Download Class 19

CS/IT 114
Introduction to Java, Part 1
FALL 2016
CLASS 19: N OV. 1 5 T H
INSTRUCTOR: JIAYIN WANG
1
Notice
Assignment
◦ ClassExercise19isassigned
◦ Homework8isassigned
◦ BothHomework8andExercise19areduethisSunday
2
Review
do/while Loop
Thewhileloopisnottheonlyindefiniteloop
Inawhileloopthetestexpressionisevaluatedbeforeenteringtheloop
Thismeansitispossiblethattheloopcodeisneverexecuted
Butsometimesweknowthatweneed...
atleastonepassthroughtheloop
Forsituationslikethisthedo/whileloopisabetterchoice
Inado/whileloopthetestexpressionisevaluated...
atthebottomoftheloop...
aftertheloopcodehasbeenexecutedatleastonce
3
do/while Loop
Ithasthefollowingsyntax
do{
STATEMENT;
...
}while(TEST_EXPRESSION);
Theloopbodywillalwaysbeexecutedatleastonce
Afterthat,theloopwillcontinue...
aslongasthetestexpressionistrue
4
do/while Loop
Let'srewritetheprogramthatrollsdice...
usingado/whileloop
importjava.util.*;
publicclassDoWhile{
publicstaticvoidmain(String[]args){
RandomranFactory=newRandom();
intsum;
do{
//rollthediceonce
intdie1=ranFactory.nextInt(6)+1;
intdie2=ranFactory.nextInt(6)+1;
sum=die1+die2;
System.out.println(die1+"+"+die2+"="+sum);
}while(sum!=7);
}
}
$javaDoWhile
5+5=10
1+1=2
1+4=5
4+5=9
4+3=7
5
The boolean Data Type
Thebooleandatatypeallowsonlytwopossiblevalue
• true
• false
Booleanexpressionsareexpressions...
whosevaluecanonlybetrueorfalse
trueandfalsearereservedwordsinJava
Theyarebooleanliterals
Youcanassignavaluetoitusingaliteral
booleantest=true;
Youcanassignavaluetoitusingacomplexexpression
booleaneven=(number%2==0);
6
Logical Operators
InJavayoucanformbooleanexpressionsusinglogicaloperators
Operator
Meaning
Example
AND
(conjunction)
(2 == 2) && (3 < 4) TRUE
||
OR (disjunction)
(1 < 2) || ( 2 == 3)
TRUE
!
NOT (negation)
! (2 == 2)
FALSE
&&
Value
TheNOToperator,!...
reversesthetruthvalueofitsoperands
Soiftheoperandistrue...
theNOToperatormakesitfalse...
andviceversa
7
Logical Operators
WecanexpressthisinatruthtableforNOT,!
HereisthetruthtableforAND,&&
andthetruthtableforOR,||
8
Updated Order of Precedence
Nowthatwehavethreenewoperators...
wehavetoseehowtheyfitintotheorderofprecedence
Description
Operators
unary operators
! ++ -- + -
multiplicative operators
* / %
additive operators
+ -
relational operators
< > <= >=
equality operators
== !=
logical AND
&&
logical OR
||
assignment operators
= += -= *= /= %= &&= ||=
Dealingwithlogicaloperators,
JavawillevaluateNOT,!,
thenAND,&&...
andfinallyOR,||
9
Short-Circuited Evaluation
JavainheritsafeatureoftheClanguagecalledshort-circuitedevaluation
WheneverabooleanexpressionusetheAND,&&,orOR,||,operators...
Javawillstoptheevaluatingtheexpressionifthevalueofthefirst
operand...
determinesthevalueoftheexpressionasawhole
Forexample,ifwehavetheexpression
stop<s.length()&&s.charAt(stop)!=''
Thereisnoneedtoevaluatethesecondoperand
s.charAt(stop)!='‘
ifweknowthefirstoperandisfalse
stop<s.length()
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Short-Circuited Evaluation
sincebothoperandsmustbetruefor&&tobetrue
Ifthefirstoperandisfalse...
weknowthewholeexpressionisfalse...
withouthavingtogofurther
Similarly,ifwehavetheexpression
height>5||width>5
Thereisnoneedtolookatthevalueofwidth...
ifweknowheightisgreaterthan5
Anothertermforthisfeatureislazyevaluation
11
Boolean Variables and Flags
Sometimes,whileprocessingdata,youwanttotakenoteofaspecial
condition
Whenthespecialconditionoccurs,aflagisset
Thisallowstheprogrammertoseparatetheactionthatsetstheflag...
