objecves - computer science department : …sprenkle/cs209/in_class/09-inclass.pdfobjecves •...

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Objec-ves• Collec-ons

Ø Maps

• Traversing• Excep-ons

Sept30,2016 Sprenkle-CSCI209 1

“Rather than teach everyone to code, �let's teach them to think. �The coding can come later; it's easier.” � - @rob_pike

On my Twitter feed:

Analysisofequals methods


public boolean equals(Object o){if(((Birthday) o).getDay() != this.getDay())

return false;

if( ((Birthday) o).getMonth() != this.getMonth())return false;

return true;}

public boolean equals(Object o) {Birthday other = (Birthday) o;

if (this.month == other.month && this.day == other.day)

return true;else

return false;}

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Usingbooleansinifstatements


if( this.equals(that) == true ) {System.out.println("equal!");

}

if( this.equals(that) ) {System.out.println("equal!");

}

Review• Whatarethe3componentsoftheJavaCollec-onFramework?

• Whatdatatypescancollec-onshold?• Howcanweconvertaprimi-vetypeintoitsrespec-veWrapperObjecttype?

• Whatisthesyntaxtosaywhattypethecollec-onholds?

• WhydidIwaitun-lnowtoshowyouEclipse?


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Eclipse• Veryhelpful–a"eryouknowwhatyou’redoing

Ø Givessugges-onsforfixes• Youneedtothinkthroughwhattheappropriatefixis


EclipseHints• A\eryouhavewri]enamethod,type

beforethemethod,andthenhitenterandtheJavadocstemplatewillbeautoma-callygeneratedforyou

• Usecommand-spacebarforpossiblecomple-ons


/**

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EclipseDiscussion• Helpfulhints

Ø Control-spacebarØ FormatthefileØ Auto-templatesforJavadocs


Review:Collec-onsFramework• Interfaces

Ø Abstractdatatypesthatrepresentcollec-onsØ Collec-onscanbemanipulatedindependentlyofimplementa-on

• Implementa.onsØ Concreteimplementa-onsofcollec-oninterfacesØ Reusabledatastructures

• AlgorithmsØ Methodsthatperformusefulcomputa-onsoncollec-ons,e.g.,searchingandsor-ng

Ø Reusablefunc-onalityØ Polymorphic:samemethodcanbeusedonmanydifferentimplementa-onsofcollec-oninterface


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Review:CoreCollec-onInterfaces• Encapsulatedifferenttypesofcollec-ons


MAPS


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Maps• Mapskeys(oftype<K>)tovalues(oftype<V>)

• NoduplicatekeysØ Eachkeymapstoatmostonevalue


Map Interface• <V> put(<K> key, <V> value)Ø Returns old value that key mapped to

• <V> get(Object key) Ø Returns value at that key (or null if no

mapping)

• Set<K> keySet() Ø Returns the set of keys


And more …

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A few Map Implementa-ons• HashMap

Ø Fast

• TreeMapØ Sor-ngØ Key-ordereditera-on

• LinkedHashMapØ FastØ Inser-on-orderitera-on


DeclaringMaps• Declaretypesforbothkeysandvalues• class HashMap<K,V>


Keys are StringsValues are Lists of Strings

Map<String, List<String>> map = new HashMap<>();

Keys are StringsValues are Integers

Map<String, Integer> map = new HashMap<>();

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ALGORITHMS


Collec-onsFramework’sAlgorithms• Polymorphicalgorithms• Reusablefunc-onality• ImplementedintheCollections class

Ø Sta-cmethods,1stargumentisthecollec-on

Ø SimilartoArraysclass,whichoperatesonarrays


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OverviewofAvailableAlgorithms• Sor-ng–op-onalComparator• Shuffling• Searching–binarySearch• Rou-nedatamanipula-on:reverse*,copy*,fill*,swap*,addAll

• Composi-on–frequency,disjoint• Findingmin,max


* Only Lists

TRAVERSINGCOLLECTIONS


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TraversingCollec-ons:For-eachLoop• For-eachloop:

• ValidforallCollectionsØ Maps (anditsimplementa-ons)arenotCollections• But,Map’skeySet()isaSet andvalues()isaCollection


for (Object o : collection) System.out.println(o);

Or whatever data type is appropriate


Iterator:LikeaCursor• Alwaysbetweentwoelements

Iterator<Integer> i = list.iterator();while( i.hasNext()) {

int value = i.next();…

}

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Iterator API • <E> next()

Ø Getthenextelement

•  boolean hasNext() Ø Aretheremoreelements?

