Introduction to Java

Java OriginsComputerlanguageinnovationanddevelopment occurs for two fundamental reasons:1) to adapt to changing environments and uses2)toimplementimprovementsintheartof programmingThedevelopmentofJavawasdrivenbybothinequal measures.anyJavafeaturesareinheritedfromtheearlier languages:!CC"" Java1Before Java: C#esigned by #ennis $itchie in 1%&'s.!eforeC(therewasnolanguagetoreconcile:ease)of)useversuspower(safetyversusefficiency(rigidity versus e*tensibility.!+,-C(C.!./(0.$T$+1(2+,C+/optimi3ed one set of traits( but not the other.C)structured(efficient(high)levellanguagethat couldreplaceassemblycodewhencreatingsystems programs.#esigned(implementedandtestedbyprogrammers( not scientists.2Before Java: C++#esigned by !4arne ,troustrup in 1%&%.$esponse to the increased comple*ity of programs and respective improvements in the programming paradigms and methods:1) assembler languages2) high)level languages5) structured programming6) ob4ect)oriented programming 7..2)..28methodologythathelpsorgani3ecomple*programs through the use of inheritance( encapsulation and polymorphism.C"" e*tends C by adding ob4ect)oriented features.3Java History#esignedbyJames9osling(2atric:1aughton(Chris;arth( -nternet version of C""?@ 1o.Java was not designed to replace C""( but to solve a different setofproblems.TherearesignificantpracticalAphilosophical differences.4Java TechnologyThere is more to Java than the language.Java Technology consists of:1) Java 2rogramming /anguage2) Java Birtual achine 7JB)5) Java +pplication 2rogramming -nterfaces 7+2-s)5Java Language Features1) simple2) object-oriented3) robust4) multithreaded5) architecture-neutral6) interpreted and high-performance7) distributed8) dynamic9) secure6Java Language Features1)simple8Javaisdesignedtobeeasyforthe professional programmer to learn and use.2)ob4ect)oriented8aclean(usable(pragmatic approachtoob4ects(notrestrictedbytheneedfor compatibility with other languages.5)robust8restrictstheprogrammertofindthe mista:esearly(performscompile)time7strong typing)andrun)time7e*ception)handling)chec:s( manages memory automatically.7Java Language Features 6) multithreaded 8 supports multi)threaded programming for writing program that perform concurrent computationsC)architecture)neutral8JavaBirtualachineprovidesa platform independent environment for the e*ecutionof Java bytecodeD)interpretedandhigh)performance8Javaprogramsare compiled into an intermediate representation 8bytecode:a) can be later interpreted by any JBb)canbealsotranslatedintothenativemachinecodefor efficiency.8Java Language Features&)distributed8JavahandlesTC2A-2protocols( accessingaresourcethroughits=$/much li:e accessing a local file.E)dynamic8substantialamountsofrun)time typeinformationtoverifyandresolve access to ob4ects at run)time.%)secure8programsareconfinedtotheJava e*ecutionenvironmentandcannotaccess other parts of the computer.9Execution Platform;hat is an e*ecution platform@1) +n e*ecution platform is the hardware orsoftware environmentinwhichaprogramruns(e.g. ;indows 2'''( /inu*( ,olaris orac.,.2)ostplatformscanbedescribedasa combination of the operating system andhardware.10Java Execution Platform;hat is Java 2latform@1) + software)only platform that runs on top of other hardware)based platforms.2) Java 2latform has two components:a)JavaBirtualachine7JB)8interpretation fortheJavabytecode(portedontovarious hardware)based platforms.b)TheJava+pplication2rogramming-nterface 7Java +2-)11Java Platform n!e"en!ence12Java Program ExecutionJava programs are both compiled and interpreted:,teps:F write the Java programF compile the program into bytecodeFe*ecute7interpret)thebytecodeonthecomputer through the Java Birtual achineCompilation happens once.