ExpressionsandDataTypesCSC121Fall2016

HowardRosenthal

LessonGoals� UnderstandthebasicconstructsofaJavaProgram� Understandhowtousebasicidentifiers� UnderstandsimpleJavadatatypesandtheoperationsonthosetypes

� Understandhowtowriteandevaluateexpressions� Understandtheconceptofcasting

2

public class Hello { public static void main ( String[] args ) { System.out.println("Hello World!"); } } • Above is a source program (source file) for a Java program. The purpose of this program is to type the characters Hello World! on the monitor.

• The file must be named Hello.java to match the name of the class. The upper and lower case characters of the file name are important. (So if the file is named hello.java with a small h it will not work). On all computers, upper and lower case inside the program are important.

• The first line class Hello says that this source program defines a class called Hello.

•  A class is a section of a program. Small programs often consist of just one class. Most programs use multiple classes to create objects

• Some classes are are imported while other are created by the programmer

• Every class is contained within a set of braces • When the program is compiled, the compiler will make a file of bytecodes called Hello.class. - This is the file that the JVM uses.

KeyTermsandDefini/ons

3

KeyTermsandDefini/ons(2)•  Methodsarebuiltoutofstatements.Thestatementsinamethodareplacedbetweenbraces{and}asinthisexample.

•  Amethodisasectionofaclassthatperformsaspecifictask•  Allprogramsstartexecutingfromthemainmethod•  Eachmethodiscontainedwithinasetofbraces

�  Braces�  Foreveryleftbrace{thereisarightbrace}thatmatches.

�  Usuallytherewillbesetsofmatchingbracesinsideothersetsofmatchingbraces.Thefirstbraceinaclass(aleftbrace)willmatchthelastbraceinthatclass(arightbrace).Abracecanmatchjustoneotherbrace.

�  Useindentingtoshowhowthebracesmatch(andtherebyshowthelogicoftheprogram).Lookattheexample.�  Increasetheindentingbythreespacesforstatementsinsidealeftandrightbrace.If

anotherpairofbracesisnestedwithinthosebraces,increasetheindentingforthestatementstheycontainbyanotherthreespaces.Lineupthebracesvertically.�  WithNotepad++theindentlevelsforbothbracesandparenthesesarecolorcoded�  Afteraleftbraceanindentwillbecreatedforyouautomatically�  Makesurethatyoustepbacktoalignyourleftandrightbraces�  Youcanalsoindentwhennecessarybyusingthetabkey

4

•  Mostclassescontainmanymorelinesthanthisone.Everythingthatmakesupaclassisplacedbetweenthefirstbrace{anditsmatchinglastbrace}.

•  Thenameoftheclass(andthereforethenameofthefile)isuptoyou.•  Byconventionthefirstletterofaclassistypicallyuppercase.If

theclasshasacompoundnameeachwordinthenamestartswithauppercaseletteri.e.NumberAdder

•  Asourcefilealwaysendwith.javainlowercase.•  ThereforethefilenameisClassname.java

•  Inprogramming,thenameforsomethinglikeaclass,amethodoravariableiscalledanidentifier.

•  Anidentifierconsistsofalphabeticalcharactersanddigits,plusthetwocharacters'_'and'$'-underscoreanddollarsign

•  Thefirstcharactermustbealphabetical,theremainingcharacterscanbemixedalphabeticcharactersanddigitsor_or$.

•  Nospacesareallowedinsidethename.•  Anexpressionisasequenceofsymbols(identifiers,operators,

constants,etc.)thatdenoteavalue3*(2*x+y)-6*z

KeyTermsandDefini/ons(3)

5

•  Areservedwordisawordlikeclassthathasaspecialmeaningtothesystem.Forexample,classmeansthatadefinitionofaclassimmediatelyfollows.Youmustusereservedwordsonlyfortheirintendedpurpose.(Forexample,youcan'tusethewordclassforanyotherpurposethandefiningaclass.)Page250fthetextlistsreservedwords

•  Astatementinaprogramminglanguageisacommandforthecomputertodosomething.Itislikeasentenceofthelanguage.

