java notes of25sessions

7/29/2019 Java Notes Of25sessions

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OOP Definition

Concepts of Classes and objects

OOPs Features

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Encapsulation / Data HidingAbstraction

Inheritance

Polymorphism

Dynamic Binding

Message Passing

Contents of this session


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OOP is a programming methodology that helps to organizea complex programs through the use of inheritance,encapsulation and polymorphism

To access the class, we need an instance which can createan logical interface in between class & user (through mainfunction). Such instance is termed as Object. That why thisnew paradigm is termed as Object Oriented ProgrammingParadigm

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OOP increasing the flexibility in coding without disturbingthe other part of the code

OOP increasing the software development speed by reusing &enhancing the existing code


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An object is like ablack box.

The internal detailsare hidden.

Identifying objectsand assigningresponsibilitiesto these objects.

Objects communicate to otherobjects by sending messages.

Messages are received by themethodsof an object

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Everything in the world is an object.

For example-

Real world objects share two characteristics: They all havestate(attributes)and behavior (action). Dogs have state(name,color etc) and behavior (barking, wagging tails etc).


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Tangible Things as a car, printer, ...

Roles as employee, boss, ...

Incidents as flight, overflow, ...

Specifications as colour, shape,

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An object represents an individual, identifiable item, unit, or

entity, either real or abstract, with a well-defined role in the

problem domain.Or

An "object" is anything to which a concept applies.

Etc.

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Defining ClassA CLASS is a template (specification, blueprint)for a collection of objects that share a commonset of attributes and operations.

HealthClubMemberClass

Objects

attributesoperations


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Class & Instance


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Class defines a new data types

Once defined, this new type can be used to create objects ofthat type

Class is a template for an object and, an object is an instanceof a class

The data or variables, defined within a class are calledinstance variables The code is contained within methods Collectively, the methods and variables defined within a class

are called members of a class

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Furniture

ChairTable Book self Common Behavior:

Keeping something

RealLife Object: State-Variable

Behavior-method

Example:-


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Modularity - large software projects can be split up insmaller pieces.

Reusability - Programs can be assembled from pre-written software components.

Extensibility - New software components can be writtenor developed from existing ones.

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Features of 100% OOP Languages:1) Encapsulation / DataHiding

2) Abstraction

3) Inheritance4) Polymorphism

5) Dynamic Binding

6) Message Passing


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ENCAPSULATION / DATAHIDINGABSTRACTION

ENCAPSULATION Binding or Wrapping of the data andmethods into a single unit is called Encapsulation.

Encapsulation is used to hide data or protectdata from outside access. So this concept is also called data-hiding.

ABSTRACTION Abstraction refers to the act of representingessential features without including the background detailsand explanation.

explanationthe capsule cures the patient.. But the patientdont know what is capsulated inside the capsule..

CAPSULEMEDICINE INSIDE THE CAP


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Encapsulation ensures that data within an object isprotected; it can be accessed only by its methods

ENCAPSULATION


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INHERITANCEInheritance is a process, through which we can create a new

class from an existing class.

In Java extends key word is used

Parent , Super, Base Class

Child , Sub , Derived Class


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Vehicle2 Wheeler 3 Wheeler 4 Wheeler

Kinetic Scooter

is-a kind of hierarchyInheritance hierarchy


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INHERITANCE Inheritance is the process by which new classes

called derived classes are created from existing classescalled baseclasses. The derived classes have all the features of

the base class and the programmer can choose to add newfeatures specific to the newly created derived class.

A car is avehicle

A teacher is aperson

A dog is ananimal


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POLYMORPHISM=PLOY MORPH(MANY FORMS)

Anything which has more than one form depending on thesituation.

Polymorphism means having many forms. It allowsdifferent objects to respond to the same message indifferent ways, the response specific to the type of theobject.

In Java, polymorphism refers to the fact that you can havemultiple methods with the same name in the same class

POLYMORPHISM


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Move ( )Move ( )

Move ( )

Move ( )

CORE JAVA-OOP CONCEPTS


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Two types of Polymorphism

1. Compile-time/Static Polymorphism early binding or staticbinding.. ex: Method Overloading

Method Overloading is a process where the same functionname is used for more than one type/Functions can beoverloaded based on the function signature.

Method Signature are:

The no of Parameters

Type of Parameters

Sequence of Parameters

**Functions can not be overloaded based on return type.


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2. Run-time/Dynamic Polymorphism late binding or dynamicbinding.. ex: Method Overriding

Replacing an inherited method with another having the samesignature

Create a method in a subclass having the same signatureas amethod in a superclass

That is, create a method in a subclass having the same nameand the same number and types of parameters

Restrictions:

The return type must be the same


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Message PassingThe most important aspect of an object is its behaviour(thethings it can do). A behaviour is initiated by sending amessageto the object (usually by calling a method).

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For example: Message Passing

In a banking system there is two object one is customer &other is account number. when any customer object sent amessage to the account object requesting for the balance, theaccount object replies with the figure..


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Code reusability

Security

Software complexity decreases

Easy Debugging


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Encapsulation code quality, ease of maintenance

Inheritance efficiency, extensibility.

Polymorphism

power!


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State the history of Java

Java Applications

Describe features of Java

Summary

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JAVE SE(Java Platform, Standard Edition)-FORMERLY J2SE Java for the desktop / workstation

JAVA EE(ENTERPRISE EDITION)-FORMERLYJ2EE Java for the server programmingJAVA ME(MOBILE EDITION)-FORMERLYJ2ME(MICRO EDITION) Java platform designedfor embedded system

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ProgramThese are the set of instructions directing the computer to dosome action

programming languagesLanguages for specifying sequences of directions to acomputer

Software:It is the logical part of computer system, which is made up byseveral programs. In short, It is set of Programs.

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Programs are written using programming languages.


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Java Authors: James Gosling , Arthur Van , and others

Originally created for consumer electronics (TV, VCR, Freeze,Washing Machine, Mobile Phone)

Many ofJavas object-oriented features were influenced byC++

Oak was renamed to Java in May 1995.

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Its the current hot language

Its almost entirely object-oriented

It has a vast library of predefined objects and operations

Its more platform independent

this makes it great for Web programming

Its more secure

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Java is a general purpose ,Object-orientedprogramming language. We can develop three typesof applications in Java

Stand alone Applications GUI Applications Applet Applications

Java Applications


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JavaSourceCode

Java Compiler

Java Enabled

Web browser Java Interpreter

Output Output

Standalone and GUIapplications

Applets

Java Applications cont


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Java Is Simple Java Is Object-Oriented

Java Is Distributed

Java Is Interpreted

Java Is Robust Java Is Secure

Java Is Architecture-Neutral

Java Is Portable

Java's Performance

Java Is Multithreaded

Java Is Dynamic

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Java is partially modeled on C++,but greatly simplified and improved.

