03-object-oriented programming in javacharlie/courses/17-214/2020-spring/slides... · 21/1/2020...
TRANSCRIPT
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PrinciplesofSoftwareConstruction:Objects,Design,andConcurrency
Object-OrientedProgramminginJava
JoshBloch CharlieGarrod
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Administrivia
• Homework1dueThursday11:59p.m.– Everyonemustreadandsignourcollaborationpolicy
• FirstreadingassignmentdueTuesday– EffectiveJavaItems15and16
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KeyconceptsfromThursday
• Bipartitetypesystem– primitives&objectrefs– Singleimplementationinheritance–Multipleinterfaceinheritance
• Easiestoutput– println ,printf• Easiestinput– Commandlineargs,Scanner• Collectionsframeworkispowerful&easytouse
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Outline
I. Object-orientedprogrammingbasicsII. InformationhidingIII. Exceptions
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Objects
• Anobject isabundleofstateandbehavior• State– thedatacontainedintheobject– InJava,thesearethefields oftheobject
• Behavior– theactionssupportedbytheobject– InJava,thesearecalledmethods–MethodisjustOO-speakforfunction– Invokeamethod=callafunction
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Classes
• Everyobjecthasaclass– Aclassdefinesmethodsandfields–Methodsandfieldscollectivelyknownasmembers
• Classdefinesbothtypeandimplementation– Type≈wheretheobjectcanbeused– Implementation≈howtheobjectdoesthings
• Looselyspeaking,themethodsofaclassareitsApplicationProgrammingInterface(API)– Defineshowusersinteractwithinstances
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Classexample– complexnumbersclass Complex {private final double re; // Real Partprivate final double im; // Imaginary Part
public Complex(double re, double im) {this.re = re;this.im = im;
}
public double realPart() { return re; }public double imaginaryPart() { return im; }public double r() { return Math.sqrt(re * re + im * im); }public double theta() { return Math.atan(im / re); }
public Complex add(Complex c) {return new Complex(re + c.re, im + c.im);
}public Complex subtract(Complex c) { ... }public Complex multiply(Complex c) { ... }public Complex divide(Complex c) { ... }
}
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Classusageexamplepublic class ComplexUser {
public static void main(String args[]) {Complex c = new Complex(-1, 0);Complex d = new Complex(0, 1);
Complex e = c.plus(d);System.out.println(e.realPart() + " + "
+ e.imaginaryPart() + "i");e = c.times(d);System.out.println(e.realPart() + " + "
+ e.imaginaryPart() + "i");}
}
Whenyourunthisprogram,itprints-1.0 + 1.0i-0.0 + -1.0i
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Interfacesandimplementations
• MultipleimplementationsofAPIcancoexist–MultipleclassescanimplementthesameAPI– Theycandifferinperformanceandbehavior
• InJava,anAPIisspecifiedbyinterfaceorclass– InterfaceprovidesonlyanAPI– ClassprovidesanAPIandanimplementation– Aclasscanimplementmultipleinterfaces
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Aninterfacetogowithourclasspublic interface Complex {
// No constructors, fields, or implementations!
double realPart();double imaginaryPart();double r();double theta();
Complex plus(Complex c);Complex minus(Complex c);Complex times(Complex c);Complex dividedBy(Complex c);
}
AninterfacedefinesbutdoesnotimplementAPI
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Modifyingclasstouseinterfaceclass OrdinaryComplex implements Complex {final double re; // Real Partfinal double im; // Imaginary Part
public OrdinaryComplex(double re, double im) {this.re = re;this.im = im;
}
public double realPart() { return re; }public double imaginaryPart() { return im; }public double r() { return Math.sqrt(re * re + im * im); }public double theta() { return Math.atan(im / re); }
public Complex add(Complex c) {return new OrdinaryComplex(re + c.realPart(), im + c.imaginaryPart());
}public Complex subtract(Complex c) { ... }public Complex multiply(Complex c) { ... }public Complex divide(Complex c) { ... }
}
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Modifyingclienttouseinterfacepublic class ComplexUser {
public static void main(String args[]) {Complex c = new OrdinaryComplex(-1, 0);Complex d = new OrdinaryComplex(0, 1);
Complex e = c.plus(d);System.out.println(e.realPart() + " + "
+ e.imaginaryPart() + "i");e = c.times(d);System.out.println(e.realPart() + " + "
+ e.imaginaryPart() + "i");}
}
Whenyourunthisprogram,itstillprints-1.0 + 1.0i-0.0 + -1.0i
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Interfacepermitsmultipleimplementationsclass PolarComplex implements Complex {final double r;final double theta;
public PolarComplex(double r, double theta) {this.r = r;this.theta = theta;
}
public double realPart() { return r * Math.cos(theta) ; }public double imaginaryPart() { return r * Math.sin(theta) ; }public double r() { return r; }public double theta() { return theta; }
public Complex plus(Complex c) { ... } // Completely different implspublic Complex minus(Complex c) { ... }public Complex times(Complex c) { ... }public Complex dividedBy(Complex c) { ... }
}
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public class ComplexUser {public static void main(String args[]) {
Complex c = new PolarComplex(1, Math.PI); // -1Complex d = new PolarComplex(1, Math.PI/2); // i
Complex e = c.plus(d);System.out.println(e.realPart() + " + "
+ e.imaginaryPart() + "i");e = c.times(d);System.out.println(e.realPart() + " + "
+ e.imaginaryPart() + "i");}
}
Whenyourunthisprogram,itstill prints-1.0 + 1.0i-0.0 + -1.0i
Interfacedecouplesclientfromimplementation
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Whymultipleimplementations?