•  AstatementinJavaisalwaysfollowedbyasemicolon.•  Agroupofstatementswithinasetofbracesiscalledablock

• Wewilllearnthateachblockleveldefinesascopeforthevariablesdefinedwithinthatscope

•  Thepart"HelloWorld!"iscalledaString.AStringisasequenceofcharacters.ThisprogramwritesaStringtothemonitorandthenstops.

KeyTermsandDefini/ons(4)

6

ReservedKeywordsInJava

7

abstract assert boolean break byte case

catch char class const* continue default

double do else enum extends false

final finally float for goto* if

implements import instanceof int interface long

native new null package private protected

public return short static strictfp super

switch synchronized this throw throws transient

true try void volatile while

The table below lists all the words that are reserved Java. Words in red are those that we will be using this semester

*Even though goto and const are no longer used in the Java programming language, they still cannot be used.

Comments

� Asinglelinecommentbeginswith//�  This//cancomeatthebeginningofthelineorafterastatementontheline:

System.out.println("Onawitheredbranch");//Writefirstlineofthepoem

� Multilinecommentsbegin/*andend*//*Program1Writeoutthreelinesofapoem.Thepoemdescribesasinglemomentintime,using17syllables.*/

�  Itisagoodideatofullycommentyourprogram�  Thisincludesdefiningthelogic,anddefiningyourvariables

8

DataTypesandOperators�  Adatatypeisasetofvaluestogetherwithanassociatedsetofoperatorsformanipulatingthosevalues.�  Whocanthinkofsomebasicdatatypesinthenumericalworld?Thelogicalworld?

� Whenanidentifierisusedasavariableitalwayshasadefineddatatype

�  Themeaningofthe0’sand1’sinthecomputerdependsonthedatatypebeingrepresented

� Wewillbeginbydefiningtheeightprimitivedatatypes�  byte,short,int,long,float,double,charandboolean–alllowercase

9

DataTypesandOperators(2)�  AlldatainJavafallsintooneoftwocategories:primitivedataand

classeswhichareusedtoinstantiateobjects.�  Thereareonlyeightprimitivedatatypes-byte,short,int,long,

float,double,char,andboolean.�  Referencevariablescontainmemoryaddressesthatrefertoobjects�  However,Javahasmanydifferentclasses,andyoucaninventasmany

othersasyouneed.�  Anydatatypeyouinventwillbeacreatedasaclassandinstantiatedasan

object.�  Muchmorewillbesaidaboutobjectsinfuturechapters(sinceJavais

aobjectorientedprogramminglanguage).Thefollowingisallyouneedtoknow,fornow:�  Aprimitivedatavalueusesasmall,fixednumberofbytes.�  Thereareonlyeightprimitivedatatypes.�  Aprogrammercannotcreatenewprimitivedatatypes.

10

SomeNotesonObjects�  Anobjectisabigblockofdata.Anobjectmayusemanybytesof

memory.�  Anobjectusuallyconsistsofmanyinternalpieces.�  Thedatatypeofanobjectiscalleditsclass.�  ManyclassesarealreadydefinedinJava.�  Aprogrammercaninventnewclassestomeettheparticularneedsof

aprogram.�  Wecreateclassesandaccessthemethodsofthoseclasses

�  Someclasseshavestaticmethodsandcanbeaccessedwithoutcreatingnewobjects

�  Wewillseethedifferencesaswemoveahead

11

Primi/veNumericDataTypes(1)� NumbersaresoimportantinJavathat6ofthe8primitivedatatypesarenumerictypes.

� Therearebothintegerandfloatingpointprimitivetypes.

� Thereare4integerdatatypes�  byte–asinglebyteusedforsmallintegers�  short–twobytes�  int–4bytes–allconstantintegersareassumedtobeoftypeint

�  long–8bytes–useforverylargenumber

12

Primi/veNumericDataTypes(2)�  Therearetwodatatypesthatareusedforfloatingpointnumber

�  float-4bytes�  double-8bytes–thisisthedefaultforallfloatingconstantsandisusedin

almostallcasesforfloatingpointarithmetic�  Floatingpointnumbers,unlikeintegers,arenotalwaysprecise.