-nterpretationoccurseachtimetheprogramis e*ecuted.13Java Execution Process14Java #P;hat is Java +2-@1)alargecollectionofready)madesoftware componentsthatprovidemanyusefulcapabilities( e.g. graphical user interface2)groupedintolibraries7pac:ages)ofrelated classes and interfaces5)togetherwithJBinsulatesJavaprograms fromthehardwareandoperatingsystem variations15Java Program Ty"esTypes of Java programs:1) applications 8 standalone 7des:top) Java programs( e*ecutedfromthecommandline(onlyneedthe Java Birtual achine to run2)applets8JavaprogramthatrunswithinaJava)enabledbrowser(invo:edthrougha>applet? reference on a web page( dynamicallydownloaded to the client computer5)servlets8Javaprogramrunningontheweb server(capableofrespondingtoGTT2requests made through the networ:16$im"le Java Program+ class to display a simple message:class y2rogram Hpublic static void main7,tring args IJ) H,ystem.out.println7>0irst Java program.K)LMM17Running the ProgramType the program( save as y2rogram.4ava.-n the command line( type:N diry2rogram.4avaN 4avac y2rogram.4avaN diry2rogram.4ava( y2rogram.classN 4ava y2rogram0irst Java program.18Ex"laining the Process1) creating a source file ) a source file contains te*t written in the Javaprogramminglanguage(createdusinganyte*teditoron any system.2)compilingthesourcefile)Javacompiler74avac)readsthe source file and translates its te*t into instructions that the Java interpretercanunderstand.Theseinstructionsarecalled bytecode.5)runningthecompiledprogram)Javainterpreter74ava) installedta:esasinputthebytecodefileandcarriesoutits instructions by translating them on the fly into instructions that your computer can understand.19Java Program Ex"laine!y2rogram is the name of the class:class y2rogram Hmain is the method with one parameter args and no results:public static void main7,tringIJ args) Hprintln is a method in the standard ,ystem class:,ystem.out.println7>0irst Java program.K)LMM20Classes an! O%&ects+ class is the basic building bloc: of Java programs.+ class encapsulates:a) data 7attributes) andb) operations on this data 7methods)and permits to create ob4ects as its instances.21'ain 'etho!The main method must be present in every Java application:1)public static void main7,tringIJ args) where:a)public means that the method can be called by any ob4ectb)static means that the method is shared by all instancesc)void means that the method does not return any value2);hentheinterpretere*ecutesanapplication(itstartsby callingitsmainmethodwhichinturninvo:esothermethods in this or other classes.5)Themainmethodacceptsasingleargument8astringarray( which holds all command)line parameters.22Exercise: Java Program1) 2ersonali3e they2rogramprogram with your name so that it tells you>Gello( my name is O?2) ;rite a program that produces the following output:;elcome to 0irst Java 2rogramP)LMMTheprogramhassomeerrors.0i*theerrorssothattheprogram successfully compiles and runs. ;hat were the errors@ 23Java $yntax.n the most basic level( Java programs consist of:a) whitespacesb) identifiersc) commentsd) literalse) separatorsf) :eywordsg) operators/? 7e.g. 125/) are of type long.31Literals: Floating)Point Ty"esTwo notations:1) standard 8 2'''.C2) scientific 8 2.'''C0? 