objecves - washington and lee universitysprenkle/cs209/pre_class/02-java_fundamentals2.pdfintro to...

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Objec,ves• MoreJavafundamentalsØ  java.langclasses:MathandStringclassØ  ControlStructuresØ  Arrays

Sept14,2016 Sprenkle-CSCI209 1

Review:PythonTransi,onWarning

• OK:

• NotOK:


int x = 3;x = -3;

int x = 3;int x = -3;…boolean x = true;

Compilererrors

You cannot redeclare a variable name in the same scope

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INTROTOJAVALIBRARIES


JavaLibraries• Organizedintoahierarchyofpackages


java

net

lang

util

Object

Arrays

Fully qualified name: java.lang.StringString

java.lang.* classes included �by default in all Java programs

javaxorg Many, many more classes and packages

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JavaAPIDocumenta,on• API:Applica,onProgrammingInterfaceØ  WhattheclasscandoforYOU!

• Completedocumenta,onofeveryclassincludedwiththeJDKØ  Everymethodandvariablecontainedinclass

http://docs.oracle.com/javase/8/docs/api• Bookmarkit!

Ø  Toomanyclasses,methodstorememberthemallØ  Refertoito\en


java.lang.Mathclass• SimilartoPython’smathmodule• IncludedbydefaultineveryJavaprogram• Containsusefulmathema,calfunc,ons(methods)andconstants(fields):

• Lookatjava.lang.MathAPIonlineØ  http://docs.oracle.com/javase/8/docs/api/Ø  NotehowAPIisspecified

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java.lang.Mathclass• ExampleUses:

double y = Math.pow(x, a);double z = Math.sin(y);double d = Math.exp(4.59) * Math.PI;

MathExample.java

methodconstant

UseClassname.methodname() tocallMath’smethodsbecausethey’restatic

static: fortheclass


java.lang.String class• Similarfunc,onalitytoPythonbutdifferentwaystouse

• Stringsarerepresentedbydoublequotes:""Ø  Singlequotesrepresentchars only

• Examples:

String emptyString = "";String niceGreeting = "Hello there.";String badGreeting = "What do you want?";

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Strings• Acharateachposi,onofStringstringvar = "The Beatles";


'T' 'h' 'e' ' ' 'B' 'e' 'a' 't' 'l' 'e' 's'0 1 2 3 4 5 6 7 8 9 10

index or position ofcharacters

chars

Length of the string: 11

stringvar.length() methodStart at 0

End at stringvar.length()-1

Use charAt method to access chars


Stringmethod:charAt• AStringisacollec,onofchars

String testString1 = "Demonstrate Strings";

char character1;char character2 = testString1.charAt(3);character1 = testString1.charAt(2);

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Stringmethods:substring• LikeslicinginPython• String substring(int beginIndex)Ø  ReturnsanewStringthatisasubstringofthisstring,

frombeginIndex toendofthisstring• String substring(int beginIndex, int endIndex)Ø  ReturnsanewStringthatisasubstringofthisstring,

frombeginIndextoendIndex-1

String language = "Java!";String subStr = language.substring(1);String subStr2 = language.substring(2, 4);

Python Gotcha: Can’t use negative numbers for indices as in Python


StringConcatena,on• Use+ operatortoconcatenateStringsString niceGreeting = "Hello";String firstName = "Clark";String lastName = "Kent";String blankSpace = " ";

String greeting = niceGreeting + "," + blankSpace + firstName +

blankSpace + lastName;

System.out.println(greeting);

Prints “Hello, Clark Kent”

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StringConcatena,on•  IfaStringisconcatenatedwithsomethingthatisnotaString,theothervariableisconvertedtoaStringautoma,cally.

int totalPoints = 110;int earnedPoints = 87;float testScore = (float) earnedPoints/totalPoints;

System.out.println("Your score is " + testScore);

ConvertedtoaString


StringBuilders vsStrings• Stringsare“read-only”orimmutable

Ø  SameasPython•  UseStringBuildertomanipulateaString•  Insteadofcrea,nganewStringusing

Ø  String str = prevStr + " more!";•  Use

• ManyStringBuildermethodsØ  toString()togettheresultantstringback

StringBuilder str = new StringBuilder( prevStr );str.append(" more!");

new keyword: �allocate memory to a new object

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Effec0veJava:CodeInefficiency• Avoidcrea,ngunnecessaryobjects:

• Dothisinstead:


String s = new String("text"); // DON’T DO THIS

String s = "text";

Why?