�  Ifyoucomparefloatingpointnumbersyoucangeterrorsorunexpectedresultswhenexecutingduetothewaythattheyarerepresentedinthecomputer

�  Duetothislackofperfectprecisionweusuallyprefertousedoubleoverfloatforrealnumbersthataren’tintegers,sincetheprecisionisgreater,althoughstillnotperfect.�  Aswewillseeshortly,therearecastingissueswhenyoumixnumbers,

especiallywithfloatingpointnumbers�  Javahasadvancedmethodsusingobjectstocalculatenumbersevenmore

precisely�  ThisclasscalledBigDecimalisusedforveryprecisemonetaryandscientific

calculations�  Inthetables,Emeans"tentothepowerof".So3.5E38means3.5x1038

13

Primi/veNumericDataTypes(3)

� Thereisafundamentaldifferencebetweenthetherepresentationsofintegersandfloatingpointnumbersinthecomputer.�  Integertypeshavenofractionalpart;floatingpointtypeshaveafractionalpart.

� Onpaper,integershavenodecimalpoint,andfloatingpointtypesdo.Butinmainmemory,therearenodecimalpoints:evenfloatingpointvaluesarerepresentedwithbitpatterns.

14

Primi/veNumericDataTypes(4)�  Eachprimitivetypeusesafixednumberofbytes.Thismeansthatifyouareusingaparticulardatatypethenthesamenumberofbyteswillbeusednomatterwhatvalueisrepresented.�  Forexample,allvaluesrepresentedusingtheshortdatatypeuse2

bytes(16bits).Thevaluezero(asashort)uses2bytesandthevaluethirtythousanduses2bytes.

�  Allvaluesrepresentedusingthelongdatatypeuse8bytes(64bits).Thevaluezero(asalong)uses8bytes,thevaluethirtythousanduses8bytes,andthevalueeighttrillionuses8bytes.

�  Valuesthatarelargeinmagnitude(negativeorpositive)needmorebitstoberepresented.Thisissimilartowritingoutnumbersonpaper:largenumbersneedmoredigits.Ifavalueneedsmorebitsthanaparticulardatatypeuses,thenitcannotberepresentedusingthatdatatype.

15

SummaryofPrimiJveNumericDataTypesInteger Primitive Data Types Type Size Range byte 1 byte (8 bits) -128 to +127 short 2 bytes (16 bits) -32,768 to +32,767

int 4 bytes (32 bits) -2 billion to +2 billion (approximately)

long 8 bytes (64 bits) -9E18 to +9E18 (approximately)

Floating Point Primitive Data Types Type Size Range float 4 bytes (32 bits) -3.4E38 to +3.4E38 double 8 bytes (64 bits) -1.7E308 to 1.7E308

Remember:Integerdatatypesreservetheleftmostbittoindicatepositive(0)ornegative(1)intwo’scomplementformat

16

Inthetables,Emeans"tentothepowerof".So3.5E38means3.5x1038

NumericOperators(1)

Operator Meaning Precedence

- unaryminus highest

+ unaryplus highest

* multiplication middle

/ division middle

% remainder/modulus middle

+ addition low

- subtraction low

17

Precedenceofoperatorscantaketheplaceofparentheses,butjustasinalgebra,youshoulduseparenthesesforclarity.Wheretherearenoparenthesesandequalprecedenceevaluationisfromlefttoright

NumericOperators(2)�  Alloftheseoperatorscanbeusedonfloatingpointnumbersandonintegernumbers.�  The%operatorisrarelyusedonfloatingpoint.(wewon’tbeusingit,buttheremainderconceptwouldbesimilar)

�  Whenmixingfloatingpointnumbersandintegernumbers,floatingpointtakesprecedence–thisiscalledcasting

�  Anintegeroperationisalwaysdonewith32bitsormore.Ifoneorbothoperandis64bits(datatypelong)thentheoperationisdonewith64bits.Otherwisetheoperationisdonewith32bits,evenifbothoperandsaresmallerthan32bits.�  Forexample,with16bitshortvariables,thearithmeticisdoneusing32bits:

18

IntegerArithmeJc�  Inintegerarithmeticyoualwaystruncate

�  7/2 = 3 �  11/4 = 2

� Themodulusoperatorgivesyoutheremainder�  7%4 = 3 �  9%2=?�  Anyideasonwherethe%canbehelpful?� Note:InJavathesignoftheresultofa%bisalwaysthesignofa(thedividend).