• Differentperformance– Chooseimplementationthatworksbestforyouruse
• Differentbehavior– Chooseimplementationthatdoeswhatyouwant– Behaviormust complywithinterfacespec(“contract”)
• Oftenperformanceandbehaviorboth vary– Providesafunctionality– performancetradeoff– Example:HashSet,LinkedHashSet,TreeSet
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Interfacesandclasses– thebigpicture
• Interfacesdefinetypes– Specifywhatfunctionalityisprovidedbyinstances– Thesearetheexpectationsimplementationsmustmeet
• Classesdefineimplementations(andtypes)– Describehow instancesmeetexpectations
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Preferinterfacestoclassesastypes…butdon’toverdoit
• Useinterfacetypesforparametersandvariablesunlessasingleimplementationwillsuffice– Supportschangeofimplementation– Preventsdependenceonimplementationdetails
• Butsometimesasingleimplementationwillsuffice
Set<Criminal> senate = new HashSet<>(); // Do this…HashSet<Criminal> senate = new HashSet<>(); // Not this
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Checkyourunderstandinginterface Animal {
void vocalize();}
class Dog implements Animal {public void vocalize() { System.out.println("Woof!"); }
}
class Cow implements Animal { public void vocalize() { moo(); }public void moo() { System.out.println("Moo!"); }
}
WhatHappens?1. Animal a = new Animal(); a.vocalize();2. Dog b = new Dog(); b.vocalize();3. Animal c = new Cow(); c.vocalize();4. Animal d = new Cow(); d.moo();
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Historicalnote:simulationandtheoriginsofOOprogramming• Simula 67wasthefirstobject-orientedlanguage
• DevelopedbyKristinNygaardandOle-JohanDahlattheNorwegianComputingCenter
• Developedtosupportdiscrete-eventsimulation– Application:operationsresearch,e.g.trafficanalysis– Extensibilitywasakeyqualityattributeforthem– Codereusewasanother
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Outline
I. Object-orientedprogrammingbasicsII. InformationhidingIII. Exceptions
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Informationhiding
• Singlemostimportantfactorthatdistinguishesawell-designedmodulefromabadoneisthedegreetowhichithidesinternaldataandotherimplementationdetailsfromothermodules
• Well-designedcodehidesall implementationdetails– CleanlyseparatesAPIfromimplementation–Modulescommunicateonly throughAPIs– Theyareoblivioustoeachothers’innerworkings
• Knownasinformationhidingorencapsulation• Fundamentaltenetofsoftwaredesign[Parnas,‘72]
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Benefitsofinformationhiding
• Decouples theclassesthatcompriseasystem– Allowsthemtobedeveloped,tested,optimized,used,understood,andmodifiedinisolation
• Speedsupsystemdevelopment– Classescanbedevelopedinparallel
• Easesburdenofmaintenance– Classescanbeunderstoodmorequicklyanddebuggedwithlittlefearofharmingothermodules
• Enableseffectiveperformancetuning– “Hot”classescanbeoptimizedinisolation
• Increasessoftwarereuse– Loosely-coupledclassesoftenproveusefulinothercontexts
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Informationhidingwithinterfaces
• Declarevariablesusinginterfacetypes• Clientcanuseonlyinterfacemethods• Fieldsnotaccessiblefromclientcode• Butthisonlytakesussofar– Clientcanaccessnon-interfacemembersdirectly– Inessence,it’svoluntaryinformationhiding
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MandatoryInformationhidingvisibilitymodifiers formembers
• private – Accessibleonly fromdeclaringclass• package-private– Accessiblefromanyclassinthepackagewhereitisdeclared– Technicallyknownasdefaultaccess– Yougetthisifnoaccessmodifierisspecified
• protected – Accessiblefromsubclassesofdeclaringclass(andwithinpackage)
• public – Accessiblefromanywhere
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HidinginteriorstateinOrdinaryComplex
class OrdinaryComplex implements Complex {private double re; // Real Partprivate double im; // Imaginary Part
public OrdinaryComplex(double re, double im) {this.re = re;this.im = im;
}
public double realPart() { return re; }public double imaginaryPart() { return im; }public double r() { return Math.sqrt(re * re + im * im); }public double theta() { return Math.atan(im / re); }
public Complex add(Complex c) {return new OrdinaryComplex(re + c.realPart(), im + c.imaginaryPart());
}public Complex subtract(Complex c) { ... }public Complex multiply(Complex c) { ... }public Complex divide(Complex c) { ... }
}
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Bestpracticesforinformationhiding
