Patterns Singleton Memento Flyweight

8/9/2019 Patterns Singleton Memento Flyweight

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TCSS 360, Spring 2005

Lecture Notes

Design Patterns:

Singleton, Memento, l!"eig#t


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$utline

Singleton pattern

Seriali%a&le inter'ace an( persistent

o&)ects

l!"eig#t pattern


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Pattern: Singleton

a class that has only oneinstance


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*estricting o&)ect creation

pro&lem: sometimes "e "ill reall! onl! e+ernee( one instance o' a particular class eamples: -e!&oar( rea(er, &an- (ata collection "e.( li-e to ma-e it illegal to #a+e more t#an one,

)ust 'or sa'et!.s sa-e

"#! "e care:

creating lots o' o&)ects can ta-e a lot o' time etra o&)ects ta-e up memor! it is a pain to (eal "it# (i/erent o&)ects oating

aroun( i' t#e! are essentiall! t#e same


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Singleton pattern

singleton: an o&)ect t#at is t#e onl! o&)ect o' itst!pe ensures t#at a class #as at most one instance pro+i(es a glo&al access point to t#at instance

ta-es responsi&ilit! o' managing t#at instancea"a! 'rom t#e programmer illegal to constructmore instances

pro+i(e accessor met#o( t#at allo"s users tosee t#e one an( onl! instance

possi&l! t#e most -no"n popular (esignpattern4 t#is s#oul( tell !ou somet#ing


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*estricting o&)ects,continue(

$ne "a! to a+oi( creating o&)ects:use static met#o(s instea( Math, System, JOptionPane

is t#is a goo( alternati+e c#oice #! or "#!not

Pro&lem: lac-s ei&ilit! 7ample: static met#o(s can.t &e passe( as

an argument to a met#o(, nor returne(

Pro&lem: cannot &e eten(e(

7ample: static met#o(s can.t &e su&classe(an( o+erri((en li-e a sin leton.s coul( &e


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Singleton seuence(iagram


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consi(er a singleton class *an(om;eneratort#at generates ran(om num&ers

public class RandomGenerator {

private static RandomGenerator gen = newRandomGenerator()

public static RandomGenerator getInstance(){ return gen

!

private RandomGenerator(){!

public double ne"t#umber() {

return Math$random()

!

!

possi&le pro&lem: al"a!s creates t#e instance,e+en i' it isn.t use(

Singleton eample


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+ariation: (on.t create t#e instance untilnee(e(

%% Generates random numbers$

public class RandomGenerator { private static RandomGenerator gen & null

public static RandomGenerator getInstance() {

i' (gen && null)

gen & ne RandomGenerator()

return gen !

!

#at coul( go "rong "it# t#is +ersion

Singleton eample 2


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+ariation: sol+e concurrenc! issue &!loc-ing


public class RandomGenerator { private static RandomGenerator gen & null

public static synchronized RandomGenerator getInstance() { i' (gen && null)

gen & ne RandomGenerator() return gen

!

!

9s an!t#ing "rong "it# t#is +ersion

Singleton eample 3


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+ariation: sol+e concurrenc! issue "it#outunnecessar! loc-ing


public class RandomGenerator {

private static RandomGenerator gen & null

public static RandomGenerator getInstance() {

i' (gen && null) {

synchronized (RandomGenerator.class) { // must test again can you see why! i" (gen == null) gen & ne RandomGenerator()

# !

return gen

!!

Singleton eample o"> to output an( 'rom inputstreams

can represent man! (ata sources: Ales on #ar( (is-

anot#er computer on net"or-

"e& page input (e+ice

-e!&oar(, mouse, etc=


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Stream #ierarc#!

java.io.InputStream

udioInputStream

*ileInputStream

Ob+ectInputStream

java.io.OutputStream,yterrayOutputStream

*ileOutputStream

Ob+ectOutputStream


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Seriali%ation eample

recommen(ation: use a Memento class t#at #assa+eloa( co(e

%% rite the ob+ect named someObjectto 'ile .'ile$dat.

try {

OutputStream os & ne *ileOutputStream(.file.dat.)

Ob+ectOutputStream oos & ne $b%ect$utput&tream(os)

oos$write$b%ect(someObject)

os$close()

! catch (IO-"ception e) { $$$ !