No pointers

Automatic garbage collection

Does not use header files andpreprocessors (like #include or #define)

Rich pre-defined class library


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Java Is Simple

Java Is Object-Oriented

Java Is Distributed

Java Is Interpreted



Java Is Portable

Java's Performance Java Is Multithreaded

Java Is Dynamic

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Focus on the data (objects) andmethods manipulating the data

Object-oriented programmingprovides great flexibility, modularity,

clarity, and reusability throughencapsulation, inheritance, andpolymorphism.


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Java Is Simple


Java Is Distributed

Java Is Interpreted



Java Is Portable

Java's Performance


Java Is Dynamic

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Works on a variety of platforms

Provides support for:Networking: via Socket ProgrammingInternet: The widely used protocolslike HTTP and FTP are developed in

java


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Java Is Simple


Java Is Distributed

Java Is Interpreted



Java Is Portable

Java's Performance


Java Is Dynamic

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You need an interpreter to run Javaprograms. The programs are compiledinto the Java Virtual Machine codecalled bytecode. The bytecode ismachine-independent and can run onany machine that has a Java interpreter,

which is part of the Java VirtualMachine (JVM).


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JAVA COMPILER

JAVA BYTE CODE

JAVA INTERPRETER

Windows 95

Macintosh Solaris Windows NT

(translator)

(same for all platforms)

(one for each different system)

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Java Is Simple


Java Is Distributed

Java Is Interpreted

Java Is Robust(Reliable) Java Is Secure


Java Is Portable


Java Is Dynamic

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Java compilers can detect manyproblems that would first show upat execution time in otherlanguages.

Memory protection andmanagement(using garbage collection)

Java has a runtime exception-

handling feature to provideprogramming support forrobustness.


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Java Is Simple


Java Is Distributed

Java Is Interpreted



Java Is Portable


Java Is Dynamic

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The Java language has built-incapabilities to ensure that violationsof security do not occur.

Access restrictions are forced(private, public)

The absence of pointers in javaensures that programs cannot gain

access to memory location withoutproper authorization.


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Java Is Simple


Java Is Distributed

Java Is Interpreted



Java Is Portable

Java's Performance


Java Is Dynamic

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Write once, run anywhere

With a Java Virtual Machine (JVM),you can write one program that willrun on any platform.


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A.classA.javaJavaCompiler

B.class

Loader

Verifier

Linker

Bytecode Interpreter

Java Virtual Machine

Compile source code

Java Virtual Machine Architecture

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Java Is Simple


Java Is Distributed

Java Is Interpreted



Java Is Portable

Java's Performance


Java Is Dynamic

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Because Java is architectureneutral, Java programs areportable. They can be run on anyplatform without being recompiled.Like

MacOSWindows XP/Vista/2007 SolarisLinux

Unix etc

Byte code

The sizes of the primitive data

types are machine independent.


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SourceCode ByteCode

.Java fileWindows

Mac

UNIX

Compile

.class file


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Java Is Simple


Java Is Distributed

Java Is Interpreted



Java Is Portable

Java's Performance


Java Is Dynamic

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Javas performance Because Java isarchitecture neutral, Java programs are

portable. They can be run on anyplatform without being recompiled.

just-in-time(Midway between compiling

and interpreting) compiling & native

code

JVM runningApplet or

Application

JITCOMPILER

.class file

machine code


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Java Is Simple


Java Is Distributed

Java Is Interpreted



Java Is Portable

Java's Performance


Java Is Dynamic

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Multithread programming issmoothly integrated in Java,whereas in other languages youhave to call procedures specific tothe operating system to enablemultithreading.

multiple concurrent threads ofexecutions can run simultaneously

Multi tasking.


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Java Is Simple


Java Is Distributed

Java Is Interpreted



Java Is Portable


Java Is Dynamic

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Java was designed to adapt to anevolving environment. New code

can be loaded on the fly withoutrecompilation. There is no need fordevelopers to create, and for usersto install, major new softwareversions. New features can be

incorporated transparently asneeded.

Java provides dynamic linking ofthe binary code at runtime.


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What did make Java so successful?

Portability

Reliability

Safety

Simplicity

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Java has emerged as a general purpose OO language.

It supports both stand alone and Internet Applications.

Makes the Web Interactive and medium for application delivery.

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The major differences between C++ & JAVA

Getting Started With Java Programming

Create, Compile and Running a Java Program

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Both Java and C++ are most popular object-orientedprogramming languages

C++ was created at AT&T Bell Labs in 1979

Java was born in Sun Microsystems in 1990

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C JavaC++

Java Vs. C++


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No Preprocessor

No Global Variables

No Goto statements

No Pointers

No Unsafe Structures

No Multiple Inheritance

No Operator Overloading No Fragile Data Types

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Java Powerful language

Programming

Clarity - Keep it Simple

Portability - Java portable, but it is an elusive goal

Performance

Interpreted programs run slower than compiled ones

Compiling has delayed execution, interpreting executesimmediately

Can compile Java programs into machine code

Runs faster, comparable to C / C++

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The secret behind the security and portability of Java is thatthe output of a Java compiler is not executable code. Rather, itis byte code

Byte codeis a highly optimized set of instructions designed tobe executed by the Java run-time system, which is called the

Java Virtual Machine (JVM)

JVM is an interpreter of byte code

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Java Vs. C++


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BytecodeJVM

OS kernelBinary codeOS kernel

JAVAsource code

C++source code

javaccompiler

C++ compilercc, turboc,turbo c++

Javainterpreter

Java compiler

Both compiled and interpreted Compiled

Java is Both compiled and interpreted

Java Vs. C++

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The .class files generated by the compiler are not executablebinaries. Instead, they contain byte-codes to be executed bythe Java Virtual Machine.

write once, run anywhere language.

Java Vs. C++

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Everything MUST be in a class. There are no global functions or global data. If you want

the equivalent of globals, make static methods andstatic data within a class.

There are no structs or enumerations or unions, onlyclasses.

Java has both kinds of comments like C++ does.

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Java Vs. C++


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Java has no preprocessor. If you want to use classes in another library, you say import

and the name of the library.

There are no preprocessor-like macros.

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All the primitive types in Java have specified sizes that aremachine independent for portability.

Java Vs. C++


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There is a garbage collection in JAVA Garbage collection means memory leaks are much harder to

cause in Java, but not impossible. (If you make nativemethod calls that allocate storage, these are typically not

tracked by the garbage collector.)

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There are no destructors in Java.- There's no need because of garbage collection.


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C++ JAVAC++ supports classes,structures orunions.

Java supports only classes.

In C++ class definition, classis closed by semicolon afterthe curly brace.

