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Ch aracteristics of JavaJava is simpleJava is object-oriented

Java is distributed

Java is interpreted

Java is robust

Java is secure

Java is arc h itecture-neutral

Java is portable

Java s performance

Java is multit h readed

Java is dynamic 2

JDK Versions

JDK 1.02 (1995)JDK 1.1 (1996)Java 2 SDK v 1.2 (a.k.a JDK 1.2, 1998)

Java 2 SDK v 1.3 (a.k.a JDK 1.3, 2000)Java 2 SDK v 1.4 (a.k.a JDK 1.4, 2002)

JDK Editions

Java Standard Edition (J2SE)± J2SE can be used to develop client-side standalone

applications or applets.

Java Enterprise Edition (J2EE)± J2EE can be used to develop server-side applications

such as Java servlets and Java ServerPages.

Java Micro Edition (J2ME).± J2ME can be used to develop applications for mobile

devices such as cell phones.

G etting Started wit h JavaProgramming

Overview of Java

Simple Java Application

Compiling ProgramsExecuting Applications

Overview of Java

H ow does Java work?

Java source code is compiled intomac h ine independent bytecodesTh e bytecodes can be run directly byan interpreterTh e bytecodes can be converted to

ine code and executed ( Just inTime (JIT) compilation).An optimizing interpreter candynamically identify program

h otspots and create code optimizedfor t h e specific mac h ine/environment.

Optimizing interpreter coming soonfrom SUN.

Java Source code

Java ³Bytecodes´

Compiler

Mac Unix PC

Bytecode

Interpreter

JITCompiler

Machine Code

Java Data Types

Sizes fully specified by Java standard.Java is a very strongly typed languageInteger types± int (4 bytes signed)± sh ort (2 bytes signed)± long (8 bytes signed) use suffix L (eg 1000000000L)± byte (1 byte signed)

Floating-point types± float (4 bytes) use suffix F (eg 1.28F)± double( 8 bytes)

Additional Data Types

ch ar± Two-byte unicode

± Assignment with

e.g. c h ar c = h ;

boolean± true or false

e.g. boolean x = true;

if (x){ };

Operators/ Control Flow

Almost exactly like regular ANSI C .+, *, -, /, %, ++, --, +=, etc.==, !=, >, < , etc.if statements, for loops, w h ile loops, do loops, switc h statements, etc.continue, break, return, System.exit(0).

Read pp 54 in Core Java.No need to spend class time going over t h ese.

Scoping

Braces are used as in C to denote begin/end of blocksBe careful of t h e following:int j = 0;

if ( j <1 ){int k = 2;

k = 3; //Error! k inaccessible h ereDeclarations do not propogate upwards.

Adding datatypes -- classes

Java h as h andful of built-in datatypes just discussed(int, float, etc.)Just like in C , user typically creates own h omemadedatatypes to work wit h particular application (iestructs and enums ).In Java t h ese are called classes .

M any class definitions come as a standard part of t h eJava distribution. M ost common Example is Stringclass.

Strings

Java provides a class definition for a type calledStringSince t h e String class is part of t h e java.lang package,no special imports are required to use it (like ah eader file in C).Just like regular datatypes (and like C), variables of type String are declared as:String s1;String s2, s3; //etc.Note t h at String is uppercase. T h is is t h e Javaconvention for classnames.

Strings

Initializing a String is painlesss1 = Th is is some java String ;

Note t h at double quotes are required.M emory is allocated dynamically.Th ink of above met h od as s h ortcut for morestandard way (assuming s1 h as been declared):

s1 = new String( T h is is some java String );new operator required to create memory for newString object.

Anatomy of a Java ProgramCommentsPackageReserved wordsM odifiersStatementsBlocks

ClassesM et h odsTh e main met h od

Comments

In Java, comments are preceded by two slashes (//) ina line, or enclosed between /* and */ in one or multiple lines. When the compiler sees //, it ignoresall text after // in the same line. When it sees /*, itscans for the next */ and ignores any text between /*and */.

Reserved Words

Reserved words or k eywords are words that have aspecific meaning to the compiler and cannot be usedfor other purposes in the program. For example, when

the compiler sees the word class, it understands thatthe word after class is the name for the class. Other reserved words in Example 1 .1 are public, static, andvoid. Their use will be introduced later in the book.

M odifiers

Java uses certain reserved words called m odifiers thatspecify the properties of the data, methods, andclasses and how they can be used. Examples of

modifiers are public and static. Other modifiers areprivate, final, abstract, and protected. A public datum,method, or class can be accessed by other programs.A private datum or method cannot be accessed by

other programs. Modifiers are discussed in Chapter 6,"Objects and Classes."

Statements

A statement represents an action or a sequence of actions. T h e statement System.out.println("Welcometo Java!") in t h e program in Example 1.1 is a

statement to display t h e greeting "Welcome to Java!"Every statement in Java ends wit h a semicolon (;).

