Java Programming - WordPress

Parameter ScopeParameters are formal arguments to methods or constructors and areused to pass values into methods and constructors.The scope of a parameter is the entire method or constructor forwhich it is a parameter.Exception-handler parameters are similar to parameters but arearguments to an exception handler rather than to a method or aconstructorCopyright 2005, Infosys TechnologiesLtd50

Final variablesYou can declare a variable in any scope to be final .The value of a final variable cannot change after it has beeninitialized.Such variables are similar to constants in other programminglanguages.To declare a final variable, use the final keyword in the variabledeclaration before the type:Copyright 2005, Infosys TechnologiesLtdfinal int Var 0;51

VisibilityVisibility is set with an access modifierApplies only to member variables and determines whether the variablecan be used from outside of the class within which it is declared.The access modifiers are public, protected, private and default(whennone specified)The default scope is Package.Copyright 2005, Infosys TechnologiesLtd52

Public-PrivatePublic variables and methods are those which can be accessed from anywhere i.e. From the class, outside the class and outside the package.Private variables are those which can be accessed only within the class.They are not visible outside that class.Copyright 2005, Infosys TechnologiesLtd53

ProtectedProtected variables re those which are visible only inside the class andthe children classes of that class.If your class extends a base class then your derived class will be able toaccess the variables and methods of the base class that are declared asprotected( and public of course .)Copyright 2005, Infosys TechnologiesLtd54

Default ScopeThe default Scope i.e. if you don’t specify any access modifiers the scopeis package scope.It means that within the package the class is it will be accessible butoutside the package it is not accessible.Copyright 2005, Infosys TechnologiesLtd55

Class Member AccessPrivateFriendlyProtected PublicSame esCopyright 2005, Infosys TechnologiesLtd56

The syntax.Java follows exactly the syntax of C with some minor differences.A happy news -------THERE IS NO POINTERS IN JAVABut we have a concept called reference that we have discussed alreadyCopyright 2005, Infosys TechnologiesLtd57

Interfaces( Inheritance in Java ]Class AClass AClass BClass BClass CClass CAllowed in JavaNot Allowed in JavaCopyright 2005, Infosys TechnologiesLtd58

InterfaceFollowing are the code for the diagram in the slide shownbefore :ClassClassBBextendsextendsAA{{ClassClass CC extendsextendsAA, ,BB{{}}}}ClassClassCCextendsextendsBB{{}}The code written above isnot acceptable by JavaCopyright 2005, Infosys TechnologiesLtd59

Implementing Multiple Inheritance in JavaInter AInter BInter CClass E can inherit frominterface A, B and C inthe following manner :INTERFACEClass EClassClass EE implementsimplements A,A,B,B,CC{{. . . . . . . . . . . . .; ;}}Copyright 2005, Infosys TechnologiesLtd60

Another way of implementing multiple InheritanceClass AEXTENDSinter BInter CClass E can inherit fromclasses A,& implements Band C in another way asshown here :INTERFACEClass EClassClassEEextendsextendsAA implementsimplementsB,B,CC{{. . . . . . . . . . . . .; ;}}Copyright 2005, Infosys TechnologiesLtd61

Creating an interface classIn Java interfacing is donein the following manner :When the code is executedas given below,“myinterface”.class file willbe created in the ntadd(intx,x,intinty)y); ;}}When the code for interface is executed as given below :javac –d c:\JavaProgs\ myinterface . javaCopyright 2005, Infosys TechnologiesLtd62

Using interface in ProgramsImporting the folderwhere myinterface.classfile is storedimportimport java.io.*java.io.*; ;importimport mypackage.*mypackage.*; intln(“ ”” ( (xx yy gmain(Stringargs[args[])]){{denodeno dd newnew demodemo( () ) ; ;d.addd.add(10(10, ,2020) ); ;}} 2005, Infosys TechnologiesCopyrightLtd63