In Java class definition is verysimilaras C++, but presence ofsemicolon is

not required.Uses scope resolutionoperator (::).

Not using scope resolutionoperator (::).

Some other differences


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Feature C++ Objective Ada Java

Encapsulation Yes Yes Yes Yes

Inheritance Yes Yes No Yes

Multiple Inherit. Yes Yes No No

Polymorphism Yes Yes Yes Yes

Binding (Early or Late) Both Both Early Late

Concurrency Poor Poor Difficult Yes

Garbage Collection No Yes No Yes

Genericity Yes No Yes Limited

Class Libraries Yes Yes Limited Yes

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class HelloWorld

{

public static void main(String args[])

{

System.out.println(Hello World);

}

}

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class HelloWorld name of the class

{


{


}

}

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class HelloWorld{


{


}

}

Public is a access specifier that declares the method adunprotected and therefore making it accessible to all other classes.

Static static is a keyowrd ,which declares this method as one thatbelongs to the entire class and not a part of any objects of the class.

Void does not return anything.66


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class HelloWorld

{


Definingmain method

{


}

}arr[]takes the values of command line arguments..Suppose we are giving a b cdarr[0]=a,arr[1]=b,arr[2]=c,arr[3]=d

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String array as argument


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class HelloWorld{


{ System.out.println ( Hello World);}

}

System System is a class under java.langpackage.java.lang.Objectjava.lang.System

outout is the object of PrintStream class.println()is the method that prints in a new line.

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Text Editor Compiler Interpreter

Programmer

Source Code.java file

Byte Code.classfile

Hardware andOperating System

javac

.exe

file


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Java is CASE SENSITIVE!!

Whitespace is ignored by compiler

Whitespace makes things easier to readhence it affects your

grade

File name has to be the same as class name in file.

Need to import necessary class definitions

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Data types

Identifier

Variable

Comments

Reserved Words

Operators

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byte, short, int, and long for integer values of various sizes

float and double for real (rational) values of differing accuracy

boolean for logical (true/false) values

char for individual characters


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PRIMITIVE SIZE IN BITS Default Valueslong 8 bytes 0L

int 4 bytes 0

short 2 bytes 0

byte 1 bytes 0

float 8 bytes 0.0f

double 4 bytes 0.0d

char 2 bytes(unicode)

\u0000

bool (boolean inJava)

1 bytes false


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Integers 4, 19, -5, 0, 1000

Doubles 3.14, 0.0, -16.123

Strings Hi Mom Enter the number :

Character 'A' 'X' '9' '$' '\n'

Boolean true, false


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An identifier is a sequence of characters that consist of letters,

digits, underscores (_), and dollar signs ($).

Identifier = the technical term for a name in a programminglanguage

Identifier naming rules:

The first character must not be a digit.

May begin with a letter or the underline character _

If these rules are broken, your program won't compile.

Must not be a Java keyword Names must be descriptive.

Identifiers


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Identifier examples

class name identifier: Hello

method name identifier: main

variable name identifier: height

Constants All caps with _ between words


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A variables can be considered as a name given to thelocation in memory where values are stored.

How does the computer know which type of data a

particular variable can hold? Before a variable is used, its typemust be declaredin a

declarationstatement.

Declaration statement syntax:

;

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Syntax:type variable_name;ortype variable_name = expression;

Note

type must be known to the compiler variable_name must be a valid identifier expression is evaluated and assigned to variable_name

location In the first form, a default value is given (0, false, or null,

depending on type)


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There are two types of comments used in Java. These are:

In Java, comments are preceded by two slashes (//) in a line

Or enclosed between /* and */ in one or multiple lines.

When the compiler sees //, it ignores all text after // in thesame line.

When it sees /*, it scans for the next */ and ignores any textbetween /* and */.


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Reserved wordsor keywordsare words that have a specificmeaning to the compiler and cannot be used for other purposesin the program.

For example, when the compiler sees the word class, itunderstands that the word after class is the name for the class.

Other reserved words are like public, static, and void. Their use

will be introduced later in the book.

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a va eywor s

abstract boolean break byte case

catch char class continue default

do double else extends false

final finally float for if

implements import instanceof int interface

long native new null package

private protected public return short

static super switch synchronized this

throw throws transient true try

void volatile while

Keywords that are reserved but not used by Javaconst goto

Keywords are words reserved for Java and cannot be used as identifiers or

variable names

Java Keywords


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Types of operatorsSimple Assignment Operator

= (Simple assignment operator)Arithmetic Operators+ (Additive operator (also used for String concatenation))

- (Subtraction operator)* (Multiplication operator)/ (Division operator)% (Remainder operator)

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Unary Operators+ (Unary plus operator; indicates positive value)- (Unary minus operator; negates an expression)++ (Increment operator; increments a value by 1)-- (Decrement operator; decrements a value by 1)! (Logical compliment operator; inverts the value of aboolean)

Equality and Relational Operators

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== (Equal to) >= (Greater than or equal to)

!= (Not equal to) < (Less than)

> (Greater than)


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Conditional Operators&& (Conditional-AND)|| (Conditional-OR)?: (Ternary (shorthand for if-then-else statement))

Type Comparison Operatorinstanceof (Compares an object to a specified type)

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Bitwise and Bit Shift Operators~ (Unary bitwise complement)> (Signed right shift)

>>> (Unsigned right shift)

& (Bitwise AND)

^ (Bitwise exclusive OR)

Operand should be integer type

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Bitwise AND

10012 & 00112 = 00012

Bit OR

10012 | 00112 = 10112 Exclusive OR

10012 ^ 00112 = 10102

1s Complement

~ 000010102

= 111101012

Operators cont


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Control statement

Array

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if statement

switch statement

conditional operator


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Writing if Statements ifstatement:

Interrogates logical expression enclosed in parentheses

boolean expressions can use just one word

example:boolean isFound=true;

if (isFound)System.out.println(The object is found.);

Determines whether it is true or false

Uses logical operators to compare values


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Writing if Statements You dont need the brackets for only one statement.

ifstatements can contain compound expressions

Two expressions joined using logical operators

OR ||

AND &&

Nested ifstatement

ifstatement written inside another ifstatement

the elsestatement always corresponds with the closest if

statement


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Using the Conditional Operator

Conditional operator (?)

Provides a shortcut to writing an if-else statement

Structure:

variable = expression ? value1:value2;

exampleint smallerNumber = (a < b) ? a : b;


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Writing switchStatements

Acts like a multiple-way ifstatement

Transfers control to one of several statements or blocksdepending on the value of a variable

Used when there are more than two values to evaluate

Restrictions:

Each case evaluates a single variable for equality only

Variable being evaluated must be: char, byte, short, or int


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char eventType;

switch (eventType){

case A:eventCoordinator = Dustin;

break;case B:eventCoordinator = Heather;

break;case C:

eventCoordinator = Will;break;default:eventCoordinator = Invalid Entry;

}


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Loops

Provides for repeated execution of one or more statementsuntil a terminating condition is reached

Three basic types:

while

do

for

What is the difference between the while and do loops?


