Objecves - Washington and Lee Universitysprenkle/cs209/pre_class/01-java_fundamentals.pdf · 9/9/16 3 Review: Java Virtual Machine • Same bytecode executes on each plaorm • Don’t

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Objec+ves• BasicsofJavaSyntax• Javafundamentals

Ø Primi+vedatatypesØ Sta+ctypingØ Arithme+coperatorsØ Rela+onaloperators

1Sept12,2016 Sprenkle-CSCI209

Review• Whatarequali+esofgoodsoOware?

• WhatisJava?Ø BenefitstousingJava?

• HowdoyoucompileaJavaprogram?Ø Howdoyourun/executeaJavaprogram?

2Purpose of questions?Sept12,2016 Sprenkle-CSCI209

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Review:BenefitsofJava• Rapiddevelopmentofprograms

Ø Largelibraryofclasses,includingGUIs,Enterprise-levelapplica+ons,Webapplica+ons

• PortabilityØ Runprogramonmul+pleplaXormswithoutrecompiling

• Sta+cally-typedlanguageØ Compiling-findsomeerrorsbeforeexecu+on,performancebenefits


Review:JavaProgrammingLanguage• En+relyobject-oriented• SimilartoPython

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Program.java

Wri[eninJavaProgrammingLanguage

Compiler

Program.class

Bytecode:machinecodeforavirtualCPU

javac

Step1:

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Review:JavaVirtualMachine

• SamebytecodeexecutesoneachplaXorm• Don’tneedtoprovidethesourcecode

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Program.class

MacJVM

BytecodeWindowsJVM

UNIXJVM

…

CPU(machinecode)

Step2:


JavaClassLibraries• Pre-definedclasses

Ø IncludedwithJavaDevelopmentKit(JDK)andJavaRun+meEnvironment(JRE)

Ø Viewtheavailableclassesonline:http://docs.oracle.com/javase/8/docs/api/

• SimilarinpurposetomodulesincludedwithPython

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SummaryofJavaPlaXormSE7.0


Imagefromhttp://docs.oracle.com/javase/8/docs/index.html

BenefitsofJava• Rapiddevelopmentofprograms

Ø Largelibraryofclasses,includingGUIs,Enterprise-levelapplica+ons,Webapplica+ons

• PortabilityØ Runprogramonmul+pleplaXormswithoutrecompiling

• Sta+cally-typedlanguageØ Compiling:findsomeerrorsbeforeexecu+on!Ø Cangiveperformanceboostbydoingop+miza+ons


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Which‘Java’isthisclassabout?• Javaprogramminglanguage• Javaclasslibraries

• UsetheJVMbutwon’tlearnabouthowitworksØ Formoreinforma+on:http://docs.oracle.com/javase/specs/


Aside:JavaScriptvsJava• JavaScriptisnotJava

Ø JavaScriptisascrip&nglanguage,primarilyembeddedinHTML,executedbyWebbrowsers


WebBrowser

JavaScript

<script type="text/javascript">function myFunction() {

return ("Hello, have a nice day!")}</script></head><body><script type="text/javascript">

document.write(myFunction())</script>

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JavaDevelopmentKit• JDK:JavaDevelopmentKit• SDK:SoOwareDevelopmentKit• FreefromOracle• Contains

Ø javac:JavacompilerØ java:JavaVirtualMachineØ Javaclasslibraries


http://www.oracle.com/technetwork/java/javase/downloads/index.html

PythonReview

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# a Python programdef main():

print("Hello")

main()

What does this program do?


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ExampleJavaProgram

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

System.out.println("Hello");}

}

What are your observations about this program?What can you figure out?


ExampleJavaProgram

• EverythinginJavaisinsideaclass Ø Javaisen&relyobject-oriented*Ø ThisclassisnamedHello

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}


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ExampleJavaProgram

•  Ingeneral,eachJavaprogramfilecontainsoneclassdefini+on*

• NameoftheclassisnameoffileØ E.g.,Hello.java

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}

Definestheclass“Hello”

Blocks of code marked with { }




}

ExampleJavaProgram

AccessModifier:controlsifotherclassescanusecodeinthisclass


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}

ExampleJavaProgram

• Classcontainsonemethod:main

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method




}

ExampleJavaProgram:mainMethod

•  SimilartomaininPythonØ  Butmustbeassociatedwithaclass

•  Musttakeoneparameter:anarrayofStringsØ  Forcommand-linearguments

•  Mustbepublic static •  Mustbevoid:datatypeofwhatmethodreturns(nothing)•  mainisautoma+callycalledwhenprogramisexecutedfrom

commandline


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}

ExampleJavaProgram

• MethodcontainsonelineofcodeØ Whatdoyouthinkitdoes?




}

ExampleJavaProgram:PrintStatements

• CallstheprintlnmethodontheSystem.out object

• println takesoneparameter,aString• Displaysstringonterminal,terminatesthelinewithnewline(\n)character


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ExampleJavaProgram:Comments

• Comments:/* */or//Ø /** */ arespecialJavaDoccomments

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/** * Our first Java class * @author Sara Sprenkle */public class Hello {

public static void main(String[] args) {//print a messageSystem.out.println("Hello");

}}


CodeStyle• Commentsattopofprogram

Ø MustincludeyournameØ High-leveldescrip+onofprogram

• ProperindentaEonØ SimilartoPythonØ Everythingwithinsetsof{}isindentedthesame

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}

/** * Our first Java class * @author Sara Sprenkle */


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WhataretheDifferences?