StringComparison:equals• boolean equals(Object anObject)Ø  Comparesthisstringtothespecifiedobject

• testisfalsebecausetheStringscontaindifferentvalues

String string1 = "Hello";String string2 = "hello";boolean test;test = string1.equals(string2);

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PythonGotcha:StringComparisons• string1 == string4willnotyieldthesameresultasstring1.equals(string4)Ø  ==testsiftheobjectsarethesame

• notifthecontentsoftheobjectsarethesameØ  Similartois operatorinPython

"same"

"same"

string1string2

string4

Memory

string1 != string4BUTstring1.equals(string4)

Equals.java


Stringmethod:equalsIgnoreCase• Doeswhatit’snamed!

• testistrue!

String string1 = "Hello";String string2 = "hello";boolean test;test = string1.equalsIgnoreCase(string2);

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Stringmethods:andmanymore!• boolean endsWith(String suffix)• boolean startsWith(String prefix)• int length()• String toLowerCase()• String trim():removetrailingandleadingwhitespace

• …• Seejava.lang.StringAPIforall

CONTROLSTRUCTURES


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Review• Whatisthesyntaxofacondi0onalstatementinPython?



ControlFlow:Condi,onalStatements

• ifstatementØ  CondiGonmustbesurroundedby() Ø  Condi,onmustevaluatetoabooleanØ  Bodyisenclosedby{} ifmul,plestatements

if (purchaseAmount < availCredit) {System.out.println("Approved");availableCredit -= purchaseAmount;

}else

System.out.println("Denied");Don’t need { } if only one statement in the body Best practice: use { }

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• ifstatement

•  Everythingbetween{ } isablockofcodeØ Hasanassociatedscope

ControlFlow:Condi,onalStatements

if (purchaseAmount < availCredit) {System.out.println("Approved");availableCredit -= purchaseAmount;

}else

System.out.println("Denied");

Condition

Block of code

ScopingIssues:PythonGotcha•  Everythingbetween{ } isablockofcode

Ø Hasanassociatedscope if (purchaseAmount < availableCredit) {

availableCredit -= purchaseAmount;boolean approved = true;

}

if( ! approved ) System.out.println("Denied");

Out of scopeWill get a compiler error(cannot find symbol)

Sept14,2016 24Sprenkle-CSCI209

How do we fix this code?

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Fixed• Moveapproved outsideoftheif statement

boolean approved = false;if (purchaseAmount < availableCredit) {

availableCredit -= purchaseAmount;approved = true;

}

if( ! approved ) System.out.println("Denied");

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LogicalOperators


OperaGon Python JavaAND &&OR ||NOT !

In Python, these are …?

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LogicalOperators


OperaGon Python JavaAND and &&OR or ||NOT not !

ControlFlow:else if•  InPython,waselif


if( x%2 == 0 ) {System.out.println("Value is even.");

}else if ( x%3 == 0 ) {

System.out.println("Value is divisible by 3.");}else {

System.out.println("Value isn’t divisible by 2 or 3.");}

What output do we get if x is 9, 13, and 6?

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ControlFlow:switchstatement• Likeabigif/else if statement• Workswithvariableswithdatatypesbyte,short,char,int, and String


int x = 3;switch(x) {

case 1:System.out.println("It's a 1.");break;

case 2:System.out.println("It's a 2.");break;

default:System.out.println("Not a 1 or 2.");

}


ControlFlow:switchstatementswitch(grade) {

case ‘a’:case ‘A’:

System.out.println("Congrats!");break;

case ‘b’:case ‘B’:

System.out.println("Not too shabby!");break;

… // Handle c, d, and f …default:

System.out.println("Error: not a grade");}

Grades.java

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ControlFlow:while Loops• while loopØ  Condi,onmustbeenclosedinparenthesesØ  Bodyofloopmustbeenclosedin {} ifmul,ple

statements


int counter = 0;while (counter < 5) {

System.out.println(counter);counter++;

}System.out.println("Done: " + counter);

shortcut


Changingcontrolflow:break• Exitsthecurrentloop

while ( <readingdata> ) {… if( <somethingbad> ) { // shouldn’t happen

break;}

}

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Review• HowdoyouwriteaforloopinPythonforcoun,ng?