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Following figure shows a while loop that prints the integersthrough 10


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Post-test loop Tests terminating condition at the end of the loop

Forces execution of statements in the loop body at leastonce

Example:

do

{System.out.println(count = + count);count++;

}

while (count


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Pre-test loop Tests terminating condition at the beginning of the loop

Includes counter initialization and incrementing code in thestatement itself

Example:

for (int count=1; count


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An array is a group of contiguous or related data items thatshare a common name.

Used when programs have to handle large amount of data

Each value is stored at a specific position

Position is called a index or superscript. Base index = 0

The ability to use a single name to represent a collection ofitems and refer to an item by specifying the item numberenables us to develop concise and efficient programs.

For example, a loop with index as the control variable can beused to read the entire array, perform calculations, and printout the results.


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Creating an array is a 2 step process

It must be declared (declaration does not specify size)

It must be created (ie. memory must be allocated for thearray)

Creating Arrays

type[] arrayName;declaration syntax:

note the location of the []

int[] grades; // declaration

grades = new int[5]; // Create array.// specify size

// assign new array to

// array variable


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int[] grades = new int[5];

When an array is created, all of its elements are automaticallyinitialized

0 for integral types

0.0 for floating point types

false for boolean types

null for object types

Creating Arrays

0

0

0

0

0

4

3

2

1

0array indices

Note: maximum array index is length -1

grades


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Because array elements are initialized to 0, the array should beinitialized with usable values before the array is used.

This can be done with a loop

Arrays have a length attribute which can be used for boundschecking

Elements are accessed using an index and []

Initializing and Using Arrays

int[] sequence = new int[5];

for (int i=0; i< sequence.length; i++)

{

sequence[i] = i * 25;

}

array length: ensures loop

won't go past end of the arrayArray element being accessed. In this

case, it is being assigned a value.


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Another way of initializing lists is by using initializer lists.

The array is automatically created

The array size is computed from the number of items in thelist.

Using initialize lists

int[] grades = {100, 96, 78, 86, 93};

type[] arrayName = {initializer_list};

String[] colours = { "Red", "Orange",

"Yellow", "Green",

"Blue", "Indigo",

"Violet"};


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Two dimensional arraysallows us to store datathat are recorded in table.For example:

Table contains 12 items,we can think of this as amatrix consisting of 4rows and 3 columns.

Item1 Item2 Item3

Salesgirl #1 10 15 30




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Sold

Person


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Declaration: int myArray [][];

Creation: myArray = new int[4][3]; // OR

int myArray [][] = new int[4][3];

Initialisation: Single Value;

myArray[0][0] = 10;

Multiple values:

int tableA[2][3] = {{10, 15, 30}, {14, 30, 33}};

int tableA[][] = {{10, 15, 30}, {14, 30, 33}};

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0


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Arrays can be used to store objects

Circle[] circleArray;circleArray = new Circle[25];

The above statement creates an array that can storereferences to 25 Circle objects.

Circle objects are not created.

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1


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Create the Circle objects and stores them in the array.

//declare an array for Circle

Circle circleArray[] = new Circle[25];

int r = 0;

// create circle objects and store in array

for (r=0; r


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Classes and objects

Constructors

this keyword

Garbage Collection

finalize() Method

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3


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Defining ClassA CLASS is a template (specification, blueprint)for a collection of objects that share a commonset of attributes and operations.

HealthClubMemberClass

Objects

attributesoperations


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5

Class & Instance


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Class defines a new data types Once defined, this new type can be used to create objects of

that type

Class is a template for an object and, an object is an instanceof a class

The data or variables, defined within a class are calledinstance variables The code is contained within methods Collectively, the methods and variables defined within a class

are called members of a class

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class classname{access modifiers:

type instance-variable1

//

type instance-variable N

access modifiers:type methodname1(parameter-list){

//body of the method

}

//

type methodname N(parameter-list){//body of the method

}

}

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General form of a class does not specify a main( ) method

Class declaration and implementation of the methods are

stored in the same place and not defined separately (Differentfrom the C++)

Sometimes .java files become very large

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class Box{

double width;

double height;

double depth;

}

This code does not cause any objects of type Box to comeinto existence

To actually create a Box object, you will use a statement like:Box mybox = new Box(); // create a Box object called mybox

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Here is a complete program that uses the Box classclass Box {

double width;

double height;

double depth;

}

// This class declares an object of type Box.

class BoxDemo {

public static void main(String args[]) {Box mybox = new Box();

double vol;

// assign values to mybox's instance variables

mybox.width = 10;

mybox.height = 20;

mybox.depth = 15;

// compute volume of boxvol = mybox.width * mybox.height * mybox.depth;

System.out.println("Volume is " + vol);

}

}

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To obtain objects of a class, we need

First declare a variable of the class type

Second, acquire an actual, physical copy of the object andassign it to that variable (by new operator)

In Java, all class objects must be dynamically allocated

Object references appear to be similar to pointers, but youcan not manipulate references as you actual pointers

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123

Statement Effect

Box mybox;

mybox

mybox=new Box( );

mybox

Box object

null

Width

HeightDepth


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mybox=new Box();

The class name followed by parentheses specifies theconstructor for the class

If a class dont explicitly define its own constructor, the Java

automatically supply a default constructor We dont need to use new for integers or characters.

Becausejavas simple types are not implemented as objects

It is possible that new will not be able to allocate memoryfor an object because of insufficient memory. In that case, arun-time exception will occur

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Box b1=mew Box();Box b2=b1;

b1 and b2 will both refer to the same object

b1

b2

Box object If b1 is assigned null (b1=null;), the b2 still points to the

original object

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WidthHeight

Depth


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class Box {

double width;

double height;

double depth;

// display volume of a box

void volume() {

System.out.print("Volume is ");

System.out.println(width * height * depth);

}

}class BoxDemo3 {

public static void main(String args[]) {

Box mybox1 = new Box();

// assign values to mybox1's instance variables

mybox1.width = 10;

mybox1.height = 20;

mybox1.depth = 15;

// display volume of first boxmybox1.volume();

}

}

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class Box {

double width;double height;

double depth;

double volume( ) { // compute and return volume

return width * height * depth;

}

}

class BoxDemo4 {public static void main(String args[ ]) {

Box mybox1 = new Box();

double vol;

mybox1.width = 10; // assign values to mybox1's instance variables

mybox1.height = 20;

mybox1.depth = 15;

vol = mybox1.volume( ); // get volume of first boxSystem.out.println("Volume is " + vol);