Interfaces Contd Comparable to a pure abstract classClasses implement, by providing definitions to the methods of theinterfaceInheritance is possible in interfaces, even multiple inheritance is possibleCopyright 2005, Infosys TechnologiesLtd64

Why use Packages ?Just think of Writing thecode from the scratch,each time you create anapplicationYou’ll end up spending yourprecious time and energy andfinally land up with a Hugecode accumulated beforeyou.Copyright 2005, Infosys TechnologiesLtd65

Reusing The Existing CodeReusability of code is one ofthe most importantrequirements in the softwareindustry.Reusability saves time, effortand also ensures consistency.A class once developedcan be reused by anynumber of programswishing to incorporatethe class in thatparticular program.Copyright 2005, Infosys TechnologiesLtd66

Concept of PackagesIn Java, the codes which can be reused byother programs is put into a “Package”.A Package is a collection of classes, interfacesand/or other packages.Packages areinterfacesessentially apackagesmeans ofclasseorganizingsclasses togetherPackaas groups.geCopyright 2005, Infosys TechnologiesLtd67

Features of PackagesPackages are useful for the following purposes: Packages allow you toorganize your classes into smaller units ( such as folders ) and make iteasy to locate and use the appropriate class file.It helps to avoid naming conflicts. When you are working with a number ofclasses, it becomes difficult to decide on names of the classes & methods.At times you would want to use the same name, which belongs to ananother class. Package, basically hides the classes and avoids conflicts innames.Packages allow you to protect your classes, data and methods in a largerway than on a class-to-class basis.Package names can be used to identify your classes.Copyright 2005, Infosys TechnologiesLtd68

An Example on the use of PackagesPackage CircleMethod tofind area ofthe circleSomeClass MethodImportSomeMethodTo find the area of a circle on the frontface of the cube, we need not write acode explicitly to find the area of thecircleWe will import the package into ourprogram and make use of the areamethod already present in the package“circle”.Copyright 2005, Infosys TechnologiesLtd69

Importing a PackageIn Java, the Packages (where the required method is already created) canbe imported into any program where the method is to be used.We can import a Package in the following manner :import package name . class name ;Suppose you wish to use a class say My Class whose location is asfollows :My PackageMy Sub PackageThis class can be imported as follows :import My Package . MySub Package . My Class ;Copyright 2005, Infosys TechnologiesLtd70My Class

Creating a PackageIn Java Packages arecreated in the followingmanner :Package package name ntadd(intx,x,intinty)y)Methodto add( ){{return(return(xx yy));;}}}}mypackageCopyright 2005, Infosys TechnologiesLtd71

Compiling the packagejavacjavac -d-d c:\c:\JavaProgsJavaProgs ulaClate .When the above commandis executed on thecommand prompt, thecompiler creates a foldercalled “mypackage” in ourJavaProgs directory andstores the“Calculate.class” into thisfolderssCopyright 2005, Infosys TechnologiesLtd72

Standard Java PackagesThe Three Java Packages that are essential to any Java programare :javajava. .langlangjavajava. .ioiojava .langContains classes that form the basis ofthe design of the programminglanguage of Javajava .iojavajava. .utilutilThe use of streams for all input outputoperations in Java is handled by thejava.io packagejava . utilContains classes and interfaces that provide additional utility butmay not be always vital.Copyright 2005, Infosys TechnologiesLtd73

java.lang packageOne of the most important classes defined in this package is Object and itrepresents the root of the java class hierarchy.This package also holds the “wrapper” classes such as Boolean,Characters, Integer, Long, Float and Double.Many a times it is necessary to treat the non-object primitive datatypes ofint, char, etc. as objects.Thus Java defines “wrapper” classes that enable us to treat even primitivedata types as objects.These wrapper classes are found in the package“java.lang”.Other classes found in this package are :Math – which provides commonly used mathematical functions like sine,cosine and square root.String & String Buffer – Encapsulate commonly used operations oncharacter strings.Copyright 2005, Infosys TechnologiesLtd74