fromtheresponsetothataction
Flagsarebooleanvariablesthatareinitiallysettofalse
Thevalueofaflagissettotrue,whensomethingspecialorunusual
happens
Flagsareoftenusedinloops
Theyareoftenusedinwhileloopstosignalthattheworkisdoneand
thattheloopshouldstop
12
Boolean Variables and Flags
Thismakethiscodeslightlylonger...
butisalsomakesitmorereadable...
becausewecanassignanametothetestcondition
Ifwewerewritingadatingprogram...
wemightassignnamestocertainconditions...
tomakehowwemakeourdecisionsclearer
booleancute=(looks>=9);
booleansmart=(IQ>125);
booleanrich=(income>100000);
booleangood_catch=cute&&smart&&rich;
Sometimesabooleanvariable...
isusedtorecordanunusualcondition
Suchvariablesarecalledflags
13
Boolean Variables and Flags
Flagsarebooleanvariablesthatareinitiallysettofalse
Thevalueofaflagissettotruewhensomethingspecialorunusualhappens
Flagsareoftenusedinloops
Theyareoftenusedinwhileloopstosignalthattheworkisdoneandthatthe
loopshouldstop
Forexample,youasktheuserstokeepenteringapositivenumberandstop
whenanegativenumberisentered.
booleanisNegative=false;
while(!isNegative){
System.out.print("Pleaseenterapositiveinteger:“);
intnumber=console.nextInt();
if(number<0){
isNegative=true;
}
}
14
New Materials
Outline
• ScannerLookahead
• DealingwithBadData
• FilesandFileObjects
• UsingaFileObject
• ReadingaFilewithaScanner
• Token-BasedProcessing
15
Scanner Lookahead
AScannerobjecthas"next"methodsthatwaitforinputfromtheuser...
andwhentheuserenterssomethingandhitsEnter..
readsthecharactersfromthecommandline...
andturnsthemintovaluesoftherightdatatype...
whicharereturnedtothecallingmethod
Hereisapartiallistofthesemethods
Method
Description
next()
reads the next token and returns it as a String
nextDouble()
reads the next token and returns it as a double value
nextInt()
reads the next token and returns it as an int value
nextBoolean() reads the next token and returns it as an boolean value
16
Scanner Lookahead
Ifwecalloneofthesemethodsandtheuserenterssomethingthatcan't
beconvertedintotherightdatatypeanexceptionisthrown
Butwellwrittencodeshouldchecktheinputbeforereadingindata
Tohelpyoudothis,aScannerobjecthasothermethods...
whichcanlookatwhattheuserhasentered...
andreturntrueifthedataentered...
canbeconvertedintoaspecificdatatype
Theydothiswithoutreadinginthedata
17
Scanner Lookahead
Foreach"next"methodthereisacorresponding"has"method
Method
Description
hasNext()
reads the next token and returns true if it can be turned into a String value
hasNextDouble()
reads the next token and returns true if it can be turned into a double value
hasNextInt()
reads the next token and returns true if it can be turned into a int value
hasNextBoolean()
reads the next token and returns true if it can be turned into a boolean value
Eachofthese"has"methodsreturnstrueifthevalueenteredcanbe
turnedintothedatatypeneeded
Thisallowsustolookatanentrywithouttryingtoconvertitintoadata
type
Thatmeanswecanavoidthrowinganexception
18
Scanner Lookahead
Soifweareexpectinganinteger....
wecancallhasNextIntbeforecallingnextInt...
andkeeppromptingforthetypeofinputweneeduntilwegetit
WhenyoucallahashNextmethoditwaits..
untiltheuserhashittheenterkey
Thenitlooksatthevalueentered...
andreturnstrueifthevaluecanbeconvertedintothepropertype
Butthevaluehasnotactuallybeenreadin
19
New Materials
Outline
• ScannerLookahead
• DealingwithBadData
• FilesandFileObjects
• UsingaFileObject
• ReadingaFilewithaScanner
• Token-BasedProcessing
20
Dealing with Bad Data
IfyouuseoneoftheScanner"has"methodsandtheuserhasentered
text...
whichcanbeturnedintothevalueyouwant...
youcalltheappropriate"next"methodtogetthedata...
andconvertitintotherightdatatype
Butwhatifthedataisofthewrongtype?
Youaskedforanintegerandtheusertypedin"fiftyseven”?