}

}

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There are two problems in the previous code Its is clumsy and error prone (you may forget to set

dimension)

Instance variables should be accessed only throughmethods defined by their class

void setDim(double w, double h, double d) {

width = w;

height = h;

depth = d;

} mybox1.setDim(10, 20, 15);

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A constructor is automatically called immediately after theobject is created, before the new operator completes

Its has no return types, not even void

Constructors have the same name as the classname

When we write Box mybox1=new Box( );actually theconstructor for the class is being called

As you know, Java creates a default constructor for the class,which initializes all instance variables to zero

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Constructor


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Box() {

System.out.println("Constructing Box");

width = 10;

height = 10;

depth = 10;}

When you write Box mybox1=new Box( ); the values of height,width and depth are automatically assigned

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Box(double w, double h, double d) {

width = w;

height = h;

depth = d;

}

Box mybox1 = new Box(10, 20, 15);

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Sometimes a method will need to refer to the object thatinvoked it. To allow this, Java defines the this keyword

this can be used inside any method to refer to the currentobject. this always refers to the object on which the methodwas invoked

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Sometimes an object will need to perform some action when

it is destroyed (i.e. release some non-java resources such asfile handle or windows character font)

finalize( ) method allows us to define specific actions that

will occurs when an object is just about to be reclaimed bythe garbage collector

protected void finalize( )

{

//finalization code here

} You cannot know when- or even if- finalize( ) will be

executed. Therefore, you must not rely on it for normalprogram execution

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Methods are distinguished by their signature:

name

number of arguments

type of arguments

position of arguments

That means, a class can also have multiple usual methodswith the same name.

Not to confuse with method overriding(coming up), methodoverloading:

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class OverloadDemo {void test() {

System.out.println("No parameters");

}

// Overload test for one integer parameter.

void test(int a) {

System.out.println("a: " + a);}

// Overload test for two integer parameters.

void test(int a, int b) {

System.out.println("a and b: " + a + " " + b);

}

// overload test for a double parameterdouble test(double a) {

System.out.println("double a: " + a);

return a*a;

}

}137


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Constructor is a special method that gets invokedautomatically at the time of object creation.

Constructor is normally used for initializing objects with defaultvalues unless different values are supplied.

Constructorhas the same name as the class name.

Constructor cannot return values.

A class can have more than one constructor as long as they havedifferent signature (i.e., different input arguments syntax).

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Sometimes want to initialize in a number of different ways,

depending on circumstance.

This can be supported by having multiple constructorshaving different input arguments.

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0


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1

public class Circle {public double x,y,r; //instance variables// Constructors

public Circle(double centreX, double cenreY, doubleradius) {

x = centreX; y = centreY; r = radius;}public Circle(double radius) { x=0; y=0; r = radius; }public Circle() { x=0; y=0; r=1.0; }

//Methods to return circumference and areapublic double circumference() { return 2*3.14*r; }public double area() { return 3.14 * r * r; }

}


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14

2

public class TestCircles {

public static void main(String args[]){Circle circleA = new Circle( 10.0, 12.0, 20.0);Circle circleB = new Circle(10.0);Circle circleC = new Circle();

}}

circleA = new Circle(10, 12, 20)circleB = new Circle(10)

Centre = (0,0)

Radius=10

circleC = new Circle()

Centre = (0,0)Radius = 1

Centre = (10,12)Radius = 20


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14

3

Method Overloading

Using one object wecan call all themethods having samename

Example:A a=new A();

a.add(12,14);

a.add(11.5,16.8);

a.add(10,15,20);

Constructor Overloading

We need different objectsto call each constructors

Example:

Area a=new Area();

Area a1=new Area(4);

Area a2=new Area(4.0);


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Java supports definition of global methods and variables that

can be accessed without creating objects of a class. Suchmembers are called Static members.

Define a variable by marking with the static methods.

This feature is useful when we want to create a variablecommon to all instances of a class.

One of the most common example is to have a variable thatcould keep a count of how many objects of a class have beencreated.

Note: Java creates only one copy for a static variable whichcan be used even if the class is never instantiated.

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Using static variables:

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5

public class Circle {// class variable, one for the Circle class, how many

circlesprivate static int numCircles = 0;private double x,y,r;

// Constructors...Circle (double x, double y, double r){

this.x = x;this.y = y;

this.r = r;numCircles++;

}}


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Using static variables:

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6

public class CountCircles {

public static void main(String args[]){

Circle circleA = new Circle( 10, 12, 20); // numCircles = 1

Circle circleB = new Circle( 5, 3, 10); // numCircles = 2

}

}

circleA = new Circle(10, 12, 20) circleB = new Circle(5, 3, 10)

numCircles


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Instance variables : One copy per object. Every object hasits own instance variable.

E.g. x, y, r (centre and radius in the circle)

Static variables : One copy per class. E.g. numCircles (total number of circle objects created)

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public class HowToAccessStaticMethod

{int i;

static int j;

public static void staticMethod(){

System.out.println("you can access a static method this way");

}

public void nonStaticMethod(){

i=100;

j=1000;

System.out.println("Don't try to access a non static method");

}

public static void main(String[] args) {

//i=100;

j=1000;

//nonStaticMethod();

staticMethod();

}}

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They can only call other static methods.

They can only access static data.

They cannot refer to this or super (more later) in anyway.

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Inheritance

Method overriding

Difference between Method overloading & Method overriding

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Inheritance is a process, through which we can create a newclass from an existing class.

In Java extends key word is used

Parent , Super, Base Class

Child , Sub , Derived Class


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A class that is inherited is called superclass

The class that inherits is called subclass

A subclass is a specialized version of a superclass

extends keyword is used to inherit superclass Java does not support multiple superclasses into single

subclass (This differs from C++)

A subclass can be a superclass of another subclass

So class can be a superclass of itself

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1.Single Inheritance:

class A

{

//code

}

class B extends A

{

//code

}

A

B


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Types:-1.Single 2.Multilevel 4.Hierarchical


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3.Multiple Inheritance:

A

BC D

Types: 1.Single 2.Multilevel 3.Multiple 4.Hierarchical


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3.Multiple Inheritance: Not supported by

Java.

Diamond ProblemBut supported by

C++To Overcome this problem we

have the concept of Interface..