ControlFlow:for Loop• VerydifferentsyntaxfromPython• Syntax:


for (<init>; <condition>; <execution_expr>)

Loop’s counter variable,Usually used in condition

Executed at end of each iteration.Typically increments or �

decrements counter

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ControlFlow:for LoopExample

• Whatisthecountervariable?• Whatisthecondi,on?• Whatistheoutput?• HowwriqeninPython?


System.out.println("Counting down…");

for (int count=5; count >= 1; count--) { System.out.println(count);}System.out.println("Blastoff!"); shortcut

Can’t print out count with Blastoff. Why?

ARRAYS


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PythonListsàJavaArrays• AJavaarrayislikeafixed-lengthlist• Todeclareanarrayofintegers: int[] arrayOfInts;Ø  Declara,ononlymakesavariablenamed

arrayOfIntsØ  Doesnotini,alizearrayorallocatememoryforthe

elements

• Todeclareandini3alizearrayofintegers:int[] arrayOfInts = new int[100];

new keyword: �allocate memory to a new object


ArrayIni,aliza,on•  Ini,alizeanarrayatitsdeclara,on:

Ø  int[] fibNums = {1, 1, 2, 3, 5, 8, 13};

Ø  Notethatwedonotusethenew keywordwhen

alloca,ngandini,alizinganarrayinthismannerØ  fibNums haslength7

1 1 2 3 5 8 130 1 2 3 4 5 6Position/index

Value

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ArrayAccess• AccessavalueinanarrayasinPython:

Ø  fibNums[0]Ø  fibNums[x] = fibNums[x-1] + fibNums[x-2]

• UnlikeinPython,cannotusenega,venumberstoindexitems



ArrayLength• AllarrayvariableshaveafieldcalledlengthØ  Note:noparenthesesbecausenotamethod

int[] array = new int[10];for (int i = 0; i < array.length; i++) {

array[i] = i * 2; }

for (int i = array.length-1; i >= 0; i--) {System.out.println(array[i]);

}

ArrayLength.java

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OversteppingArrayLength• JavasafeguardsagainstoversteppinglengthofarrayØ  Run,meExcep,on:“Arrayindexoutofbounds”Ø  Moreonexcep,onslater…

• Exampleint[] array = new int[100];Ø  Aqemptstoaccessorwritetoindex<0orindex>=

array.length(100)willgenerateexcep,on


Arrays• AssigningonearrayvariabletoanotherèbothvariablesrefertothesamearrayØ  SimilartoPython

• Drawpictureofbelowcode:int [] fibNums = {1, 1, 2, 3, 5, 8, 13};int [] otherFibNums;

otherFibNums = fibNums;otherFibNums[2] = 99;

System.out.println(otherFibNums[2]);System.out.println(fibNums[2]);

fibNums[2]andotherFibNums[2]arebothequalto99

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ArrayCopying•  Copyanarray(element-by-element)usingthearraycopy methodintheSystemclass

•  Forexample:

fibNums[2] = 2, otherNums[2]= 99

System.arraycopy(from, fromIndex, to, toIndex, count);

int [] fibNums = {1, 1, 2, 3, 5, 8, 13};int [] otherNums = new int[fibNums.length];System.arraycopy(fibNums,0,otherNums,0,fibNums.length);otherNums[2] = 99;System.out.println(otherNums[2]);System.out.println(fibNums[2]);


ControlFlow:foreachLoop•  IntroducedinJava5Ø  Suncalls“enhancedfor”loop

•  Iterateoverallelementsinanarray(orCollec,on)Ø  SimilartoPython’sfor loopint[] a;int result = 0; . . .for (int i : a){ result += i; }

for each int element iin the array a,the loop body is visited once for each element of a.

“in”

http://docs.oracle.com/javase/1.5.0/docs/guide/language/foreach.html

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java.util.Arrays• Arraysisaclassinjava.util• Methodsforsor,ng,searching,deepEquals,fillarrays

• Touseclass,needimport statementØ  Goesattopofprogram,beforeclassdefini,on

import java.util.Arrays;

ArraysExample.java

Summary:PythontoJavaGotchas• Everyvariableneedstobedeclaredbeforeitisused

• Everyvariableneedsasta,cally-declareddatatype

• Scopeofvariables• SyntaxØ  SemicolonsattheendofstatementsØ  BracesaroundblocksofcodeØ  Keywords


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