• CarefullydesignyourAPI• Provideonly functionalityrequiredbyclients– All othermembersshouldbeprivate
• Youcanalwaysmakeaprivatememberpubliclaterwithoutbreakingclients– Butnotvice-versa!
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Outline
I. Object-orientedprogrammingbasicsII. InformationhidingIII. Exceptions
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Whatdoesthiscodedo?FileInputStream fIn = new FileInputStream(fileName);if (fIn == null) {switch (errno) {case _ENOFILE:
System.err.println(“File not found: “ + …);return -1;
default:System.err.println(“Something else bad happened: “ + …);return -1;
}}DataInput dataInput = new DataInputStream(fIn);if (dataInput == null) {System.err.println(“Unknown internal error.”);return -1; // errno > 0 set by new DataInputStream
}int i = dataInput.readInt();if (errno > 0) {System.err.println(“Error reading binary data from file”);return -1;
} // The Slide lacks space to close the file. Oh well.return i;
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Whatdoesthiscodedo?FileInputStream fIn = new FileInputStream(fileName);if (fIn == null) {switch (errno) {case _ENOFILE:
System.err.println(“File not found: “ + …);return -1;
default:System.err.println(“Something else bad happened: “ + …);return -1;
}}DataInput dataInput = new DataInputStream(fIn);if (dataInput == null) {System.err.println(“Unknown internal error.”);return -1; // errno > 0 set by new DataInputStream
}int i = dataInput.readInt();if (errno > 0) {System.err.println(“Error reading binary data from file”);return -1;
} // The Slide lacks space to close the file. Oh well.return i;
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There’sabetterway:exceptionsFileInputStream fileInput = null;
try {fileInput = new FileInputStream(fileName);DataInput dataInput = new DataInputStream(fileInput);return dataInput.readInt();
} catch (IOException e) {System.err.println("Could not read file: " + e);return DEFAULT_VALUE;
}
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Exceptions
• Informcallerofproblembytransferofcontrol• Semantics– Propagatesupstackuntilmainmethodisreached(terminatesprogram),orexceptioniscaught
• Sources– Programcanthrowexplicitly– Underlyingvirtualmachine(JVM)cangenerate
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Control-flowofexceptionspublic static void main(String[] args) {
try {test();
} catch (IndexOutOfBoundsException e) {System.out.println"("Caught index out of bounds");
}}
public static void test() {try {
System.out.println("Top");int[] a = new int[10];a[42] = 42; // Index is too high; throws exceptionSystem.out.println("Bottom");
} catch (NegativeArraySizeException e) {System.out.println("Caught negative array size");
}}
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Benefitsofexceptions
• Youcan’tforgettohandlecommonfailuremodes– Compare:usingaflagorspecialreturnvalue
• Providehigh-levelsummaryoferror,andstacktrace– Compare:coredumpinC
• Improvecodestructure– Separatenormalcodepathfromexceptional– Easetaskofrecoveringfromfailure
• Easetaskofwritingrobust,maintainablecode
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Checkedvs.uncheckedexceptions
• Checkedexception–Mustbecaughtorpropagated,orprogramwon’tcompile– Exceptionalconditionthatprogrammermustdealwith
• Uncheckedexception– Noactionisrequiredforprogramtocompile…• Butuncaughtexceptionwillcausefailureatruntime
– Usuallyindicatesaprogrammingerror
• Error– SpecialuncheckedexceptionthrownbyJVM*– Recoveryisimpossible*
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Java’sexceptionhierarchy
Throwable
Exception
RuntimeException
IOException
EOFException
FileNotFoundException
NullPointerException
IndexOutOfBoundsException
ClassNotFoundException
Object
Error
StackOverflowError
…
…
…
…
Checked Exceptions
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Designchoice:checkedexceptions,uncheckedexceptions,anderrorcodes• Uncheckedexception– Programmingerror,otherunrecoverablefailure
• Checkedexception– Anerrorthateverycallershouldbeawareofandhandle
• Specialreturnvalue(e.g.,null fromMap.get)– Commonbutatypicalresult
• Donotuseerrorcodes– tooeasytoignore• Donotreturnnull toindicatezero-lengthresult– Useazero-lengthlistorarrayinstead
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Usingyourownexceptiontypesclass SpanishInquisitionException extends RuntimeException {
SpanishInquisitionException(String detail) {super(detail);
}}
public class HolyGrail {public void seek() {
...if (heresyByWord() || heresyByDeed())
throw new SpanishInquisitionException("heresy");...