A

B

C D


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Inheritance: Inheritance is a process, through which we can create a new class from an

existing class.4.Hierarchical Inheritance:

class A

{

//code

}

class B extends A

{//code

}

class C extends A

{

//code

}

class D extends A{

//code

}

A

BC D


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If a superclass keeps its data members private, then therewould be no way for subclass to directly access or initializeits parents instance variables

The solution is the use of keyword super Whenever a subclass needs to refer to its immediate

superclass, it can do so by use of the keyword super Super has two general forms,

For accessing constructors For accessing hidden member of superclass

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super (parameter-list)

super( ) must always be the first statement executed inside asubclass constructor

// BoxWeight now uses super to initialize its Box attributes.

class BoxWeight extends Box {

double weight; // weight of box

// initialize width, height, and depth using super()

BoxWeight(double w, double h, double d, double m) {

super(w, h, d); // call superclass constructor

weight = m;}

}

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Here super acts as somewhat like thissuper.member

Mostly applicable when member names of a subclass hidemembers by the same name in the superclass

// Using super to overcome name hiding.class A {

int i;}// Create a subclass by extending class A.class B extends A {

int i; // this i hides the i in A

B(int a, int b) {super.i = a; // i in Ai = b; // i in B

}

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Method Overriding is a process through which a base classmethod overridden by a derived class method

For method Overriding Method name and signature shouldbe same.

When a overridden method is called from within a subclass, itwill always refer to the version of that method defined by thesubclass

Like virtual functions in C++

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class B extends A {

int k;

B(int a, int b, int c) {

super(a, b);

k = c;

}

void show() {

super.show( ); // this calls A's show()

System.out.println("k: " + k);

}}

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Method overriding occurs only when the names and the typesignatures of the two methods are identical

If they are not, then two methods are simply overloaded

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Methods declared as final cannot be overriddenclass A {

final void meth() {

System.out.println("This is a final method.");

}

}

class B extends A {void meth() { // ERROR! Can't override.

System.out.println("Illegal!");

}

}

Normally Java resolves calls to methods dynamically, atrun time (late binding)

Since, final methods cannot be overridden, a call to onecan be resolved at compile time (early binding)

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Final classes

Abstract classes

String Class

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0


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We can prevent an inheritance of classes by other classes bydeclaring them as final classes.

This is achieved in Java by using the keyword final as follows:final class Marks{ // members}final class Student extends Person{ // members}

Any attempt to inherit these classes will cause an error.

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What about the object of abstract classes?

There can not be any object of any Abstract Classes.

i.e-An abstract class can not be directly instantiated with thenew operator

Why?

As the abstract class is not fully defined..


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A class with one or more abstract methods is automaticallyabstract and it cannot be instantiated.

A class declared abstract, even with no abstract methods cannot be instantiated.

A subclass of an abstract class can be instantiated if itoverrides all abstract methods by implementation them.

A subclass that does not implement all of the superclassabstract methods is itself abstract; and it cannot beinstantiated.

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Every string you create is actually an object of type String.Even string constants are actually String objects

String objects are immutable; once a String object is created,its contents cannot be altered

Java defines a peer class of String, called StringBuffer, whichallows strings to be altered

One way to create a string is:

String mystring=this is a test

+ operator is used to concatenate two strings

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equals( ) is used to test two string for equality

length( ) is used to obtain the length of string

charAt( ) is used to obtain the character at a specified indexwithin a string

Also you can have arrays of string

String str[ ]={one, two, three};

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If you do not want (properties of) your class to be extended


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If you do not want (properties of) your class to be extendedor inherited by other classes, define it as a final class. Java supports this is through the keyword final. This is applied to classes.

You can also apply the final to only methods if you do notwant anyone to override them.

If you want your class (properties/methods) to be extendedby all those who want to use, then define it as an abstractclass or define one or more of its methods as abstractmethods. Java supports this is through the keyword abstract. This is applied to methods only. Subclasses should implement abstract methods; otherwise,

they cannot be instantiated.

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4

Contents


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Understanding the concepts of :

Introduction to Package

Creating package

Importing package

Using package in java programs

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Packages:

Putting Classes Together

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6


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18

7

The features in basic form limited to reusing the classeswithin a program. What if we need to use classes from other programs withoutphysically copying them into the program under development

? In Java, this is achieved by using what is known as packages,a concept similar to classlibraries in other languages.


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Types of packages


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package

System packagesEx:

java.lang.*java.util.*java.io.*java.sql.*

User DefinedPackages

Created by user .

We have to follow

some steps .

package

Key word is used

to create a

package


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Java provides a large number of classes groped into differentpackages based on their functionality. The six foundation Java packages are:

java.lang Contains classes for primitive types, strings, mathfunctions, threads, and exception

java.util Contains classes such as vectors, hash tables, date etc.

java.io Stream classes for I/O

java.awt Classes for implementing GUI windows, buttons, menusetc.

java.net Classes for networking java.applet Classes for creating and implementing applets

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Java supports a keyword called package for creating user-defined packages. The package statement must be the firststatement in a Java source file (except comments and whitespaces) followed by one or more classes.

Package name is myPackage and classes are considred aspart of this package; The code is saved in a file calledClassA.java and located in a directory called myPackage.

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3

package myPackage;public class ClassA {

// class body}class ClassB {// class body

}


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Within the current directory (abc) store the following code in


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Within the current directory ( abc ) store the following code in

a file named ClassX.java

19

7

import myPackage.ClassA;

public class ClassX{

public static void main(String args[]){

ClassA objA = new ClassA();objA.display();

}}


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Let us store the code listing below in a file namedClassA.java within subdirectory named secondPackagewithin the current directory (say abc).

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9

package secondPackage;public class ClassC {// class body

public void display(){

System.out.println("Hello, I am ClassC");}

}


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Public keyword applied to a class, makes it available/visible

everywhere. Applied to a method or variable, completelyvisible.

Privatefields or methods for a class only visible within thatclass. Private members are notvisible within subclasses, andare notinherited.

Protectedmembers of a class are visible within the class,subclasses and alsowithin all classes that are in the same

package as that class.

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20

4

Accessible to: public protected Package

(default)

private

Same Class Yes Yes Yes Yes

Class in package Yes Yes Yes No

Subclass in

different package

Yes Yes No No

Non-subclass

different package

Yes No No No


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When packages are developed by different organizations it is


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When packages are developed by different organizations, it is

possible that multiple packages will have classes with the samename, leading to name classing.

We can import and use these packages like:

import pack1.*; import pack2.*; Student student1; // Generates compilation error

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7

class Teacher

package pack1;

class Student

class Student

package pack2;

class Courses


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Packages allow grouping of related classes into a single

united.

Packages are organised in hierarchical structure.

Packages handle name classing issues.

Packages can be accessed or inherited without actual copy of

code to each program.

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0

Contents


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Introduction about interface

Defining an interface

Use of interface?