7e.g. 2.'''Cglobal? scope2) variables defined by a method have a >local? scope;e consider the scope of method variables onlyL class variables will be considered later.743aria%le $co"e75$co"e 1efinition+ scope is defined by a bloc::HOM+ variable declared inside the scope is not visible outside:Hint nLMn Y 1L76Exam"le: 3aria%le $co"eclass ,cope Hpublic static void main7,tring argsIJ) Hint *L* Y 1'Lif 7* YY 1') Hint y Y 2'L ,ystem.out.println7K* and y: K " * " >K " y)L* Y y S 2LM,ystem.out.println7K* is K " * " >y is? " y)LMM773aria%le LifetimeBariablesarecreatedwhentheirscopeisenteredby control flow and destroyed when their scope is left:1)+variabledeclaredinamethodwillnotholdits value between different invocations of this method.2) + variabledeclaredinabloc:loosesitsvaluewhen the bloc: is left.5) -nitiali3ed in a bloc:( a variable will be re)initiali3ed with every re)entry.Bariable5s lifetime is confined to its scopeP78Exam"le: 3aria%le Lifetimeclass LifeTime {public static void main(String args[]) {int x;for (x = 0; x < 3; x++) {int y = -1;System.out.println("y is: " + y);y = 100;System.out.println("y is now: " + y);}}}79Ty"e 1ifferences,uppose a value of one type is assigned to a variable of another type.T1 t1LT2 t2 Y t1L;hat happens@ #ifferent situations:1) types T1 and T2 are incompatible2) types T1 and T2 are compatible:a) T1 and T2 are the sameb) T1 is larger than T2c) T2 is larger than T180Ty"e Com"ati%ility;hen types are compatible:1)integertypesandfloating)pointtypesarecompatible with each other2) numeric types are not compatible with char or boolean5) char and boolean are not compatible with each other a is ) ve numK)LMMM964este! if $tatement+ nestedifisanifstatementthatisthetarget of another if or else. 1ested ifs are verycommon in programming. ;hen you nest ifs( the main thing to remember isthatanelsestatementalwaysreferstothe nearestifstatementthatiswithinthesame bloc:astheelseandthatisnotalready associated with an else.974este! if $tatementclass i is : > " i)LM-tispossibletoinitiali3emultiplevariableinthe initiali3ation bloc: of the for loop by separating it by commaasgiveninthebelowe*ample.0or7iY'(4YCLi^CLi"")-t is also possible to have more than one increment or decrementsectionaswellasgivenbelow.for7int iY'L i ^ C L i""( 4)))112The /hile Loo"The general form of the while loop iswhile7condition) HstatementLMThe condition can be any!oolean e*pression. The body of the loop will be e*ecuted as long as the conditional e*pression is true. ;henconditionbecomesfalse(controlpasses to the ne*t line of code immediately following the loop. Thecurlybracesareunnecessaryifonlya single statement is being repeated.113The /hile Loo"114The /hile Loo"class ;hile Hpublic static void main7,tring argsIJ) Hint n Y 1'Lwhile7n N ') H,ystem.out.println7Ktic: K " n)Ln))LMMM115!o)/hile Loo"general form isdo HAA body of loopM while 7condition)Lgoto?statement( whichtendstoproducecodethatishardto understand and maintain.Gowever(insomeplaces(theuseofgotosiswell 4ustified.-nparticular(whenbrea:ingoutfromthe deeply nested bloc:s of code.brea: occurs in two versions:1) unlabelled2) labeledThelabeledbrea:statementisa>civili3ed? replacement for goto.124La%ele! %rea*9eneral form:brea: labelLwherelabelisthenameofalabelthat identifies a bloc: of code:label: H O MTheeffectofe*ecutingbrea:labelListo transfer control immediately after the bloc: of code identified by label.125Exam"le: La%ele! %rea*class !rea: Hpublic static void main7,tring argsIJ) Hboolean t Y trueLfirst: H second: Hthird: H,ystem.out.println7K!efore the brea:.K)Lif 7t) brea: secondL,ystem.out.println7KThis won\t e*ecuteK)LM,ystem.out.println7KThis won\t e*ecuteK)L M,ystem.out.println7>+fter second bloc:.K)LMM M126Exam"le: 4este! Loo" %rea*class 1ested/oop!rea: H public static void main7,tring argsIJ) Houter: for 7int iY'L i^5L i"") H,ystem.out.print7K2ass K " i " K: K)L for 7int 4Y'L 4^1''L 4"") Hif 74 YY 1') brea: outerL AA e*it both loops,ystem.out.print74 " K K)LM,ystem.out.println7KThis will not printK)LM,ystem.out.println7K/oops complete.K)LMM127%rea* (ithout La%el-t is not possible to brea: to any label which is not defined for an enclosing bloc:. Trying to do so will result in a compiler error.class !rea: