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Design Patterns

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Title: Design Patterns

1
Design Patterns

SWE 619
Software Construction
Fall 2008

2
Design Patterns

Why should we study patterns?
Standard solutions to common problems
Provide a vocabulary for programmers
Bag of tricks!
Essential for every programmer

3
What is a design pattern?

It is a design technique geared to a particular,
common problem
Example suppose you need to limit objects to
only single, logicially distinct instances
What does it provide?
Optimization (faster execution, lesser memory
storage)
Flexibility (remove dependency)
Easier for clients to use your classes

4
Downside?

Increased complexity
Patterns are not for free
Contrast better performance and flexibility of
use with increased difficulty in
implementing/maintaining your code
Judgment call, when to use patterns

5
Use of substitution principal

Many patterns use subtypes while relying on
substitution principal
Assume that subtypes will conform to super type
specs
Clients need not know which subtype is being used
Types can be changed during execution without
requiring client code to change

6
Types of patterns

Originally proposed by Erich Gamma et al. in GOF
book
Three main types of patterns
Creational patterns Factory pattern (PolyGen),
Singleton, Prototype etc.
Structural patterns Flyweight, Bridge, Adaptor,
Proxy, Façade etc.
Behavioral patterns Iterator, Command, Strategy,
Template etc.
Well look at patterns covered in Liskov 15

7
Factory Pattern

Hides details of object creation.
Why?
Do not want clients burdened with deciding which
object to use.
Client code is based on supertype specs
Easier to replace types
Use a factory method to create objects

8
Factory method

Is it a constructor? NO
Factory may call a constructor
Earlier client code
Poly p new DensePoly()
Poly q new SparsePoly()
New client code
Poly p PolyProcs.makePoly()
Poly q PolyProcs.makePoly()

P
Q
Client
P
Factory
Q
Client
9
Factory Method Benefits

Eliminates the needs to bind application classes
into your code.
Client deals with interfaces instead of specific
objects
Client may use any class that implements that
interface
Enables subclasses to provide extended versions
of an object
creating an object inside a factory is more
flexible than creating an object directly in the
client.

10
When to Use

When a class cannot anticipate the class of
objects it must create.
When a class wants its subclasses to specify the
objects it creates.
Classes delegate responsibility to one of several
helper subclasses, and you want to localize the
knowledge of which helper subclasses is the
delegate.

11
Creation of compatible types

Example
There are 4 types of objects- SparsePoly,
DensePoly, SparseMatrix and DenseMatrix
SparsePoly can only be used with SparseMatrix
DensePoly can only be used with DenseMatrix
How can we restrict client code to only use
compatible objects?
Solution Use related factories (f1, f2)
f1 only creates SPoly and SMatrix and f2 only
creates DPoly and DMatrix Figure 15.3

12
Compatible types fig 15.3
M
P
Poly
Matrix
Factory
F1
F2
SPoly
DPoly
SMatrix
DMatrix
13
Other Factory Patterns

Builder
Constructs a collection of objects
Prototype
Provides method to obtain new object of same type
as prototype
Differs from clone(), in that objects are
typically in an initial state
Iteration
Every Iterator is a factory method
Produces a generator object satisfying Iterator
interface

14
Flyweight Pattern

When
Lots of occurrences, need to be shared
Storage costs are high because of the quantity of
objects
The application doesn't depend on object
identity.
Naïve approach create new instance each time
Smarter approach Create only the first time
Benefit
(Possibly) Less memory usage
(Possibly) Better performance

15
Liskov example 15.4

public class Word
// Overview
private static Map map // maps strings to words
public static Word makeWord (String s) //
factory
//E returns the word corresponding to s
private Word (String s) // private to exclude
client
//E Makes this to be the word corr. to s
public String mapWord (Context c)
//E Returns the string corr. to this
// in the form suitable for context c

16
2 variations

Key table is hidden from the user
Client need not know if objects are being shared
Example on previous slide
Key table is visible to clients
When there are other uses for the table
E.g. clients need to iterate over all instances.
Example on next slide

17
Figure 15.5

public class Ident
// O
Ident (String s) // package private constructor
public class IdentTable
//O
public IdentTable() // Note public constructor
//E makes this empty IdentTable
public Ident makeReserved(String s) //Nonstatic
Ident factory
public Ident makeNonReserved (String s)
public Iterator idents() // publicly accessible
iterator

18
State Pattern

When to use
Change the rep periodically for your data
abstraction
E.g. Liskovs sets are small initially, so use
Vector for rep, but if grow to large sizes, use
HashSet for rep
How to handle this composite representation?

19
LiskovSet example

Rep
Before Vector els
Now Vector els HashSet h
One (undesirable) approach
Insert elements into both these objects gag!
Another (undesirable) approach
Declare els as
Object els (not Vector els or HashTable els).
Depending upon instance, call the right method.

20
Whats wrong with this approach?

Ugly!
Programmer is doing the job of runtime system
Need a better approach State pattern
Define a new interface SetState
Needs all the functionality to manipulate Set
SmallSet and BigSet implement the interface
Rep SetState els
Now, dispatching is done by runtime sys ?

21
Figure 15.7

public class Set //rename LiskovSet
private SetState els
private int t1 // threshold for growing
private int t2 // threshold for shrinking
public Set () els new SmallSet() // set
the thresholds
public boolean empy() return els.size() 0
public void insert (Object x)
int sz els.size()
els.insert(x)
if (sz t1 els.size() gtt1 !(els
instanceOf BigSet)
els new BigSet(els)
// note state switch in context object, eg Set

22
Fig 15.7 continued

public interface SetState
//O SetState is used to implement LiskovSet it
provides most
// of the Set methods. SetState objects are
mutable
public class SmallSet implements SetState
// implementation similar to that shown in fig
5.6
public class BigSet implements SetState

23
Alternate Approach Switch State in the State
Objects

Subtypes of state type could detect need for
change and modify containing object directly.
Implies state object (eg BigState) would need to
alter context object rep (eg els in Set)
Not necessarily a desirable dependency
if (state too small) // in BigState remove()
return new SmallState()
else
return this

24
Bridge Pattern