Different between interface and abstract class

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1


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21

2212

Interfaces

Design Abstraction and a way forloosing realizing Multiple Inheritance


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2

1

5

speak()

Politician Priest

Speaker

speak() speak()

Lecturer

speak()

Syntax (appears like abstract class):


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Example:

21

6

interface InterfaceName {

// Constant/Final Variable Declaration

// Methods Declaration only method body

}

interface Speaker {

public void speak( );

}


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Interfaces are used like super-classes who properties areinherited by classes. This is achieved by creating a class thatimplements the given interface as follows:

21

7

class ClassName implements InterfaceName [, InterfaceName2, ]

{

// Body of Class

}


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A general form of interface implementation:


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This shows a class can extended another class whileimplementing one or more interfaces. It appears like amultiple inheritance (if we consider interfaces as special kindof classes with certain restrictions or special features).

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0

class ClassName extends SuperClass implements InterfaceName [,

InterfaceName2, ]

{

// Body of Class}

Consider a university where students who participate in the


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national games or Olympics are given some grace marks.Therefore, the final marks awarded = Exam_Marks +Sports_Grace_Marks. A class diagram representing thisscenario is as follow:

22

1

Student Sports

Exam

Results

extends

extendsimplements


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interface A{ class D


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void display();}

interface B extends A{

void show();

}

class C implements B{

void display(){

System.out.println(interface A);

}

void show(){

System.out.println(Interface B);

}}

{public static void main(String arr[])

{

C c=new C();

c.display();

c.show();

}

}


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Like abstract class,

1. if a class implement an interface, you have to override theinterfaces methods in the class.

2. You cannot create instances from an interface by using newoperator.

3. Interface can be a type as well.

Runnable r;

4. the purpose of creating interface is because ofpolymorphism.

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4. In the relationships, we say that:4.1. A relationship between class/abstract class and class isa strong relationship. It is known as IS-A relationship.E.g: A duck is a bird. It clearly means the duck is really abird. So the bird can be a superclass of a duck. It could be

either concrete or abstract class.4.2. A relationship between class and interface is a weakrelationship. It is known as Is-kind-of relationship.E.g: A duck is flyable. Flyable can never ever be thesuperclass of the duck. It just means this duck can fly. So

flyable is interface.

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To explain Exception in java

Show how programmer-defined exceptions are created,

thrown and caught

To provide the concepts of exception handling

Explain the use of try, catch, finally, throw, throws keywords.

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8


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The compilation process will detect programming mistakessuch as syntax errors and type mismatches. Such errors cantherefore be referred to as compilation errors.

But once code is compiled and running, it will have to face thereal world of erroneous input, inexistent files, hardwarefailure Such problems are commonly known as runtimeerrors, which are likely to cause the program to abort.

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9

An exception is a condition that is caused by a runtime error


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e cept o s a co d t o t at s caused by a u t e e oin the program.

Need of Exception Handling If a program does not handle the exception at all, the

program will terminate abnormally and produce a messagethat describes what exception occurred, and where it wasproduced.

The purpose of exception handling is to provide a means todetect and report an exceptional circumstance so thatappropriate action can be taken.


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A Java exception is an object that describes an exceptionalcondition that has occurred in a piece of code

When an exceptional condition arises, an object representingthat exception is created and thrownin the method thatcaused the error

An exception can be caught to handle it or pass it on

Exceptions can be generated by the Java run-time system, orthey can be manually generated by your code


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Any code that absolutely must be executed before a method


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returns is put in a finally block General form of an exception-handling blocktry{

// block of code to monitor for errors}catch (ExceptionType1 exOb){

// exception handler for ExceptionType1}catch (ExceptionType2 exOb){

// exception handler for ExceptionType2}//

finally{// block of code to be executed before try block ends

}

If an exception is not caught by user program, then executionof the program stops and it is caught by the default handler


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provided by the Java run-time system Default handler prints a stack trace from the point at which

the exception occurred, and terminates the program

Ex:

class Exc0 {

public static void main(String args[]) {int d = 0;

int a = 42 / d;

}

}Output:java.lang.ArithmeticException: / by zero

at Exc0.main(Exc0.java:4)

Exception in thread "main"


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Handling an exception has two benefits,

It allows you to fix the error

It prevents the program from automatically terminating

The catch clause should follow immediately the try block


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PROGRAM:

Output:

Division by zero.

After catch statement.
http://c/excep/Handle.javahttp://c/excep/Handle.java


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Program Output
http://c/excep/MultiCatch.javahttp://c/excep/MultiCatch.java


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If no command line argument is provided, then you will seethe following output:

a = 0

Divide by 0: java.lang.ArithmeticException: / by zero

After try/catch blocks

If any command line argument is provided, then you will seethe following output:a = 1Array index oob: java.lang.ArrayIndexOutOfBoundsException

After try/catch blocks.


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Remember that, exception subclass must come before any ofof their superclasses

Because, a catch statement that uses a superclass will catchexceptions of that type plus any of its subclasses. So, thesubclass would never be reached if it come after its

superclass

For example, ArithmeticException is a subclass ofException Moreover, unreachable code in Java generates error

Multiple Catch Statements (Contd.):Catch-all block

Catch-all block- Accepts more generic Exception argument type

catch(Exception e)


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- Good approach to handle multiple exception throws by a code block

public class TwoMistakes3

{ public static void main(String[] args)

{ int num[] = {4, 0, 0};

try{ num[2] = num[0] / num[1];

num[2] = num[3] / num[0];

}

catch( Exception e) // multiple exceptions can be handle by this block

{ System.out.println(e.getMessage());

}

}

}14

EXAMPLE PROGRAM

Throwing an Exception

Throw
http://c/excep/catchall.javahttp://c/excep/catchall.java


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- Mostly exceptions are implicitly thrown by the Java run-time system

- Programmers can throw an exception explicitly too

throw ThrowableInstance;

//ThrowableInstance must be the object of class which is derived from Throwable

e.g. throw new MyException();

derived from

When exception thrown

- Control exits current try block

Throwable

- Proceeds to catch handler (if exists)

21

Preceding step


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try block

throw

statement

unmatched catch

matching catch

unmatched catch

next step

EXAMPLE PROGRAM

Example: Throw Statementclass ThrowDemo

{ static void demoproc()

{

try

{

Output:Caught inside demoproc.Recaught: java.lang.NullPointerException: demothrow new NullPointerException("demo");
http://c/excep/Throwte.javahttp://c/excep/Throwte.java


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}

catch(NullPointerException e)

{

System.out.println("Caught inside demoproc.");

throw e; // re-throw the exception

}

}


{

try

{ //Re-throwingdemoproc();

}

catch(NullPointerException e)

{ System.out.println("Recaught: " + e); }

}

}


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Preceding step


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250

try block

throw

statement

unmatchedcatch

matching catch

unmatchedcatch

next step

finally

try

try{


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{..

...}

finally{..

...}

.....}catch(.){ . }

catch(..){ . }....finally{

....