%% load the ob+ect named someObject'rom 'ile .'ile$dat.

try {

InputStream is & ne *ileInputStream(.file.dat.)

Ob+ectInputStream ois & ne $b%ectInput&tream(is)

rray/ist some/ist & (rray/ist)ois$read$b%ect()

is$close()! catch (-"ception e) { $$$ !


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Memento seuence(iagram


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must implement t#e met#o(less+ava$io$Seriali0ableinter'ace 'or !our class to &ecompati&le "it# o&)ect inputoutput streams

public class ,an1ccount implements &erializable

{

$$$

ensure t#at all instance +aria&les insi(e !ourclass are eit#er seriali%a&le or (eclare(transient

transient Ael(s "on.t &e sa+e( "#en o&)ect is seriali%e(

Ma-ing !our classesseriali%a&le


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Let.s ma-e our singleton game mo(elseriali%a&le

#at can #appen i' a singleton is sa+e(an( loa(e( 9s it possi&le to #a+e more t#an one instance

o' t#e singleton.s t!pe in our s!stem Does t#is +iolate t#e Singleton pattern ill

it &rea- our co(e 9' so, #o" can "e A it

Bac- to singleton===


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Singleton eample 5

+ariation: #as strict c#ec-s to ma-e suret#at "e #a+e not sa+e( a stale re'erenceto t#e singleton o&)ect

%% Generates random numbers$public class RandomGenerator {

private static RandomGenerator gen & null

$$$

public double ne"t#umber() {

%% put code li1e this in methods that use%modi'y it i" (this '= gen) throw new Illegal&tateception(*not singleton*)+

return Math$random()

!

!


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Pattern: l!"eig#t

a class that has only one instance for eachunique state


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l!"eig#t an( Strings

l!"eig#te( strings a+a Strings are !"eig#te( &! t#e compiler "#ere+er

possi&le

can &e !"eig#te( at runtime "it# t#e internmet#o(

public class String2est { public static void main(String34 args) { String 'ly & .'ly.5 eight & .eight. String 'ly6 & .'ly.5 eight6 & .eight.

System$out$println('ly && 'ly6) %% true System$out$println(eight && eight6) %% true

String distinctString & 'ly 7 eight System$out$println(distinctString && .'lyeight.) %% 'alse

String 'lyeight & ('ly 7 eight)$intern() System$out$println('lyeight && .'lyeight.) %% true !!


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9mplementing a l!"eig#t

!"eig#ting "or-s &est on immutable o&)ects immutable: cannot &e c#ange( once constructe(

class pseu(o8co(e s-etc#:

pu&lic class FlyweightedE static map or ta&le o' instances

pri+ate constructor

static met#o( to get an instance i' "e #a+e create( t#is t!pe o' instance &e'ore, get it 'rom

map an( return it ot#er"ise, ma-e t#e ne" instance, store an( return it

F


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l!"eig#t seuence(iagram


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9mplementing a l!"eig#t

public class Flyweighted{

Map or table o' instances

private Flyweighted() {!

public static synchroni0ed FlyweightedgetInstance(Ob+ect 1ey) {

i' (8myInstances$contains(1ey)) {

Flyweighted' & ne Flyweighted(1ey)

myInstances$put(1ey5 ')

return ' ! else

return (Flyweighted)myInstances$get(1ey)

!

!


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Class &e'ore !"eig#ting

G class to &e !"eig#te(public class Point {

private int "5 y

public Point(int "5 int y) { this$" & " this$y & y

!

public int get9() { return this$" !

public int get:() { return this$y !

public String toString() {

return .(. 7 this$" 7 .5 . 7 this$y 7 .).

!

!


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Class a'ter !"eig#ting

G class t#at #as &een !"eig#te(4public class Point {

private static ,ap instances = new -ash,ap()+

public static oint getInstance(int int y) {

String 1ey & " 7 .5 . 7 y i' (instances$contains;ey(1ey)) %% re


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*e'erences

T#e a+a Tutorial: 9$ #ttp:)a+a=sun=com(ocs&oo-stutorialessentialio

a+a GP9 pages #ttp:)a+a=sun=com)2se1=

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Patterns Singleton Memento Flyweight