}}
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Guidelinesforusingexceptions(1)
• Avoidunnecessarycheckedexceptions(EJItem71)• Favorstandardexceptions(EJItem72)– IllegalArgumentException – invalidparametervalue– IllegalStateException – invalidobjectstate– NullPointerException – nullparamwhereprohibited– IndexOutOfBoundsException – invalidindexparam
• Throwexceptionsappropriatetoabstraction(EJItem73)
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Guidelinesforusingexceptions(2)
• Documentallexceptionsthrownbyeachmethod– Checkedandunchecked(EJItem74)– Butdon’tdeclare uncheckedexceptions!
• Includefailure-captureinfoindetailmessage(Item75)– throw new IlegalArgumentException(
"Modulus must be prime: " + modulus);
• Don’tignoreexceptions(EJItem77)// Empty catch block IGNORES exception – Bad smell in code!try {
...} catch (SomeException e) { }
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Rememberthisslide?There’sonepartwedidn’tshowyou:cleanupFileInputStream fileInput = null;
try {fileInput = new FileInputStream(fileName);DataInput dataInput = new DataInputStream(fileInput);return dataInput.readInt();
} catch (IOException e) {System.err.println("Could not read file: " + e);return DEFAULT_VALUE;
} finally { // Close file if it’s openif (fileInput != null) {
try {fileInput.close();
} catch (IOException ignored) {// No recovery necessary (or possible)
}}
}
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Manualresourceterminationisuglyanderrorprone,esp.formultipleresources• Evengoodprogrammersusuallygetitwrong– Sun’sGuidetoPersistentConnectionsgotitwrongincodethatclaimedtobeexemplary
– Solutiononpage88ofBlochandGafter’s JavaPuzzlersisbadlybroken;noonenoticedforyears
• 70%oftheusesoftheclosemethodintheJDKitselfwerewrongin2008(!)
• Even“correct”idiomsformanualresourcemanagementaredeficient
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Thesolution:try-with-resourcesAutomaticallyclosesresources!try (DataInput dataInput =
new DataInputStream(new FileInputStream(fileName))) {return dataInput.readInt();
} catch (IOException e) {System.err.println("Could not read file: " + e);return DEFAULT_VALUE;
}
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Filecopywithoutmanualterminationstatic void copy(String src, String dest) throws IOException {
InputStream in = new FileInputStream(src);try {
OutputStream out = new FileOutputStream(dest);try {
byte[] buf = new byte[8 * 1024];int n;while ((n = in.read(buf)) >= 0)
out.write(buf, 0, n);} finally {
out.close();}
} finally {in.close();
}}
}
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Filecopywithtry-with-resourcesstatic void copy(String src, String dest) throws IOException {
try (InputStream in = new FileInputStream(src);OutputStream out = new FileOutputStream(dest)) {byte[] buf = new byte[8 * 1024];int n;while ((n = in.read(buf)) >= 0)
out.write(buf, 0, n);}
}
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Summary
• Interface-baseddesignshandlechangewell• Informationhidingiscrucialtogooddesign• Exceptionsarefarbetterthanerrorcodes• Theneedforcheckedexceptionsisrare• try-with-resourcesisabigwin;alwaysuseit