}

Finally Example Program
http://c/excep/Finally2.javahttp://c/excep/Finally2.java


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25

2

A t t t b i id th bl k f th t


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A try statement can be inside the block of another try Each time a try statement is entered, the context of that

exception is pushed on the stack

If an inner try statement does not have a catch, then the nexttrystatements catch handlers are inspected for a match If a method call within a try block has try block within it, then

then it is still nested try

try

{


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{ try

{

}

catch(Exception e)

{

}

}

catch(Exception e)

{

}

Example Program
http://c/excep/NestTry.javahttp://c/excep/NestTry.java


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FORMAT:type mthod-name(parameter list) throws exception-list

{

//body of method.}

Example Program

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6

Ja a has its b ilt in capabilit to ens re that e ceptions are
http://c/excep/ThrowsDemo.javahttp://c/excep/ThrowsDemo.java


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Java has its built-in capability to ensure that exceptions arehandled within the java program.

Exceptions handled by:

Programmer

JVM(Java Runtime System)

Object


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25

8

Throwable

Error

LinkageError

ThreadDeath

VirtualMachineError

AWTError

Exception

RunTimeException

ArithmeticException

IndexOutOfBoundsException

NullPointerEXception

IllegaAccessException

NoSuchMethodException

ClassNotFoundException


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Throwable The base class for all exceptions, it is required for a class to bethe rvalue to a throw statement.

Error Any exception so severe it should be allowed to pass uncaughtto the Java runtime.

Exception Anything which should be handled by the invoker is of this type,and all but five exceptions are.

Checked Exceptions Unchecked Exceptions

Checked exceptions inability to acquire system

( h i ffi i t fil d t


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resources (such as insufficient memory, file does not

exist)

Java checks at compile time that some mechanism is explicitly in

place to receive and process an exception object that may be created

during runtime due to one of these exceptions occurring.Unchecked exceptions exceptions that occur because of the user

entering bad data, or failing to enter data at all.

Unchecked exceptions can be avoided by writing more robust code that

protects against bad input values. Java does not check at compile timeto ensure that there is a mechanism in place to handle such errors.

Java Built-in Exceptions: Checked Exceptions

Table: Javas Checked Exceptions defined in java.lang

Exception Meaning


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Exception MeaningClassNotFoundException Class not foundCloneNotSupportedException Attempt to clone an object that does not implement

the Cloneable interface.llegalAccessException Access to a class is denied.InstantiationException Attempt to create an object of an abstract class orinterface.nterruptedException One thread has been interrupted by another thread.NoSuchFieldException A requested field does not exist.NoSuchMethodException A requested method does not exist.

Java Built-in Exceptions: Unchecked Exceptions

Table: Javas Unchecked RuntimeException Subclasses

Exception Meaning


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Exception MeaningArithmeticException Arithmetic error, such as divide-by-zero

ArrayIndexOutOfBoundsException Array index is out-of-bounds

ArrayStoreException Assignment to an array element of an incompatible type

ClassCastException Invalid cast

IllegalArgumentException Illegal argument used to invoke a method

IllegalMonitorStateException Illegal monitor operation, such as waiting on an unlocked thread

IllegalStateException Environment or application is in incorrect state

IllegalThreadStateException Requested operation not compatible with current thread state

IndexOutOfBoundsException Some type of index is out-of-bounds

NegativeArraySizeException Array created with a negative size

NullPointerException Invalid use of a null reference

NumberFormatException Invalid conversion of a string to a numeric format

SecurityException Attempt to violate security

StringIndexOutOfBounds Attempt to index outside the bounds of a string

UnsupportedOperationException An unsupported operation was encountered20


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We can also create our own exceptions specific to ourapplications.

It is quite easy, just we create a class by extending

Exception class. Since it is user defined we must tell the situation

where it will be occurred and the exception objectmust be created and throw by ourselves.

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3

Problem Statement :

Consider the example of the Circle class


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Consider the example of the Circle class Circle class had the following constructor

public Circle(double radius){r = radius;

}

How would we ensure that the radius is not zero

or negative?

26

4

import java.lang.Exception;

class InvalidRadiusException extends Exception {


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26

5

class InvalidRadiusException extends Exception {

public InvalidRadiusException (String msg){

super(msg);

}

}

class Circle {double r;


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6

public Circle (double radius ) {if (radius


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26

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public static void main(String[] args){try{

Circle c1 = new Circle(-1);System.out.println("Circle created");

}catch(InvalidRadiusException e){

System.out.println(e.getMessage());}

}}

Example
http://c/excep/Usere.javahttp://c/excep/Usere.java


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26

8


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Multitasking:

refers to a computer's ability to perform multiple jobs concurrently

Multitasking is divided into two types:


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Multitasking is divided into two types:

Process-based: Here two or more programs runs concurrently. You can

run Windows calculator and a Text editor (Notepad) at the same time.

Thread-based: A single program can perform two or more tasks

simultaneously. For example, text editor can print while formatting is

being done.

more than one program are running concurrently

270


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What is Thread?

A thread is a single sequential flow of control

within a program


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within a program. Thread does not have its own address space but

uses the memory and other resources of theprocess in which it executes.

There may be several threads in one process The Java Virtual Machine (JVM) manages these

and schedules them for execution.


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A thread is a single sequence of execution within

a program


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a program

refers to multiple threads of control within asingle program

each program can run multiple threads of controlwithin it, e.g., Web Browser

Multithreading enables programmers to do multiple

things at one time


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things at one time.

For ex, we can send tasks such as printing into thebackground and continue to perform some othertask in the foreground.

CPU CPU1 CPU2


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CPU


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Process 1 Process 3Process 2 Process 4

main

run

GC

Processes & Threads


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THREADS


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4: Threads279

When multiple events/actions need to occur

at the same time


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at the same time Examples:

Download 10 pages.

Single-threaded program: sequentially

Multithreaded: all at the same time save time Download data from the network and respond to

mouse at the same time

class ABC

{


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{.

.....

.

.....

.

.....

}

Beginning

Single-threaded

Body of

Execution

End

Main Method


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start start start

switchingswitching

Module

Threads are implemented as objects that contains a method called run()

class MyThread extends Thread

{

public void run()

{// th d b d f ti

:: Extending the thread class

EXAMPLE
http://localhost/var/MultiT/ThreadEx1.javahttp://localhost/var/MultiT/ThreadEx1.java


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// thread body of execution

}

}

Create a thread:

MyThread thr1 = new MyThread();

Start Execution of threads:

thr1.start();

Create and Execute:

new MyThread().start();

ThreadEx1 .java
http://localhost/var/MultiT/ThreadEx1.javahttp://localhost/var/MultiT/ThreadEx1.java


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class MyThread implements Runnable

{

.....

java notes of25sessions

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