19

CasJng�  Javaisahighlytypesensitivelanguage�  WhenevaluatinganyexpressionwithoperandsofdifferenttypesJava

firstpromotesorcaststheoperandofthe“smaller”datatype�  Bysmallerwemeantherange�  byteissmallerthanshortwhichissmallerthanintwhichissmaller

thanlongwhichissmallerthanfloatwhichissmallerthandouble�  booleanexpressionsarenevercast�  charautomaticallycastsuptoint,nottoshort

�  Youcanonlycastdownwardsexplicitly,otherwiseyoumaycreateanerror

�  Example:inta=10;shortb=5;a=b;Thisiscastingupwards–itisimplicitandautomaticb=(short)(a);Thisiscastingdownwards–mustbeexplicit

20

MixingNumericDataTypes(1)�  Ifbothoperandsareintegers,thentheoperationisanintegeroperation.Ifanyoperandisdouble,thentheoperationisdouble.7.1+7.4 = 14.5 �  7.0+7.4 = 14.4 �  7+7.4 = 14.4 �  (15/2) +7.4 = ? �  (15%2) + 7.4 = ?

�  Thenumbersare“casted”upwards�  Thisbecomesmoreimportantinthenextchapterwhenwelearnabouttypingvariables

�  Note: Unless otherwise declared all decimals are assumed to be of type double

21

MixingNumericDataTypes(2)�  Rememberthatwithoutparenthesesyoufollowthehierarchy�  Mixedinteger/decimaladditioniscasttodecimalwhenthemixing

occurs(10.0+5)=15.010/4*(18.0)=36.0(5/9)*(212.0-32.0)=0.0

�  Note:Integerscanbeoftypebyte,short,int,long,butdefaulttointHoweveryoucandirectlyassignaninttoashortorbytevariable(ifitfitswiththerange)shortb=5;works

�  Decimalcanbeoftypedoubleorfloat,butdefaulttodoubleExample:floatz;z=2.0+3.0;thiscreatesanerror–Why?Javadoesn’tallowthedoubletocastdownbecauseofprecisionissuesz=(float)(2.0+3.0);-Thisiscorrect–weexplicitlycastdown.

22

Typeboolean�  Typebooleanidentifierscanhaveonlyoneoftwovalues–trueorfalse.(1or0)�  Abooleanvaluetakesupasinglebyte.

�  Therearethreeoperators�  &&-meansand–bothoperandsmustbetrueforthevalueoftheexpressiontobetrue

�  ||-meansor–oneoftheoperandsmustbetrueforthevalueoftheexpressiontobetrue

�  !-meansnot

p q p&&q (and) p||q (or) !p (not) true true true true false true false false true false false true false true true false false false false true

23

Typeboolean–ShortCircuiJng�  Therearealsobooleanoperators&and|�  What’sthedifference?

�  Whenyouuse&&or||thecompilerismoreefficient,itcanshort-circuitwhennecessary

�  Thismeansthatonceitdeterminesifastatementistrueorfalseitstopsevaluatingi.e.:

(true||false)||false–itisevaluatedtrueafterthefirst||isevaluated(true&&false)&&true–sameidea,butevaluatesasfalseafterfirst&&

�  Sowhenisthereaproblem:�  Ifyoutrytoassignalogicalvalue(allowed,thismightnottakeplaceifthere

isshortcircuiting:�  (true||false)||(a=true)–adoesn’tgetassignedthevaluetrue(a=true)is

allowedastheexpressionevaluatesastrue�  (true|false)|(a=true)–adoesgetassignedthevaluetrue

�  Don’tusethesetypesofassignmentstatementsinsideofbooleanstatements–itwillinevitablyleadtoerrors

24

SwitchesandBooleanLogic

25

XA BY

TogetfromAtoBbothXandYmustbeclosed–X&&Y

A B

X

Y

TogetfromAtoBeitherXorYmustbeclosed–X||Y

A

A

RelaJonalOperatorsOperator Description Example (with A=2, B=5

== Checks if the value of two operands are equal or not, if yes then condition becomes true. (A == B) is not true.

