Chapter 25 More Design Patterns

Polymorphism Issue: Conditional variation –If-then-else or switch statements –New variation or case: Conditional statements change Often in many places (methods, classes) –Makes it difficult to extend program, add new variations Examples –Alternatives based on type –Pluggable software components: Replace one server with another Without affecting the clients

Polymorphism Solution –Do not test for the type of an object and use conditional logic –Use polymorphic operations instead Example: Third-party tax calculator programs in the NextGen POS system We want to use an “adapter class” –We call adapter’s methods, adapter converts it to tax calculator’s methods We’d like our code not to depend on which tax calculator we use We’d like to avoid conditional statements in the adapter class Solution: Use one adapter class per tax calculator –Use polymorphism

Adapter classes implement iTaxCalculatorAdapter interface

Polymorphism in Monopoly Issue: Different actions need to be performed when player lands on different types of square Examples: –Go square: Get $200 –Tax square: Pay income tax (10% or $200) –Regular square: Do nothing (for now) Solution: Use abstract superclass, Square –RegularSquare, TaxSquare, IncomeTaxSquare subclasses

Polymorphism in Monopoly

Issue: How to adapt interaction diagrams to this change? May use “sd” and “ref” frames here to be more formal

One Polymorphic Case

Another polymorphic case

Notice design change: –Now Player knows location, instead of Piece –Decreases coupling –In fact, we realize “piece” class not really needed

When to use interfaces vs. abstract superclasses? Use interfaces whenever you can –Because of single inheritance, abstract classes are restrictive –Unless there are a lot of default method implementations inherited – then use abstract classes

Design Pattern: Pure Fabrication Idea: Make up (“fabricate”) new class with no direct equivalent concept in the domain –When other design patterns do not offer solutions with low coupling and high cohesion Recall example: Saving a Sale object in a database What goes wrong if “Sale” saves itself? –Task requires a lot of supporting database operations –Sale has to be coupled to the db interface –Saving to db a general task A lot of other classes need the same support Lots of duplication in each object

Solution: New “PersistentStorage” class Persistent storage not a domain concept We grouped a number of responsibilities into this class Similar issue in Monopoly: –Player rolls dice: Not reusable in another game –Summing of dice done by player –Can’t ask for dice total without rolling again Improved design: Fabricate “Cup” class that does the jobs above

Fabrication in Monopoly: The “Cup” class

Design of Objects Design of objects done in one of two ways –Objects chosen by representational decomposition Representing domain objects Example: –TableOfContents –Objects chosen by behavioral decomposition Group a number of related operations into a class Example: –Algorithm object: TableOfContentsGenerator Don’t overdo fabrication –If you put all functionality in one class, you end up writing a C program

Updated Monopoly Interaction Diagram

Design Pattern: Indirection Issue: How to avoid coupling between two or more things –Why? One is likely to change a lot. Don’t want the other to depend on it. Solution: Use indirection. Create intermediate object. Example: TaxCalculatorAdapter –Our design is independent from the third party tax program API –If the API changes, we only modify the adapter. Example: PersistentStorage –Just change PersistentStorage to use a different database

Indirection via the adapter class/object

Design Pattern: Protected Variations Issue: How to design objects, subsystems and systems so that variations or instability in these elements do not have an undesirable impact on others? Solution: –Identify points of predicted variations and instability –Put a stable interface around them to protect the rest of the design Example: External tax calculator and the class hierarchy of adapters

Examples of Mechanisms Motivated by Protected Variations Data-driven designs: –Example: Web-page style sheet read separately from the.html file Service look-up mechanisms: JNDI, Jini, UDDI –Protects designs from where services are located Interpreter-Driven Designs –Virtual machines, language interpreters,... Reflective or Meta-Level Designs –Java Beans –A way to learn what attribute or bean property a “Bean” has and what method to use to access it

Law of Demeter In any method M attached to a Class C, only methods defined by the following classes may be used: The instance variable classes of C the argument classes of the method M including C itself Note: Global objects or the local objects of M are considered arguments of M

Law of Demeter (weak) In any method M attached to a Class C, data can be accessed and messages can be sent to only the following objects: the arguments of the method M including self The instance variables for the receiver of the message. Global objects the local objects of M

Law of Demeter (strong) In any method M attached to a Class C, data can be accessed and messages can be sent to only the following objects: the arguments of the method M including self The instance variables defined in the class containing the method being executed. (no direct access to inherited instance variables.) Global objects the local objects of M

Law of Demeter (from Dr. Gordon) In any method M attached to a Class C, data can be accessed and messages can be sent to only the following objects: the arguments of the method M including self the local objects of M (no direct access to instance variables.)

Cohesion Revisited Varieties of Cohesion –Coincidental Cohesion –Logical Cohesion –Temporal Cohesion –Communication Cohesion –Sequential Cohesion –Functional Cohesion –Data Cohesion

Coincidental Cohesion This cohesion occurs when the components have no apparent reason for being grouped together In OO framework, we say that it occurs when a class consists of methods that are unrelated It is usually a sign of poor design in any software design approach.

Logical Cohesion This occurs when there is a logical connection among the elements of the software but no actual connection A library of mathematical functions might exhibit logical cohesion

Temporal Cohesion This occurs when the elements are bound together because they are all used at approximately the same time. The initialization routines of a compiler or other such software are usually a good example of temporal cohesion. A better OO approach would be to spread the initialization work over classes that would more closely be charged with subsequent behavior.

Communication Cohesion This occurs when methods are grouped because they all access the same data or devices. The “class” acts as a manager for the data or devices.

Sequential Cohesion This occurs when elements in a class are linked by the necessity to be activated in a particular order. A better design would be to raise the level of abstraction Since the sequentiality must be captured at some level, the goal is to hide this as much as possible from other levels of abstraction.

Functional Cohesion This is a desirable type of cohesion in which the elements of the component all relate to the achievement of a single goal. The measure of quality design at the level of a method is functional cohesion. Each method should have a clearly stated unifunctional goal achieved by that method.

Data Cohesion At the OO level, this is the measure of the quality of design of a class A class defines a set of data values and exports routines that manipulate the data. Data Cohesion occurs when a class is designed to implement a data abstraction.

Estimating Cohesion Write a brief statement of the Class (method). If the statement that describes the purpose of a class (method) is a compound statement containing more that one verb, the class is probably performing more than one function. It probably has sequential or communicating cohesion If the statement contains words relating to time, it probably has sequential or temporal cohesion. “Wait for... “ If the predicate of the statement does not contain a single specific object following the verb, it probably has logical cohesion. “Edit all data” If the statement contains words such as initialize or clean-up, it is probably temporal cohesion.