•  void remove() Ø RemovethepreviouselementØ Onlysafewaytoremoveelementsduringitera-on

• Notknownwhatwillhappenifremoveelementsinfor-eachloop



PolymorphicFilterAlgorithm

static void filter(Collection c) { Iterator i = c.iterator();

while( i.hasNext() ) {// if the next element does not// adhere to the condition, remove it if ( ! condition(i.next()) ) {

i.remove();}

}}

Polymorphic: works regardless of Collection implementation

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TraversingLists:ListIterator• Methodstotraverselistbackwardstoo

Ø hasPrevious()Ø previous()

• To get a ListIterator: Ø listIterator(int position)

• Passinsize()asposi-ontogetatendoflist

Key difference

HowNottoIterate• Don’tusegettoaccessList

Ø Ifimplementa-onisaLinkedList,performanceisreeeeeallyslow


for (int i = 0; i < list.size(); i++) {count += list.get(i); // do something

}

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BenefitsofCollec-onsFramework• ?


BenefitsofCollec-onsFramework• Providescommon,well-knowninterface

Ø  AllowsinteroperabilityamongunrelatedAPIsØ  ReducesefforttolearnandtousenewAPIsfordifferent

implementa-ons• Reducesprogrammingeffort:providesuseful,reusabledatastructuresandalgorithms

•  Increasesprogramspeedandquality:provideshigh-performance,high-qualityimplementa-onsofdatastructuresandalgorithms;interchangeableimplementa-onsàtuning

• ReducesefforttodesignnewAPIs:usestandardcollec-oninterfaceforyourcollec-on

• FosterssoMwarereuse:Newdatastructures/algorithmsthatconformtothestandardcollec-oninterfacesarereusable


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EXCEPTIONS



Errors• Programsencountererrorswhentheyrun

Ø UsersmayenterdatainthewrongformØ FilesmaynotexistØ Programcodehasbugs!*

• Whenanerroroccurs,aprogramshoulddooneoftwothings:Ø Reverttoastablestateandcon-nueØ Allowtheusertosavedataandthenexittheprogramgracefully

* (Of course, not your programs)

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JavaMethodBehavior• Normal/correctcase:returnspecifiedreturntype• Errorcase:doesnotreturnanything,throws anExceptionØ Anexcep-onisaneventthatoccursduringexecu-onofaprogramthatdisruptsnormalflowofprogram'sinstruc-ons

Ø Exception: objectthatencapsulateserrorinforma-on

Similar to Python


Prin-ngStackTraceExample

How helpful is this output?How user friendly is it?

java.io.FileNotFoundException: fred.txtat java.io.FileInputStream.<init>(FileInputStream.java)at java.io.FileInputStream.<init>(FileInputStream.java)at ExTest.readMyFile(ExTest.java:19)at ExTest.main(ExTest.java:7)

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Prin-ngStackTraceExample

•  Usefulfordebuggingyourcode•  Generate/displayuser-friendlyerrorsinfinishedproduct

•  O\enrequires“higher-levelcode”tohandleexcep-on


java.io.FileNotFoundException: fred.txtat java.io.FileInputStream.<init>(FileInputStream.java)at java.io.FileInputStream.<init>(FileInputStream.java)at ExTest.readMyFile(ExTest.java:19)at ExTest.main(ExTest.java:7)

How helpful is this output?How user friendly is it?


Excep-onClassifica-on:Error• Aninternalerror• Strongconven-on:reservedforJVM

Ø JVM-generatedwhenresourceexhaus-onoraninternalproblem• Example:OutofMemoryerror

• Program’scodeshouldnotandcannotthrowanobjectofthistype

• Uncheckedexcep-on

When can that happen in Java?

