattern has

A short name A brief description of the context A description of the problem that it solves A prescription for a solution

_



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Building architect Christopher Alexander discovered over 250 patterns for architectural design. Co-authored A Pattern Language: Towns, Buildings,

Construction, published in 1977.

In 1995, four authors, Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides (AKA “The Gang of Four”) published the classic software book Design Patterns. Original 23 design patterns



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The Strategy Design Pattern

Context

There are different algorithms (“strategies”) to solve a particular problem.

Description The algorithms all have similar public interfaces,

but each solves the problem in a different way.



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The Strategy Design Pattern, cont’d

Solution Create a strategy interface that is

an abstraction of the algorithm.

Declare the interface shared by the algorithms.

Code each strategy in a class that implements the strategy interface.

At run time, select one of the strategies and call its interface methods._



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The Strategy Design Pattern, cont’d

<<interface>>Shape

draw() : void

Triangle

draw() : void

Rectangle

draw() : void

Circle

draw() : void

Strategy interface

Strategy Strategy Strategy



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The Factory Method Design Pattern

Context

An application can instantiate any one of several classes that implement an interface.

Description

You know that your application needs to instantiate one of the classes. But you won’t know which class until run time.

You need to provide a means to instantiate the classas determined by the application at run time. Therefore, your code must have the flexibility

to instantiate and work with any of the classes.



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The Factory Method Design Pattern

Solution

Design a factory method that will, based on its parameters, create and return an object that is instantiated from the correct class.



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Factory Method Example

public class GreeterFactory1 { public static Greeter make(String language) throws Exception { if (language.equals("English")) { return new English(); } else if (language.equals("French")) { return new French(); } else { throw new Exception(); } }}

<<interface>>Greeter

English French

public interface Greeter { String greet();}

public class English implements Greeter{ public String greet() { return "Hello!"; }}

Demo



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Factory Method Example, cont’d

public class GreeterFactory1 { public static Greeter make(String language) throws Exception { if (language.equals("English")) { return new English(); } else if (language.equals("French")) { return new French(); } else { throw new Exception(); } }}

How can we make this factory method even more flexible?

Hardcoded to work only with English and French.



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Dynamic Class Loading

Suppose that when we write the factory method,we don’t know which languages are available.

At run time, the factory method can dynamically load a language class if it is given the name of the language:

where language is a string containing the name of a language.

Class.forName(language)



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Dynamic Class Loading, cont’d

public class GreeterFactory2 { public static Greeter make(String language) throws ClassNotFoundException, InstantiationException, IllegalAccessException { return (Greeter) Class.forName(language).newInstance(); }} Dynamically load a language class,

and instantiate and return a language object.

Demo

Documents

CS 160: Software Engineering October 22 Class Meeting Department of Computer Science San Jose State University Fall 2014 Instructor: Ron Mak mak