Some of the important methods of Math class int abs(int i) -- returns the absolute value of I long abs(long l) -- returns the absolute value of l float abs(float f) -- returns the absolute value of f double abs(double d) -- returns the absolute value of d double ceil(double d) -- returns as a double the smallest integer thatis not less than d double floor(double d) --- returns as a double the largest integerCopyright 2005, Infosys TechnologiesLtd75

java.io packageThis package has two very important abstract classes :Input Stream – This class defines the basic behavior required for input.Output stream – This class is the basis of all the classes that deal withoutput operations in Java.Since these are abstract classes, they cannot be used directly but mustbe inherited, so that the abstract methods can be implemented.All I/O stream classes are derived from either of these classes.Copyright 2005, Infosys TechnologiesLtd76

java.io packageThe classes derived in Inputstream and Outputstream can only read fromor write to the respective files.We cannot use the same class for both reading and writing operations.An exception to this rule is the class “RandomAccessFile”.This is the class used to handle files that allow random access and iscapable of mixed reading and writing operations on a file.There are two additional interface to this package : Data input Data output These classes are used to transfer data other than bytes orcharactersCopyright 2005, Infosys TechnologiesLtd77

Java.util packageOne of the most important package in this package is the class “Date”,which can be used to represent or manipulate date and time information.In addition, the class also enable us to account for time zones .Java helps us to change the size of an array which is usually fixed, bymaking use of the class “Vector”. This class also enable us to add,remove and search for items in the array.Copyright 2005, Infosys TechnologiesLtd78

Tips on using packagesThe statement :import java.awt.* ;Will include all the classes available in the “awt” subdirectorypresent in the java directory.While creating a package, care should be taken that thestatement for creating a package must be written before anyother import statementsLEGALILLEGALpackage mypackage ;import java . io;import java . io;package mypackage ;Copyright 2005, Infosys TechnologiesLtd79

Important Packages in Javajava.langjava.iojava.appletYou don’t need to explicitly import this package. It is alwaysimported for you.This package consists of classes that help you for all theInput and Output operations.This package consists of classes that you need, to executean applet in the browser or an appletviewer.java.awtThis package is useful to create GUI applications.java.utilThis package provides a variety of classes and interfacesfor creating lists, calendar, date, etc.java.netThis package provides classes and interfaces for TCP/IPnetwork programming.Copyright 2005, Infosys TechnologiesLtd80

Declaring and Access Control

ArraysAn array is a data structure which defines an ordered collection of a fixednumber of homogeneous data elementsThe size of an array is fixed and cannot increase to accommodate moreelementsIn Java, array are objects and can be of primitive data types or referencetypesAll elements in the array must be of the same data typeCopyright 2005, Infosys TechnologiesLtd82

ArraysDeclaring Arrays Variables elementType [] arrayName ;or elementType arrayName [];where elementType can be any primitive data type or reference typeExample:int IntArray[];Pizza[] mediumPizza, largePizza;Copyright 2005, Infosys TechnologiesLtd83

ArraysConstructing an Array arrayName new elementType [ noOfElements ];Example:IntArray new int[10];mediumPizza new Pizza[5];largePizza new Pizza[2];Declaration and Construction combinedint IntArray new int[10];Pizza mediumPizza new Pizza[5];Copyright 2005, Infosys TechnologiesLtd84

ArraysInitializing an Array elementType [] arayName { arrayInitializerCode };Example:int IntArray[] {1, 2, 3, 4};char charArray[] {‘a’, ‘b’, ‘c’};Object obj[] {new Pizza(), new Pizza()};String pets[] {“cats”, “dogs”};Copyright 2005, Infosys TechnologiesLtd85

IO Facilities in Java

OverviewIO Streams in JavaUnderstanding some fundamental streamsCreating streams for required functionalitySome advanced streamsCopyright 2005, Infosys TechnologiesLtd87