Ifthevalueisofthewrongtype...
youhavetogetridofthetexttheuserentered...
inordertomakewayforanewentry
21
Dealing with Bad Data
Thetexttheusertypedinisnotuseduntilyoucalloneofthe"next"methods
Ifyoudon't"consume"thisinput,youwillgetaninfiniteloop
Ifwerunthefollowingprogramanddon’ttypeinaninteger,wewillstuckin
infiniteloop
importjava.util.*;
publicclassReadIntBad{
publicstaticvoidmain(String[]args){
Scannerinput=newScanner(System.in);
System.out.print("Pleaseenteraninteger");
while(!input.hasNextInt()){
System.out.print("Pleaseenteraninteger");
}
intnumber=input.nextInt();
System.out.println("Youentered"+number);
}
}
$javaReadIntBad
Pleaseenteraninteger:
asdf
Pleaseenteraninteger:
Pleaseenteraninteger:
Pleaseenteraninteger:
Pleaseenteraninteger:
…
22
Dealing with Bad Data
Sincewedidnotgetridofthetextenteredbytheuser...
hasNextInt()keepsreadingthesamevalueagainandagain...
whichneverchanges
Weneedtoreadinthevalueenteredbytheuserandthrowitaway
Wedothisbycallingthenext()method
next()readsintheentirelineoftextenteredbytheuser
next()doesnotconvertwhattheuserentered
Wecallnext()to"consume"thebadentry...
tomakewayforanotherentry
23
Dealing with Bad Data
Thisishowwedoit
importjava.util.*;
publicclassReadIntGood{
publicstaticvoidmain(String[]args){
Scannerinput=newScanner(System.in);
System.out.println("Pleaseenteraninteger:");
while(!input.hasNextInt()){
$javaReadIntGood
input.next();//readintheentrybutdon'tuseitPleaseenteraninteger:
System.out.println("Pleaseenteraninteger:");
asdfa
}
Pleaseenteraninteger:
intnumber=input.nextInt();
sdfgdfgh
System.out.println("Youentered"+number);
}
Pleaseenteraninteger:
}
57
Youentered57
24
New Materials
Outline
• ScannerLookahead
• DealingwithBadData
• FilesandFileObjects
• UsingaFileObject
• ReadingaFilewithaScanner
• Token-BasedProcessing
25
Files and File Objects
Oneofthemostimportantcomputerconceptsisthatofafile
Afilestoredinacomputerhasanameandalocation
TogetthecontentsofafileinJava,youneedtoconstructaFileobject
Filef=newFile("red_sox.txt");
TheargumenttotheFileconstuctorisapathname
Thepathnameuniquelyidentifiesafile
Itconsistsof
• Apath
• Thenameofthefile
Thepathisthelocationofafilespecifiedbythelistofthedirectoriesthatyoumustgo
throughtogettothefile
Inthiscoursewewillonlyusefilesinthecurrentdirectory
Thepathnameforfilesinthecurrentdirectoryissimplythenameofthefile
26
New Materials
Outline
• ScannerLookahead
• DealingwithBadData
• FilesandFileObjects
• UsingaFileObject
• ReadingaFilewithaScanner
• Token-BasedProcessing
27
Using a File Object
OnceyouhavecreatedtheFileobject...
youcancallitsmethodstogetsomeinformationaboutthefileLikethis
importjava.io.*;//fortheFileclass
publicclassFileInfo{
publicstaticvoidmain(String[]args){
Filef=newFile("red_sox.txt");
System.out.println("existsreturns"+f.exists());
System.out.println("canReadreturns"+f.canRead());
System.out.println("lengthreturns"+f.length());
System.out.println("getAbsolutePathreturns"+f.getAbsolutePath());
}
}
$javaFileInfo
existsreturnstrue
existsreturnstrue
canReadreturnstrue
lengthreturns629
getAbsolutePathreturns/Users/jiayinwang/Documents/drjava/class19/
red_sox.txtexample_code_cs114/6_chapter_examples_cs114/red_sox.txt
28
Using a File Object
HerewecalledtheFileconstructorwithjustthenameofthefile
Thisworksonlyifthefileisinthecurrentdirectory
TheFileclassispartofthejava.iopackage,sowemusthave
importjava.io.*;
atthetopofanycodethatusesthisclass.”io"isshortfor"input/output"
Thesyntaxofthefileconstructoris:
Filef=newFile("red_sox.txt");
wearenotcreatinganewfile
InsteadwearecreatinganewFileobject
togetthedatainanexistingfile
29
Using a File Object
HerearesomeofthemoreusefulmethodsoftheFileclass
Method
Description
canRead()
Returns true if the file exists and can be read
delete()
Deletes the file
exists()
Returns true if the file exists on the system
getAbsolutePath()
Returns the full path showing where this file is located
getName()
Returns the name of the file as a String, without any path attached
isDirectory()
Returns true if the "file" is a directory (folder) on the system
isFile()
Returns true if the file is a file (not a directory) on the system
length()
Returns the number of characters in this file
renameTo(newName)
Changes this file’s name to that of the parameter given
30
New Materials
Outline
• ScannerLookahead
• DealingwithBadData
• FilesandFileObjects
• UsingaFileObject
• ReadingaFilewithaScanner
• Token-BasedProcessing
31
Reading a File with a Scanner object
Whenweneedtogetinputfromtheuser,wecreateaScannerobject
likethis
Scannerconsole=newScanner(System.in);
ButwecanalsouseaScannerobjecttoreadindatafromothersources
AScannerobjectcanalsobeusedtoreaddatafromafile
Todothis,wefirstneedtocreateaFileobjectwhichgivesusaccessto
thefile
OncewehavedonethiswecanthenpassthisFileobject..