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# a Python programdef main():

print(“Hello”)

main()/** * Our first Java class * @author Sara Sprenkle */public class Hello {


}}Sept12,2016 Sprenkle-CSCI209

Javavs.Python,sofar…• Seman&csthesame,syntaxdifferent

Ø BlocksofcodeØ Endstatements

• Accessmodifiers• Datatypedeclara+ons• Class-basedprograms• Compiled

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We’ll see more differences as we go…


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LiteralTransla+ontoPythonProgram?

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/** * Our first Java class * @author Sara Sprenkle */public class Hello {


}}


Transla+ontoPythonProgram

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class Hello:"""Our first Python class"""

def __init__(self):# fill in later…

def main(self):print ("Hello")

Semi-literaltransla+on


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JAVAFUNDAMENTALS


PrintStatement• Syntax:

• SimilartoPython’sfile.write() methodØ NeedtocombineparameterintooneStringusing+’s• Python’sprint usedcommas

Ø MoreonStringopera+onslater

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System.out.println(<String>);System.out.print(<String>);

No newline


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StringConcatena+on•  Ifastringisconcatenatedwithsomethingthatisnotastring,theothervariableisconvertedtoastring.

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System.out.println("The answer is " + 78);

Automatically converted to a String

Note +


EscapeSequences• SameasPython:

• InJava,youcanprinta'withoutescaping• Whatdoesthefollowingdisplay?

System.out.println("To print a \\, you must use \"\\\\\"");

30First.java

Meaning Sequence

Newlinecharacter(carriagereturn)

\n

Tab \tQuote \"Backslash \\


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Javakeywords/reservedwords• Case-sensi+ve• Can’tbeusedforvariableorclassnames• Reservedwordsseensofar…

Ø publicØ classØ staticØ void

• Exhaus+velistØ http://docs.oracle.com/javase/tutorial/java/nutsandbolts/_keywords.html


DataTypes•  Javaisstrongly-typed

Ø  Everyvariablemustbeadeclaredtype•  AlldatainJavaisanobjectexceptfortheprimiEvedatatypes:

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int 4bytes(-2,147,483,648->2,147,483,647)short 2bytes(-32,768->32,767)long 8bytes(reallybigintegers)byte 1byte(-128->127)float 4bytes(floa+ngpoint)double 8bytes(floa+ngpoint)char 2bytes(Unicoderepresenta+on),singlequotesboolean false ortrueSept12,2016 Sprenkle-CSCI209

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Variables• Mustbedeclaredbeforeused

Ø Syntax:<datatype> <name> [= value];

• Variablenamestypicallystartwithlowercasele[erØ _(underscore)alsoavalidfirstcharacterØ Conven+on:Subsequentwordsarecapitalized

•  Called“CamelCasing”

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Op+onalassignment


VariableExamples• Mustbedeclaredbeforeused

Ø Syntax:<datatype> <name> [= value];

• Examples:Ø int x;Ø double pi = 3.14;Ø char exit = 'q';Ø boolean isValid = false;

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CamelCasing

Note must use single quotes for chars


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FloatsinJava• Decimalliteralsareconsidereddoubles• Thiscodewon’tcompile:

• Compilererrormessage:

• Tofixcode,addanftospecifica+onofnumberordeclareasdouble

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Float.java:13: possible loss of precisionfound : doublerequired: float float f = 3.14;

float f = 3.14;

Float.java

Compiler reads 3.14as a double


PythonTransi+onWarning

• OK:

• NotOK:

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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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MoreDataTypeInforma+on• Defaultdatatypes

Ø SameasPython2,notPython3Ø Resultofintegerdivisionisanint

• Example:4/3 = ??

• Cas+ngØ SimilartoPythonforprimi+vetypesØ Example:4/(double) 3

37Sept12,2016 Sprenkle-CSCI209TestScore.java

Forma[edOutput• printforformat

Ø System.outisaPrintStreamobject

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double d1=3.14159, d2=1.45, total=9.43;

// simple formatting...System.out.printf("%6.5f and %5.2f ", d1, d2);

// %n is platform-specific line separator, // e.g., \n or \r\nSystem.out.printf("%-6s%5.2f%n", "Tax:", total);

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Constants• Read-onlyvariables

Ø Cannotbeassignednewvalues

• Keywordfinal precedesdatatypeØ Examplewithinamethod:

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final double CM_PER_INCH = 2.540;

Why might we want to use constants within a method?


ClassConstants• Constantvariableforallmethodsinclassorformul+pleclassesØ Muchmorecommonthanconstantinstancevariables

• Requiresstatic keywordØ static:“forclass”Ø Alsousedformethods(willseemorelater)

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static final double CM_PER_INCH = 2.540;


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Arithme+c,Rela+onalOperators• JavahasmostofthesameoperatorsasPython:

Ø Arithme+coperators:+, -, *, /, %• Nopoweroperator:**

Ø Rela+onaloperators:==, !=, <, >, <=, >=• Evaluatetoaboolean value

Ø Incrementanddecrement•  += x, -= y, etc.• Addi+onalshortcutfor+=1,-=1:++ , --

41Conversion.javaSept12,2016 Sprenkle-CSCI209

UnixOutputRedirec+on:>• Wecanredirectoutputtoafile

Ø Forexample

Ø Abovecommandsavestheoutputfromthels commandintothefilenamedjava_files.out

• ThisishowyouwillsaveoutputfromyourJavaprogramsini+allyØ Forexample

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java Intro > out

ls *.java > java_files.out

Please follow instructions on namesSept12,2016 Sprenkle-CSCI209

Documents

Objecves - Washington and Lee Universitysprenkle/cs209/pre_class/01-java_fundamentals.pdf · 9/9/16 3 Review: Java Virtual Machine • Same bytecode executes on each plaorm • Don’t