!= Checks if the value of two operands are equal or not, if values are not equal then condition becomes true. (A != B) is true.

> Checks if the value of left operand is greater than the value of right operand, if yes then condition becomes true.

(A > B) is not true.

< Checks if the value of left operand is less than the value of right operand, if yes then condition becomes true. (A < B) is true.

>= Checks if the value of left operand is greater than or equal to the value of right operand, if yes then condition becomes true.

(A >= B) is not true.

<= Checks if the value of left operand is less than or equal to the value of right operand, if yes then condition becomes true.

(A <= B) is true.

� Theresultofapplyingarelationaloperatorisatrueorfalsevalue

26

OperatorHierarchyPriority Operators Operation Associativity

1 [] arrayindex

left () methodcall . memberaccess

2

++ pre-orpostfixincrement

right

-- pre-orpostfixdecrement +- unaryplus,minus ~ bitwiseNOT ! boolean(logical)NOT (type) typecast new objectcreation

3 */% multiplication,division,remainder left

4 +- addition,subtraction left + Stringconcatenation

5 << signedbitshiftleft

left >> signedbitshiftright >>> unsignedbitshiftright

6 <<= lessthan,lessthanorequalto

left >>= greaterthan,greaterthanorequalto instanceof referencetest

7 == equalto left != notequalto

8 & bitwiseAND left & boolean(logical)AND

9 ^ bitwiseXOR left ^ boolean(logical)XOR

10 | bitwiseOR left | boolean(logical)OR 11 && boolean(logical)AND left 12 || boolean(logical)OR left 13 ?: conditional right

14

= assignment

right

*=/=+=-=%=<<=>>=>>>=&=^=|=

combinatedassignment

•  Thehierarchyisverysimilartowhatyouknowfromalgebra

• Whenthereisanequivalenthierarchylevelandnoparenthesesyouevaluatefromlefttoright

• Whenindoubtuseparentheses

27

SomeExtraExamples�  5>4 – true �  4>5 – false �  (5>4) || (4>5) - ? �  (5>4) && (4>5) - ?

28

AnotherExampleEvaluateastrueorfalsetrue || false && 3 < 4 || !(5==7) (true || (false && (3 < 4))) || !(5==7) – putting in the parentheses correctly always helps (true||(false&&true))||!false(true||false)||truetrue||truetrue

29

Typechar�  Typecharisthesetofallcharactersfoundonthestandardkeyboard,and

thousandsofothercharactersaswell.�  charisaprimitivedatatype�  Typecharisdenotedusingsinglequotes�  ‘A’,‘5’�  JavausesUnicode–2byterepresentationsthatincreasesthenumberof

charactersthatcanberepresentedfrom255to65536uniquecharacters�  Note:KeyboardlettersinASCIICodeandUnicodehavethesamevalue–i.e.‘A’=65

ie01000001inASCIIor0000000001000001inUnicode�  Youcanaddandsubtracttypechar–theyareactuallytreatedlikeintegers

whenadding�  i.e.‘A’+1 = 660r0000000001000010 - char would automatically cast up to int

�  You could cast back down to char by saying (char)(66) which yields ‘B’ �  They are added as type int – 4bytes – to accommodate all Unicode characters

�  The most common characters and their Unicode values are found in Appendix B �  You can also compare type char values – they compare based on their ASCII value

�  ‘A’ < ‘B’ – true

30

String�  StringisaclassinJavawithlotsofdifferentmethodsthatallowsyoutomanipulate

them�  AnindividualStringisanobject,notabasicdatatype.�  AStringisasequenceofcharactersenclosedindoublequotes

�  JavaprovidesaStringclassandwecancreateStringobjects�  WhydoesStringhaveacapitalSwhileprimitivedatatypeshavelowercasefirstletters–