totheScannerconstructor
Filef=newFile("red_sox.txt");
Scannerinput=newScanner(f);
32
Reading a File with a Scanner object
Afterdoingthisweneverneedfagain,sowecanshortenthecodeasfollows
Scannerinput=newScanner(newFile("red_sox.txt"));
Butifwetrytorunthiswegetasurprise
importjava.util.*;//forScannerclass
importjava.io.*;//forFileclass
publicclassFileReaderBad{
publicstaticvoidmain(String[]args){
Scannerinput=newScanner(newFile("red_sox.txt"));
System.out.print("JustcreatedScannerobjectforfile");
}
}
$javacFileReaderBad.java
FileReaderBad.java:7:unreportedexceptionjava.io.FileNotFoundException;mustbecaughtortobethrown
Scannerinput=newScanner(newFile("red_sox.txt"));
^
1error
33
Reading a File with a Scanner object
YouwouldthinkthataFileNotFoundExceptionwouldbesomethingyou
wouldonlyseeatruntime
ButaFileNotFoundExceptionisadifferentkindofexception
Itisacheckedexception
thatJavademandsspecialtreatmentforthem
Whenyourcodecouldcreateoneoftheseexceptions...
Javainsiststhatyourcodecatchthisexception...
or,themethodheadermustdeclare...
thatitmightthrowtheexception
34
Reading a File with a Scanner object
Youdothisbyincludingathrowsclauseinthemethodheader
Thiswillallowthecompiler...
tocheckeverymethodthatcallsthisparticularmethod...
tomakesureiteithercatchestheexception...
orhasthesamethrowsclauseinit'smethodheader
Athrowsclauselookslikethis
publicstaticvoidmain(String[]args)throwsFileNotFoundException{
Scannerinput=newScanner(newFile("hamlet.txt"));
...
}
35
Reading a File with a Scanner object
Hereisanexample
$catCountWords.java
//Countsthenumberofwordsinafile.
importjava.io.*;
importjava.util.*;
publicclassCountWords{
publicstaticvoidmain(String[]args)throwsFileNotFoundException{
Scannerinput=newScanner(newFile("red_sox.txt"));
intcount=0;
while(input.hasNext()){
Stringword=input.next();
count++;
}
System.out.println("Totalwords="+count);
}
}
$javaCountWords
Totalwords=181
36
New Materials
Outline
• ScannerLookahead
• DealingwithBadData
• FilesandFileObjects
• UsingaFileObject
• ReadingaFilewithaScanner
• Token-BasedProcessing
37
Token-Based Processing
Whenprogrammerstalkaboutreadinginputfromatextfile...
theytalkabouttokens
Atokenisastringofcharacterintheinput...
setoffbyspecialcharacters
Usuallytokensaresetofffromeachotherbywhitespace
Thewhitespacecharactersare
• Space
• Tab
• Newline
Readingthecontentofafiletokenbytoken...
iscalledtoken-basedprocessing
38
Token-Based Processing
Ifwehaveafilethatcontainsthefollowing
308.214.97.4
2.8
3.94.7-15.4
2.8
Thefilecontains8tokens...
eachofthetokensconsistsofdecimalnumber...
sowecanusehasNextDouble()toseewhetherthereismoreinputto
process...
andnextDouble()toreadinthevalueofthetokenasadouble
39
Token-Based Processing
//Readsaninputfileofnumbersandprintsthenumbersand
//theirsum.
importjava.io.*;
importjava.util.*;
publicclassShowSum{
publicstaticvoidmain(String[]args)throwsFileNotFoundException{
Scannerinput=newScanner(newFile("numbers.txt"));
doublesum=0.0;
intcount=0;
while(input.hasNextDouble()){
doublenext=input.nextDouble();
count++;
System.out.println("number"+count+"="+next);
sum+=next;
}
System.out.println("Sum="+sum);
}
}
$javaShowSum
number1=308.2
number2=14.9
number3=7.4
number4=2.8
number5=3.9
number6=4.7
number7=-15.4
number8=2.8
Sum=329.29999999999995
40