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Excep-onClassifica-on:Exception1.  RuntimeException:somethingthat

happensbecauseofaprogrammingerrorØ Uncheckedexcep-onØ Examples:ArrayOutOfBoundsException, NullPointerException, ClassCastException

2.   Checkedexcep-onsØ Awell-wri]enapplica-onshouldan-cipateandrecoverfrom• Compilerenforces

Ø Examples:IOException, SQLException

Error


Excep-onClassifica-onThrowable

Exception

IOExceptionRuntimeException

SQLException

Others…

UncheckedCh

ecked

Checked

Checked: All non- RuntimeExceptions

Partofjava.lang package

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TypesofExcep-onsUnchecked• Anyexcep-onthatderivesfromError orRuntimeException

•  Programmerdoesnotcreate/handle

•  Trytomakesurethattheydon’toccur

• O\enindicatesprogrammererrorØ  E.g.,precondi-onviola-ons,

notusingAPIcorrectly

Checked• Anyotherexcep-on•  Programmercreatesandhandlescheckedexcep-ons

• Compiler-enforcedcheckingØ  Improvesreliability*

•  Forcondi-onsfromwhichcallercanreasonablybeexpectedtorecover


TypesofUncheckedExcep-ons1.  DerivedfromtheclassError

Ø Anylineofcodecangeneratebecauseitisaninternalerror

Ø Don’tworryaboutwhattodoifthishappens2.  DerivedfromtheclassRuntimeException

Ø IndicatesabugintheprogramØ FixthebugØ Examples:ArrayOutOfBoundsException, NullPointerException, ClassCastException


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CheckedExcep-ons• Needtobehandledbyyourprogram

Ø Compiler-enforcedØ Improvesreliability*

• Foreachmethod,tellthecompiler:Ø WhatthemethodreturnsØ Whatcouldpossiblygowrong

• Adver9setheexcep-onsthatamethodthrows• Helpsusersofyourinterfaceknowwhatmethoddoesandletsthemdecidehowtohandleexcep-ons


Discussion:WhyCheckedandUncheckedExcep-ons?

• Whydowehaveexcep-onsthatthecompilerdoesn’tforcetheprogrammertocheck?Ø Thinkaboutexamplesofuncheckedexcep-ons(ArrayOutOfBoundsException, NullPointerException, ClassCastException)andwhenthoseexcep-onscanoccur


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THROWINGEXCEPTIONS


CommonExcep-onsName PurposeIllegalArgumentException Whencallerpassesininappropriate

argumentIllegalStateException Invoca-onisillegalbecauseofreceiving

object’sstate.(Ex:closingaclosedwindow)

• BothinheritfromRuntimeException• Mayseemlikethesecovereverythingbutonlyusedforcertainkindsofillegalargumentsandexcep-ons

• NotusedwhenØ  Anullargumentpassedin;shouldbeaNullPointerException

Ø  Passininvalidindexforanarray;shouldbeanIndexOutOfBoundsException


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FactorialAlterna-ves


public static double factorial( int x ) {if( x < 0 )

return 0.0;double fact = 1.0;while( x > 1 ) {

fact *= x;x--;

}return fact;

}




throw new IllegalArgumentException("x" +"must be >= 0");

double fact = 1.0;while( x > 1 ) {

fact *= x;x--;

}return fact;

}

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throw new IllegalArgumentException("x" +"must be >= 0");

double fact = 1.0;while( x > 1 ) {

fact *= x;x--;

}return fact;

}

What are the pros and cons of these approaches?

IllegalArgumentException:Thrown to indicate that a method has been passed an illegal or inappropriate

argument

Note, no throws clause in method signature. Why?

Goal:FailureAtomicity• A\eranobjectthrowsanexcep-on,theobjectshouldbeinawell-defined,usablestateØ Afailedmethodinvoca-onshouldleaveobjectinstatepriortoinvoca-on

• Approaches:Ø Checkparameters/statebeforeperformingopera-on(s)Ø Dothefailure-proneopera-onsfirstØ Userecoverycodeto“rollback”stateØ Applytotemporaryobjectfirst,thencopyovervalues


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Prac-ce

• Howshouldweimplementthismethod?• Whataresomeproblemswecouldface?


public void setBirthday(int month, int day) { }

Prac-ce

• Howshouldweimplementthismethod?Ø Ruleofthumb:Handleerrorcheckingfirst


public void setBirthday(int month, int day) { }

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CATCHINGEXCEPTIONS



Try/CatchBlock

• Thesimplestwaytocatchanexcep-on• Syntax:

try {code;more code;

} catch (ExceptionType e) {

error code for ExceptionType;}catch (ExceptionType2 e) {

error code for ExceptionType2;}…

Python equivalent?