Writing first program

To keep t h ings simple our first few programswill all be written as just a single mainprogram.In java, t h e file in w h ich your program residesmust contain a .java extension (e.g.M yFirstProgram.java).

Writing first program

Th en, t h e program must be wrapped in a classdefinition w h ich is t h e same as t h e filebasename ( M yFirstProgram). Careful, Java iscase-sensitive.

class M yFirstProgram { }

Finally, main is defined similar to C , but wit h afew more modifiers:

public static void main(String[] args){ }Th ese are all required . No s h ortcuts.

A Simple Application

Example 1.1//This application program prints Welcome//to Java!

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

System.out.println("Welcome to Java!");}

Compiling/running first java program

Create source code file (call it for exampleM yFirstProgram.java).To compile:prompt >> javac M yFirstProgram.java

Th is produces byte code file namedM yFirstProgram.class

To run:prompt >> java M yFirstProgram

Creating and Compiling Programs

On command line± j avac file. j ava

ource ¡ ode

¡ reate/ ¢ odi£

ource ¡ ode

ompile

ource¡

odei.e.¤

avac Welcome.¤

¥ ytecode

yteodei.e.¤

ava Welcome

Result

compilation err ors

runtime errors or incorrect result

Executing Applications

On command line± j ava classname

JavaInterpreter

on Windows

JavaInterpreter

on Sun Solaris

JavaInterpreter on Linux

Bytecode

Example

j avac Welcome. j ava

j ava Welcome

output:...

Observations

.class file is not mac h ine code. It is intermediate formcalled Java Byte code. Can run on any platform aslong as platform h as a Java Virtual M ac h ine (JVM ).

Th e second step on previous slide invokes t h e JVM tointerpret t h e byte code on t h e given platform.In th eory, byte code can be moved to anot h erplatform and be run t h ere wit h out recompiling t h is

e magic of applets.Leave off t h e .class part w h en invoking t h e JVM .

Observations

Th is is an old-fas h ioned command-lineprogram. Java also supports G UI applicationsand web-browser h osted programs calledapplets .After t h e first couple of weeks we will usegrap h ical rat h er t h an scripting front-ends.

Parsing Strings

Recall t h at t h e args array is an array of Strings. T h us,to accept key input as integers, float, etc. we mustconvert.

To convert to int, use t h e Integer.parseInt(String)function.Ex. String s1 = 32 ;

int j = Integer.parseInt(s1); // j now h olds 32

Converting to double is analogous:Ex. String s1 = 32 ;double j = Double.parseDouble(s1); // j now h olds 32

Parsing Strings

Note t h at t h e conversion met h ods are just regularprocedural functions. T h at is, no object was createdin order to call t h e met h od. T h e entity before t h e . is

_not_ an object.M

ore on th

is later.Wh at if t h e String is unparseable? (e.g. andrewrat h er t h an 32 ).Study t h e Integer and Double classes and look for a

description of th

e errors th

at are throw n by th

evarious met h ods. Does anyt h ing stand out? Seeexample Looper.java. M ore on t h is later.

Th e Virtual M ac h ine

Java is both compiled and interpreted± Source code is compiled into Java b ytecode± Which is then interpreted by the Ja va Virtu al M a chine

(JVM)± Therefore bytecode is machine code for the JVM

Java bytecode can run on any JVM, on any platform± «including mobile phones and other hand-held devices

Networking and distribution are core features± In other languages these are additional APIs± Makes Java very good for building networked applications,

server side components, etc.

Features of t h e JVM

Th e G arbage Collector± Java manages memory for you, t h e developer h as no control over t h e

allocation of memory (unlike in C/ C++).± Th is is muc h simpler and more robust (no c h ance of memory leaks or

corruption)± Runs in t h e background and cleans up memory w h ile application is

running

Th e Just In Time compiler (JIT)± Also known as H ot Spot± Continually optimises running code to improve performance± Can approac h th e speed of C++ even t h oug h its interpreted

Blocks

A pair of braces in a program forms a block thatgroups components of a program.

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

System.out.println("Welcome to Java!");}

lass block

ethod block

Classes

The cla ss is the essential Java construct. A class is atemplate or blueprint for objects. To program in Java,you must understand classes and be able to write and

use them. The mystery of the class will continue to beunveiled throughout this book. For now, though,understand that a program is defined by using one or more classes.

M et h ods

What is System.out.println? It is a m ethod : acollection of statements that performs a sequence of operations to display a message on the console. It canbe used even without fully understanding the detailsof how it works. It is used by invoking a statementwith a string argument. The string argument isenclosed within parentheses. In this case, theargument is "Welcome to Java!" You can call thesame println method with a different argument toprint a different message.

main M et h od

The main method provides the control of programflow. The Java interpreter executes the application byinvoking the main method.

The main method looks like this :

public static void main(String[] args) {// Statements;

Th e exit M et h od

Use Exit to terminate the program and stop allthreads.

NOTE : When your program starts, a thread isspawned to run the program. When theshowMessageDialog is invoked, a separate threadis spawned to run this method. The thread is not

terminated even you close the dialog box. Toterminate the thread, you have to invoke the exitmethod.

