Transcript cs441-03-dpexl-ch5

CS 4240: Introduction to Design
Patterns
Readings:
Chap. 5 of Design Patterns Explained
 Also, Chap. 6 and 7
 Also, on-line info on Composite pattern
(and others) at http://www.oodesign.com
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Readings
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Chapter 5 of our textbook
Handouts on various patterns
Background, Definition
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From Alexander’s work in architecture
Alexander: “a solution to a problem in
context.”
Design pattern: a description of a problem
that reoccurs and an outline of an approach
to solving that problem
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Generally domain, language independent
Also, analysis patterns
Design Patterns: Essential
Elements
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Pattern name
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Intent
Problem
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When to apply the pattern, what context.
How to represent, organize components
Conditions to be met before using
Solution
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A vocabulary of patterns is beneficial
Design elements: relationships, responsibilities,
collaborations
A template for a solution that you implement
Consequences
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Results and trade-offs that result from using the pattern
Needed to evaluate design alternatives
Essential Elements (con’td)
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Participants and collaborators
Implementation
Generic Structure
Patterns Are (and Aren’t)
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Name and description of a proven solution to
a problem
Documentation of a design decision
They’re not:
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Reusable code, class libraries, etc. (At a higher
level)
Do not require complex implementations
Always the best solution to a given situation
Simply “a good thing to do”
Describing Design Patterns
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The GoF defined a standard format
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Generally followed
Not just a UML diagram!
Pattern Format (13 sections):
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Pattern name and classification
Intent: what’s it do? Rationale?
Also known as
Motivation
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Applicability
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A scenario that illustrates a sample problem and how this
patterns helps solve it.
For which situations can this be applied?
Structure
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Graphical representation (e.g. UML)
Pattern Format (cont’d)
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Participants
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Collaborations
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Code fragments that illustrate the pattern
Known uses
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Pitfalls, hints, techniques, language issues
Sample code
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How participants interact
Consequences
Implementation
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Classes and objects, their responsibilities
From real systems
Related patterns
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Similar patterns, collaborating patterns
Example 1: Singleton Pattern
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Global variables are bad!
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Why?
We often want to use global variables!
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Why?
Example 1: Singleton Pattern
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Context: Only one instance of a class is created.
Everything in the system that needs this class
interacts with that one object.
Controlling access: Make this instance accessible to
all clients
Solution:
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The class has a static variable called theInstance (etc)
The constructor is made private (or protected)
Clients call a public operation getInstance() that returns the
one instance
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This may construct the instance the very first time or be given
an initializer
Singleton: Java implementation
public class MySingleton {
private static MySingleton theInstance =
new MySingleton();
private MySingleton() { // constructor
…
}
public static MySingleton getInstance() {
return theInstance;
}
}
Static Factory Methods
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Singleton patterns uses a static factory method
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Advantages over a public constructor
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Factory: something that creates an instance
They have names. Example:
BigInteger(int, int, random) vs.
BigInteger.probablePrime()
Might need more than one constructor with same/similar
signatures
Can return objects of a subtype (if needed)
Wrapper class example:
Double d1 = Double .valueOf(“3.14”);
Double d2 = new Double (“3.14”);
More info: Bloch’s Effective Java
Examples from Java Library
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In Swing, you can create a Border and
add it to a component
Border b =
BorderFactory.createBevelBorder(3);
 Need the exact same border again?
 The above method doesn’t create a new
one. Just gives you the same one
 Note: works because the bevel is
immutable
Example from the Java Library
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Java can manipulate its own class
definitions
Example:
Class classObj =
Class.forName(“edu.uva.cs441.MyClass”);
 Then we can do things to the Class
object
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What if we call this again?
Only one Class object per Java-class
Example #2: PlayLists and Songs
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We saw this in CS201 (maybe)
Could PlayLists “Nest”?
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Could a PlayList contain Songs and
other PlayLists?
This is the “Composite Design Pattern”
Make PlayList contain
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A list of PlayableItems, not Songs
PlayList.play() calls play() on each item
Polymorphism at work
Class Diagram for This Composite
General Class Diagram
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Could be many Leaf classes
Polymorphism in Action
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Playlist contains a list of PlayableItems
PlayList.play() calls play() on each item
for (int i=0; i<list.size(); ++i) {
( (PlayableItem) list.get(i) ).play();
}
Will call the Song.play() or PlayList.play()
depending on what item i really is
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Review question: why is the cast needed?
Façade Pattern
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Intent:
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Problem:
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Simplify interface to set of interfaces in an
existing subsystem.
Or, need to define new/variant interface
Only need part of a complex system.
Or, need to interact in a particular way.