StreamsStreams are channels of communicationProvide a good abstraction between the source and destinationCould also act as a shield to lower transport implementationMost of Java’s IO is based on streams– Byte-oriented streams– Character-oriented streamsCopyright 2005, Infosys TechnologiesLtd88

Concatenating StreamsDataInputStreamFileInputStreamCopyright 2005, Infosys TechnologiesLtd89FileObject

Input & Output Streams

StreamsA stream can be thought of as a Conduit (pipe) for data between a sourceand the destination.Two types of Streams are1. Low level streams2. High level streamsCopyright 2005, Infosys TechnologiesLtd91

Low level streams & High level streamsStreams which carries bytes are called low level streams.Examples are FileInputStream and FileOutputStream.Streams which carries primitive data types are called highlevel streams. Examples are DataInputStream andDataOutputStream.Copyright 2005, Infosys TechnologiesLtd92

InputStream & OutputStreamInputStreams are used for reading the data from the source.OutputStreams are used for writing the data to thedestination.Copyright 2005, Infosys TechnologiesLtd93

utputStreambytesCopyright 2005, Infosys TechnologiesLtd94Javaprogram

Writing Primitives to a FileDataOutputStream dos new DataOutputStream( No);Copyright 2005, Infosys TechnologiesLtd95

Filter StreamsFilter contents as they pass through the streamFilters can be concatenated as seen beforeSome filter streams– Buffered Streams– LineNumberInputStream– PushBackInputStream– PrintStreamCopyright 2005, Infosys TechnologiesLtd96

Conversion StreamsInputStreamReader: bridge from byte streams to character streamsBufferedReader in newBufferedReader( r: bridge from chararcter streams to byte streamsCopyright 2005, Infosys TechnologiesLtd97

ReviewStreams are the basis of Java’s IOPre-defined streams for many situationsStreams need to be concatenatedConversion streams available for byte to character and vice-versaconversionCopyright 2005, Infosys TechnologiesLtd98

String classA string is a collection of charactersHas equals( ) method that should be used to compare the actual stringvaluesLot of other methods are available which are for the manipulation ofcharacters of the stringCopyright 2005, Infosys TechnologiesLtd99

public class Stringcomparison{public static void main(String args[]) {String ss1 new String("Rafiq");String ss2 new String("Rafiq");String s1 "Rafiq";String s2 "Rafiq";System.out.println(" comparison for StringObjects: " (ss1 ss2));System.out.println(" comparison for StringLiterals: " (s1 s2));System.out.println(" equals( ) comparison for StringObjects:" (ss1.equals(ss2)));System.out.println(" equals( ) comparison for StringLiterals:" (s1.equals(s2)));}}Copyright 2005, Infosys TechnologiesLtd100

class checkstring{ public static void main(String args[]){String str "HELLO guys & girls";System.out.println("The String is:" str);System.out.println("Length of the String is:" str.length());System.out.println("Character at specifiedposition:" str.charAt(4));System.out.println("substring of the Stringis:" str.substring(6,10));System.out.println("Index of the specifiedcharacter:" str.indexOf("g"));System.out.println("conversion touppercase:" str.toUpperCase());System.out.println("conversion touppercase:" str.toLowerCase());}}Copyright 2005, Infosys TechnologiesLtd101

String Buffer The prime difference between String & StringBuffer class is that thestringBuffer represents a string that can be dynamically modified. StringBuffer’s capacity could be dynamically increased eventhough it’scapacity is specified in the run time.ConstructorsStringBuffer()StringBuffer(int capacity)StringBuffer(String str)Methodsint length()int capacity()void setLength(int len)Copyright 2005, Infosys TechnologiesLtd102

String Bufferch

The Java Platform The Java platform has two components: The Java Virtual Machine (Java VM) The Java Application Programming Interface(Java API) The Java API is a large collection of ready-made software components that provide many useful capa