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PrinciplesofSoftwareConstruction:Objects,Design,andConcurrency

IntroductiontoJava

JoshBlochCharlieGarrod

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Administrivia

• Homework1duenextThursday11:59p.m.– Everyonemustreadandsignourcollaborationpolicy

• FirstreadingassignmentdueTuesday– EffectiveJavaItems15and16

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Outline

I. "HelloWorld!"explainedII. ThetypesystemIII. Quick‘n’dirtyI/OIV. Abriefintroductiontocollections

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The“simplest”JavaProgramclass HelloWorld {

public static void main(String[] args) {System.out.println("Hello world!");

}}

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Complication1:youmustuseaclasseven ifyouaren’tdoingOOprogrammingclass HelloWorld {

public static void main(String[] args) {System.out.println("Hello world!");

}}

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Complication2:mainmustbepublicclass HelloWorld {

public static void main(String[] args) {System.out.println("Hello world!");

}}

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Complication3:mainmustbestaticclass HelloWorld {

public static void main(String[] args) {System.out.println("Hello world!");

}}

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Complication4:mainmustreturnvoidclass HelloWorld {

public static void main(String[] args) {System.out.println("Hello world!");

}}

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Complication5:mainmustdeclarecommandlineargumentsevenifunusedclass HelloWorld {

public static void main(String[] args) {System.out.println("Hello world!");

}}

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Complication6:standardI/Orequiresuseofstatic fieldofSystemclass HelloWorld {

public static void main(String[] args) {System.out.println("Hello world!");

}}

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Executionisabitcomplicated

• Firstyoucompile thesourcefile– javac HelloWorld.java– ProducesclassfileHelloWorld.class

• Thenyoulaunchtheprogram– java HelloWorld– JavaVirtualMachine(JVM)executesmainmethod

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Onthebrightside…

• Hasmanygoodpointstobalanceshortcomings• Someverbosityisnotabadthing– Canreduceerrorsandincreasereadability

• ModernIDEseliminatemuchofthepain– Typepsvm insteadofpublic static void main

• Managedruntimehasmanyadvantages– Safe,flexible,enablesgarbagecollection

• ItmaynotbebestlanguageforHelloWorld…– ButJavaisverygoodforlarge-scaleprogramming!

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Outline

I. “HelloWorld!”explainedII. ThetypesystemIII. Quick‘n’dirtyI/OIV. Abriefintroductiontocollections

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Javahasabipartite (2-part)typesystem

Primitives ObjectReferenceTypesint, long, byte, short, char, float, double, boolean

Classes,interfaces,arrays,enums,annotations

Noidentityexcepttheirvalue HaveidentitydistinctfromvalueImmutable Somemutable,someimmutableOnstack, existonlywheninuse Onheap,garbagecollectedCan’tachieveunityofexpression UnityofexpressionwithgenericsDirtcheap More costly

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ProgrammingwithprimitivesAlotlikeC!

public class TrailingZeros {public static void main(String[] args) {

int i = Integer.parseInt(args[0]);System.out.println(trailingZerosInFactorial(i));

}

static int trailingZerosInFactorial(int i) {int result = 0; // Conventional name for return value

while (i >= 5) {i /= 5; // Same as i = i / 5; Remainder discardedresult += i;

}return result;

}}

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Primitivetypesummary

• int 32-bitsignedinteger• long 64-bitsignedinteger• byte 8-bitsignedinteger• short 16-bitsignedinteger• char 16-bitunsignedinteger/character• float 32-bitIEEE754floatingpointnumber• double 64-bitIEEE754floatingpointnumber• boolean Boolean value:true orfalse

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Deficientprimitivetypes

• byte,short – useint instead!– byte isbroken– shouldhavebeenunsigned

• float – usedouble instead!– Providestoolittleprecision– Onlycompellingusecaseislargearrays,especiallyinresource-constrainedenvironments

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PopQuiz!

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Whatdoesthisfragmentprint?int[] a = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

int i;int sum1 = 0;for (i = 0; i < a.length; i++) {

sum1 += a[i];}

int j;int sum2 = 0;for (j = 0; i < a.length; j++) {

sum2 += a[j];}

System.out.println(sum1 - sum2);

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Maybenotwhatyouexpect!int[] a = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

int i;int sum1 = 0;for (i = 0; i < a.length; i++) {

sum1 += a[i];}int j;int sum2 = 0;for (j = 0; i < a.length; j++) { // Copy/paste error!

sum2 += a[j];}System.out.println(sum1 - sum2);

Youmightexpectittoprint0,butitprints55

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Youcouldfixitlikethis…int[] a = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

int i;int sum1 = 0;for (i = 0; i < a.length; i++) {

sum1 += a[i];}int j;int sum2 = 0;for (j = 0; j < a.length; j++) {

sum2 += a[j];}

System.out.println(sum1 - sum2); // Now prints 0, as expected

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Butthisfixisfarbetter…int sum1 = 0;for (int i = 0; i < a.length; i++) {

sum1 += a[i];}

int sum2 = 0;for (int i = 0; i < a.length; i++) {

sum2 += a[i];}

System.out.println(sum1 - sum2); // Prints 0

• Reducesscopeofindexvariabletoloop• Shorterandlesserrorprone

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Thisfixisbetterstill!int sum1 = 0;for (int x : a) {

sum1 += x;}

int sum2 = 0;for (int x : a) {

sum2 += x;}

System.out.println(sum1 - sum2); // Prints 0

• Eliminatesscopeofindexvariableentirely!• Evenshorterandlesserrorprone

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Lessonsfromthequiz

• Minimizescopeoflocalvariables[EJItem57]– Declarevariablesatpointofuse

• Initializevariablesindeclaration• Preferfor-eachloopstoregularfor-loops• Usecommonidioms• Watchoutforbadsmellsincode– Suchasindexvariabledeclaredoutsideloop

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Objects

• Allnon-primitivesarerepresentedbyobjects.• 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)– Defineshowusersinteractwithitsinstances

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Theclasshierarchy

• TherootisObject(allnon-primitivesareobjects)• AllclassesexceptObjecthaveoneparentclass– Specifiedwithanextends clauseclass Guitar extends Instrument { ... }

– Ifextends clauseomitted,defaultstoObject• Aclassisaninstanceofallitssuperclasses

Object

ToyInstrument

YoyoGuitar

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Implementationinheritance

• Aclass:– Inheritsvisiblefieldsandmethodsfromitssuperclasses– Canoverridemethodstochangetheirbehavior

• Overridingmethodimplementationmustobeycontract(s)ofitssuperclass(es)– Ensuressubclasscanbeusedanywheresuperclass can– Liskov SubstitutionPrinciple(LSP)– Wewilltalkmoreaboutthisinalaterclass

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Interfacetypes

• Definesatypewithoutanimplementation• Muchmoreflexiblethanclasstypes– Aninterfacecanextendoneormoreothers– Aclasscanimplementmultipleinterfaces

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Enum types

• Javahasobject-orientedenums• Insimpleform,theylookjustlikeCenums:

enum Planet { MERCURY, VENUS, EARTH, MARS,JUPITER, SATURN, URANUS, NEPTUNE }

• Buttheyhavemany advantages!– Compile-timetypesafety– Multipleenum typescansharevaluenames– Canaddorreorderwithoutbreakingexistinguses– High-qualityObjectmethodsareprovided– Screamingfastcollections(EnumSet,EnumMap)– Caniterateoverallconstantsofanenum

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Boxedprimitives

• Immutablecontainersforprimitivetypes• Boolean,Integer,Short,Long,Character,Float,Double

• Letyou“use”primitivesincontextsrequiringobjects• Canonicalusecaseiscollections• Don’tuseboxedprimitivesunlessyouhaveto!• Languagedoesautoboxing andauto-unboxing– Blursbutdoesnoteliminatedistinction– Therebedragons!

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Comparingvalues

x == y comparesthecontents ofx andyprimitivevalues: returnstrueifxandyhavethesamevalueobjectsrefs: returnstrueifxandyrefertosameobject

x.equals(y) comparesthevaluesoftheobjectsreferredto byx andy

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Trueorfalse?

int i = 5;int j = 5;System.out.println(i == j);---------------------------

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Trueorfalse?

int i = 5;int j = 5;System.out.println(i == j);---------------------------true

5j

i 5

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Trueorfalse?

int i = 5;int j = 5;System.out.println(i == j);---------------------------true

String s = "foo";String t = s;System.out.println(s == t);---------------------------

5j

i 5

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Trueorfalse?

int i = 5;int j = 5;System.out.println(i == j);---------------------------true

String s = "foo";String t = s;System.out.println(s == t);---------------------------true

5j

i 5

"foo"

t

s

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Trueorfalse?

int i = 5;int j = 5;System.out.println(i == j);---------------------------true

String s = "foo";String t = s;System.out.println(s == t);---------------------------true

String u = "iPhone";String v = u.toLowerCase();String w = "iphone";System.out.println(v == w);---------------------------

5j

i 5

"foo"

t

s

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Trueorfalse?

int i = 5;int j = 5;System.out.println(i == j);---------------------------true

String s = "foo";String t = s;System.out.println(s == t);---------------------------true

String u = "iPhone";String v = u.toLowerCase();String w = "iphone";System.out.println(v == w);---------------------------Undefined!(false inpractice)

5j

"foo"

t v

u

w

"iPhone"si 5

"iphone"

"iphone"?

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Themoral

• Alwaysuse.equals tocompareobjectrefs– (Exceptforenums,whicharespecial)– The==operatorcanfailsilentlyandunpredictably

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Outline

I. “HelloWorld!”explainedII. TheJavatypesystemIII. Quick‘n’dirtyI/OIV. Abriefintroductiontocollections

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Output

• UnformattedSystem.out.println("Hello World");System.out.println("Radius: " + r);System.out.println(r * Math.cos(theta));System.out.println();System.out.print("*");

• FormattedSystem.out.printf("%d * %d = %d%n", a, b, a * b); // Varargs

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Commandlineinputexample

Echos allcommandlineargumentsclass Echo {

public static void main(String[] args) {for (String arg : args) {

System.out.print(arg + " ");}

}}

$ java Echo Woke up this morning, had them weary blues

Woke up this morning, had them weary blues

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Commandlineinputwithparsing

PrintsGCDoftwocommandlineargumentsclass Gcd {

public static void main(String[] args) {int i = Integer.parseInt(args[0]);int j = Integer.parseInt(args[1]);System.out.println(gcd(i, j));

}

static int gcd(int i, int j) {return i == 0 ? j : gcd(j % i, i);

}}

$ java Gcd 11322 35298666

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Scannerinput

Countsthewordsonstandardinputclass Wc {

public static void main(String[] args) {Scanner sc = new Scanner(System.in);long result = 0;while (sc.hasNext()) {

sc.next(); // Swallow tokenresult++;

}System.out.println(result);

}}

$ java Wc < Wc.java32

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Outline

I. “HelloWorld!”explainedII. ThetypesystemIII. Quick‘n’dirtyI/OIV. Abriefintroductiontocollections

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Primarycollectioninterfaces

Collection

QueueSet List

Deque

Map

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“Primary”collectionimplementations

Interface ImplementationSet HashSetList ArrayListQueue ArrayDequeDeque ArrayDeque(stack) ArrayDequeMap HashMap

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Othernoteworthycollectionimplementations

Interface Implementation(s)

Set LinkedHashSetTreeSetEnumSet

Queue PriorityQueue

Map LinkedHashMapTreeMapEnumMap

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Collectionsusageexample1

Squeezeduplicatewordsoutofcommandline

public class Squeeze {public static void main(String[] args) {

Set<String> s = new LinkedHashSet<>();for (String word : args)

s.add(word);System.out.println(s);

}}

$ java Squeeze I came I saw I conquered[I, came, saw, conquered]

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Collectionsusageexample2

Printuniquewordsinlexicographicorderpublic class Lexicon {

public static void main(String[] args) {Set<String> s = new TreeSet<>();for (String word : args)

s.add(word);System.out.println(s);

}}

$ java Lexicon I came I saw I conquered[I, came, conquered, saw]

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Collectionsusageexample3

Printindexoffirstoccurrenceofeachwordclass Index {

public static void main(String[] args) {Map<String, Integer> index = new TreeMap<>();

// Iterate backwards so first occurrence winsfor (int i = args.length - 1; i >= 0; i--) {

index.put(args[i], i);}System.out.println(index);

}}

$ java Index if it is to be it is up to me to do it{be=4, do=11, if=0, is=2, it=1, me=9, to=3, up=7}

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Moreinformationoncollections

• Formuchmoreinformationoncollections,seetheannotatedoutline:

https://docs.oracle.com/javase/11/docs/technotes/guides/collections/reference.html

• Formoreinfoonany libraryclass,seejavadoc– Searchwebfor<fullyqualifiedclassname>8– e.g.,java.util.scanner 8

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Whataboutarrays?

• Arraysaren’treallyapartofthecollectionsframework

• Butthereisanadapter:Arrays.asList• Arraysandcollectionsdon’tmix• Ifyoutrytomixthemandgetcompilerwarnings,takethemseriously

• Generallyspeaking,prefercollectionstoarrays• See EffectiveJava Item28fordetails

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TolearnJavaquickly

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Summary

• Javaiswellsuitedtolargeprograms;smallonesmayseemabitverbose

• Bipartitetypesystem– primitives&objectrefs– Singleimplementationinheritance–Multipleinterfaceinheritance

• AfewsimpleI/Otechniqueswillgetyoustarted• Collectionsframeworkispowerful&easytouse