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Try/CatchBlock

• Codeintryblockrunsfirst• Iftryblockcompleteswithoutanexcep-on,catchblock(s)arenotexecuted

• Iftrycodegeneratesanexcep-onØ AcatchblockrunsØ Remainingcodeintryblockisnotexecuted

• Ifanexcep-onofatypeotherthanExceptionType isthrowninsidetryblock,methodexitsimmediately*



error code forExceptionType

}


Try/CatchBlock

• YoucanhavemorethanonecatchblockØ Tohandle>1typeofexcep-on

• Ifexcep-onisnotoftypeExceptionType1,fallstoExceptionType2,andsoforthØ Runthefirstmatchingcatchblock



error code forExceptionType

}catch (ExceptionType2 e) {

error code for ExceptionType2

}

Can catch any exception with Exception e but won’t have customized messages

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Try/CatchExamplepublic void read(BufferedReader in) {

try {boolean done = false;while (!done) {

String line=in.readLine();// above could throw IOException!if (line == null)

done = true;}

} catch (IOException ex) {

ex.printStackTrace();}

}Prints out stack trace to method call

that caused the error


Try/CatchExamplepublic void read(BufferedReader in) {

try {boolean done = false;while (!done) {

String line=in.readLine();// above could throw IOException!if (line == null)

done = true;}

} catch (IOException ex) {

ex.printStackTrace();}

}More precise catch may help pinpoint error

But could result in messier code

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Thefinally Block

• Allowsyoutocleanupordomaintenancebeforemethodends(onewayortheother)Ø E.g.,closingfilesordatabaseconnec-ons

try {…

} catch (Exception e) {

…}finally {

…}

• Op-onal:addafinallyblocka\erallcatchblocksØ Codeinfinally blockalwaysrunsa\ercodeintry and/orcatchblocks• A\ertry blockfinishesor,ifanexcep-onoccurs,a\erthecatchblockfinishes

FinallyTest.java


Prac-ce:try/catch/finally Blocks

try {statement1;statement2;

} catch (EOFException e) {

statement3;statement4;

}finally {

statement5;}

• Whichstatementsrunif:Ø Neitherstatement1norstatement2throwsanexcep-on

Ø statement1throwsanEOFException

Ø statement2throwsanEOFException

Ø statement1throwsanIOException

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WhattodowithaCaughtExcep-on?• Dumpthestacka\ertheexcep-onoccurs

Ø Whatelsecanwedo?

• Generally,twoop-ons:1.  Catchtheexcep-onandrecoverfromit2.  Passexcep-onuptowhoevercalledit


Summary:MethodsThrowingExcep-ons• APIdocumenta-ontellsyouifamethodcanthrowanexcep-onØ Ifso,youmusthandleit

• Ifyourmethodcouldpossiblythrowanexcep-on(bygenera-ngitorbycallinganothermethodthatcould),adver-seit!Ø Ifyoucan’thandleeveryerror,that’sOK…letwhoeveriscallingyouworryaboutit

Ø However,theycanonlyhandletheerrorifyouadver-setheexcep-onsyoucan’tdealwith


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ProgrammingwithExcep-ons• Excep-onhandlingisslow

• Useonebigtry blockinsteadofnes-ngtry-catch blocksØ SpeedsupExcep-onHandlingØ Otherwise,codegetstoomessy

• Don'tignoreexcep-ons(e.g.,catch blockdoesnothing)Ø Be]ertopassthemalongtohighercalls


try {}catch () { }try {}catch () { }

try {try {}catch () { }

}catch () { }

try {……

}catch () { }


BenefitsofExcep-ons?

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BenefitsofExcep-ons• Forceerrorchecking/handling

Ø Otherwise,won’tcompileØ Doesnotguarantee“good”excep-onhandling

• EasedebuggingØ Stacktrace

• Separateserror-handlingcodefrom“regular”codeØ ErrorcodeisincatchblocksatendØ Descrip-vemessageswithexcep-ons

• PropagatemethodsupcallstackØ Letwhoever“cares”abouterrorhandleit

• Groupanddifferen-ateerrortypes

JavadocGuidelinesabout@throws• Alwaysreportifthrowcheckedexcep-ons• Reportanyuncheckedexcep-onsthatthecallermightreasonablywanttocatchØ Excep-on:NullPointerExceptionØ Allowscallertohandle(ornot)Ø Documentexcep-onsthatareindependentoftheunderlyingimplementa-on

• Errorsshouldnotbedocumentedastheyareunpredictable


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TODO• Assignment6:DueWednesday

Ø ModifyingMediaItemclasses• Addingimplementa-onofComparable• UsingsomeCollec-on(ofyourchoice)tomaintainlibraryØ Jus-fy/explainyourchoice

• Exam1Ø Prepara-ondocumentposted


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