Participants, collaborators:
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Façade class, subsystem classes, client
class
Façade Pattern (cont’d)
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Consequences:
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Certain subsystem operations may not be
available in the façade
Clients may “go around” the facade
Note in textbook
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Reduce coupling for client
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Existing interfaces requires client to
access many objects to carry out task
Façade object may be one object that’s a
“gateway” to multiple objects
In other words: the Façade encapsulates
the system
Another motto: find what varies and
encapsulate it
Examples:
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Java’s URL class
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Really hides a set of objects and methods
But not easy to see that it does this IMHO
An example from our recent CS1110
textbook
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Make I/O easier for beginners
Class StdIn: static methods to read
Class In: create object to read from
keyboard, file or URL
Class In from Princeton course text
See Javadoc here:
http://www.cs.princeton.edu/introcs/stdlib/javadoc/In.html
Encapsulate Scanner, simplify methods
public final class In {
private Scanner scanner;
...
public In() {
scanner = new Scanner(
new BufferedInputStream(System.in), charsetName);
scanner.useLocale(usLocale);
}
...
/**
* Is the input stream empty?
*/
public boolean isEmpty() {
return !scanner.hasNext();
}
Create “reader” from filename or URL
public In(String s) {
try {
// first try to read file from local file system
File file = new File(s);
if (file.exists()) {
scanner = new Scanner(file, charsetName);
scanner.useLocale(usLocale);
return;
}
// next try for files included in jar
URL url = getClass().getResource(s);
// or URL from web
if (url == null) { url = new URL(s); }
URLConnection site = url.openConnection();
InputStream is
= site.getInputStream();
scanner
= new Scanner(
new BufferedInputStream(is), charsetName);
scanner.useLocale(usLocale);
}
catch (IOException ioe) {
System.err.println("Could not open " + s);
}
}
Think About and Discuss
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Façade: any relation to the Law of
Demeter?
“Principle of Least Knowledge”
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Talk only your immediate friends
Adaptor Pattern
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Intent:
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Problem:
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Make an existing object match an existing
interface that it doesn’t match
Existing component has right functionality but
doesn’t match the interface used
Interface here? Abstract class (or Java-interface)
used polymorphically
Solution:
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Wrapper class with delegation etc.
Two flavors: Object Adaptor vs. Class Adaptor
Adaptor Pattern (cont’d)
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Participants and Collaborators
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Adapter class: “new” interface for…
Adaptee class (existing class)
Target: the needed interface
Client who uses objects of type Target
Consequences:
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The good: solves problem and meets
intent above!
Some other wrinkles…
Class Diagram for Object Adaptor
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Object Adaptor pattern
Note that Adapter is an
example of a “wrapper class”
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A common implementation technique
used in several design patterns
Book’s Example
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Abstract class (or interface) Shape
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We need a Circle class
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Methods include: display(), fill()
Our code relies on Shape hierarchy and
polymorphism
Sweet! Another one exists! XXCircle
Bummer! Method names are different
Solution: see book or notes on board
Class Adaptor Pattern
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Don’t create a separate object that
wraps the existing object
Instead, create new class that
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Includes what existing class has (inherit)
Also meets Target interface
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With a Java interface, or multiple inheritance in
other languages
Class Adaptor example
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Problem:
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Existing code has a CommandBuffer class
Doesn’t have an Iterator
You have code that could do great things if
only you could use CommandBuffer as if it
were Iterable
Example of Class Adaptor pattern
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The new class implements Iterator methods in terms
of getCommand() etc defined in existing class
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No pair of objects: just an instance of new
class
Think About and Discuss
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Class adaptors are different than object
adaptors
Does this difference relate to a “rule” or
“principle” of OO design we’ve seen?
Comparing Façade and Adaptor
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Similarities:
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Adaptor: existing interface we must design to
(not for Façade)
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Both designed to help us use existing classes
better
Both can use wrapper objects
Similar implementation solves different problems
New object used polymorphically
Façade: need simpler interface (not a goal
for Adaptor)
See table 7-1 on page 110
Summary of Intro to Design
Patterns
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Understand background, general
principles of design patterns
Examples:
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Singleton
Composite
Façade
Adaptor
Recognize them, apply them in simple
situations
Exercise: Can you Adapt?
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Deprecated Java interface Enumerator
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hasMoreElements()
nextElement()
Replaced by interface Iterator
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hasNext()
next()
remove()
Your task
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Create a class: EnumerationIterator
This let us design and write new code
that works with Iterators, and
also interact with legacy code that
exposes Enumerators for use to use
First question: class or object adaptor?
Questions to Answer
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Question: which classes play which roles in
the design pattern? Write this down.
Final question: write Java class with
constructor and next() and hasNext().