Stringisthenameofaclass�  Stringscanbeconcatenated

�  “Iam”+“aman”=“Iamaman”�  Stringsandvalues

�  Everythingdependsontheorder�  “Thesumoftwonumbersis”+ (5*2) prints as The sum of two numbers is 10 Why? You always work from inside the parentheses outwards �  However (“The sum of two numbers is 5”) + 2 prints as The sum of two numbers is 52

�  In Chapter 9 we do a lot more with String objects

31

Cas/ngWithStringsandCharacters‘A’+‘B’=131(integer)‘A’+“B”=AB(String)“A”+“B”=AB(String)“”+‘A’+‘B’=AB(String)–‘A’getscasttoString‘A’+‘B’+“”=131(String)3+4+“”=7(String)“”+3+4=34(String)Keyisthatwithoutparentheseswearereadinglefttoright

32

PrinJngandSpecialCharacters�  TheSystem.outobjectispredefinedandisincludedwiththebasic

java.langandthereforeisalwaysavailableforuse.�  Itiscalledthestandardoutputobject,andprintstotheterminal�  WewilllearnhowtoprinttootherFileobjectslaterinthisterm

�  Wewillbeusingtwobasicstaticmethodsfromthisclass�  System.out.println(“abc”)//printstheStringandacharacterreturn�  System.out.print(“abc”)//printsaStringwithoutacharacterreturn

�  EscapesequencesinsidetheStringcanbeusedtocontrolprinting

EscapeSequence Character

\n newline

\t tab

\b backspace

\f formfeed

\r return

\" "(doublequote)

\' '(singlequote)

\\ \(backslash)

\uDDDD characterfromtheUnicodecharacterset(DDDDisfourhexdigits)–usedwhenthecharacterisn’tavailableduringinput

33

PrinJngExamplepublicclassPrintAPoem{publicstaticvoidmain(String[]args){System.out.println(“Hewrotehiscode”);System.out.print(“\tHeindentedwell\n”);

System.out.println(“\tTillhewasdone”);System.out.print(“\nTheAuthor\n”);}}Hewrotehiscode

HeindentedwellTillhewasdone

TheAuthor

34

System.out.println(concatenatedString)–printsandgoestothenextlineSystem.out.print(concatenatedString)–printsandstaysonthesameline

PrinJngExample–ConcatenaJoninPrintStatements�  WhenyouwriteexpressionsinaSystem.out.println()statementtheexpressionmayormaynotbecalculatedfirst,dependingonifandwhereyouputtheparentheses�  Therulesareexactlythesameasusedwhenconcatenating

String(s)�  UltimatelytheprintlnmethodwilloutputasingleString

publicclassPrintingNumbers//Classname{

publicstaticvoidmain(String[]args){ System.out.println("Thesumof5+6is"+(5+6));

System.out.println("Thesumof5+6is"+5+6); System.out.println("Thesumof"+5+"+"+6+"is"+(5+6));}}

35

ProgrammingExercise-Class(1)Modifythefollowingprogramsothatit’soutputis:WelcomeToJavaProgrammingpublicclassWelcome1{

publicstaticvoidmain(String[]args){ System.out.println(“WelcomeToJavaProgramming”);}

}

36

ProgrammingExercise-Class(2)Exercise6.TriangleWriteaprogramthatprintsthetriangle:*********************Usethisframework:public class Pname //Class name { public static void main ( String[] args ) //main method header { Code goes here //Body } } CompilewithjavacPname.javaExecutewithjavaPnameonceyoucompilesuccessfully

37

ProgrammingExercise-Lab(1)Exercise7.TriangleWithInitialsWriteaprogramthatprintsthetriangle:****H***G****R*******

38

ProgrammingExercise-Lab(2)Exercise2.UptimeTheuptimecommandoftheUNIXoperatingsystemdisplaysthenumberofdays,hoursandminutessincetheoperatingsystemwasstarted.ForexampletheUNIXcommanduptimemightreturnthestringUp53days12:39.Writeaprogramthatconvertsthe53days,12hoursand39minutestothenumberofsecondsthathaveelapsedsincetheoperatingsincewaslaststarted.

39