Object Orientation

Introduction to OOP

Java is an object orientedprogramming language, so somerudimentary knowledge of OOterminology is required

Classes (e.g. Employee , Track )Objects

specific instances of a class (e.g.Tony Joh nson , Track 5 )

In Java all functions are containedwit h in classesFor largely h istorical reasonsfunctions are often called met h ods or

member functionsInh eritance (aka subclassing)

e.g. Associate Director , DriftCh amber Track

EncapsulationVariables/Functions can be declared:

privateonly functions wit h in same classor superclass can access t h em(c.f. C++ protected)

publicany function can access t h em

Java also implementsstatic

Applies to class not objectfinal

Cannot be overridden bysubclass

Wh at is OOP?

M odelling real-world objects in softwareWh y design applications in t h is way?± We naturally class ify objects into different types .

± By attempting to do t h is wit h software aim to make itmore maintainable, understandable and easier to reuse

In a conventional application we typically:± decompose it into a series of functions,

± define data structures th

ose functions act upon± th ere is no relations h ip between t h e two ot h er t h an t h efunctions act on t h e data

Wh at is OOP?

H ow is OOP different to conventional programming?± Decompose t h e application into abstract data types by

identifying some useful entities/abstractions

± An abstract type is made up of a series of be h aviours andth e data t h at t h ose be h aviours use.

Similar to database modelling, only t h e types h avebot h be h aviour and state (data)

Wh at is an OO program?

Wh at does an OO program consist of?± A series of objects t h at use eac h ot h ers be h aviours in order to carry

out some desired functionality± Wh en one object invokes some be h aviour of anot h er it sends it a

message± In Java terms it invokes a method of t h e ot h er object± A met h od is t h e implementation of a given be h aviour.

OO programs are intrinsically modular± Objects are only related by t h eir public be h aviour (met h ods)

± Th erefore objects can be swapped in and out as required (e.g. for amore efficient version)± Th is is anot h er advantage of OO systems

Object-Oriented Programming

Understanding OOP is fundamental to writing good Javaapplications± Improves design of your code± Improves understanding of t h e Java APIs

Th ere are several concepts underlying OOP:± Abstract Types ( Classes)± Encapsulation (or Information H iding)± Aggregation

± Inh eritance± Polymorp h ism

Classesclass Employee{

public Employee (String n, double s){

name = n;salary = s;

}public Employee (String n)

{ name = n;salary = 0;

}public void raiseSalary(double byPercent){

salary *= 1 + byPercent / 100;

}public double getSalary(){

return salary;}private String name;private double salary;

Class Declaration

Private Member Variables

Constructor

Overloaded Constructor

Access Method

ObjectsObjects represent instances of classes

Employee j avaExpert = new Employee(³Joe´,100000);

j avaExpert.RaiseSalary(10); // Raise salary by 10%

system.out.println(³Salary is´ + j avaExpert.GetSalary());

Note t h at Java uses t h e keyword new for creation of new objects.Unlike C++ th ere is no delete keyword for t h e deletion of objects.

± Java h andles deletion of objects (and recovery of t h e memory t h ey occupy)automatically as soon as t h ere are no longer any references to t h em.

± Th is makes writing code muc h easier, it is impossible to create a memoryleak in Java..

± Th e process of keeping track of w h en objects are unreferenced and deleting

em is called garbage collection . It does impose some processing overh

eadwh ich slows down Java programs compared to equivalent programs in C orC++.

Inh eritanceclass Manager Extends Employee{

public M anager (String n, double s, Employee e){

name = n;salary = s;secretary = e;

}public String GetSecretary(){

return secretary;}public void RaiseSalary(double byPercent){

double bonus = 10;super.RaiseSalary(byPercent + bonus);}private Employee secretary;

Class Declaration

New member variables

Constructor

New Access Method

Overridden Method

InterfacesInterfaces in Java are a replacementfor t h e concept of multipleinh eritance in C++.Suppose we want to sort managersby t h e number of employees t h eyh ave.

Interfaces± Objects can implement any number of

interfaces± Can h ave h ierarc h ies of interfaces± Interfaces cannot h ave met h ods± Cleaner but not quite as powerful as

multiple in h eritance± Only way to implement callbacks in

JavaEmployee

SortableManager

In C++ both Employee and Sortablecan be classesIn Java one (probably sortable) mustbe an ³interface´

Abstract Data Types

Identifying abstract types is part of t h e modelling/designprocess± Th e types t h at are useful to model may vary according to t h e

individual application± For example a payroll system mig h t need to know about Departments,

Employees, M anagers, Salaries, etc± An E-Commerce application may need to know about Users, S h opping

Carts, Products, etcObject-oriented languages provide a way to define abstractdata types, and t h en create objects from t h em

± It s a template (or cookie cutter ) from w h ich we can create new objects± For example, a Car class mig h t h ave attributes of speed, colour, and

be h aviours of accelerate, brake, etc± An individual Car object will h ave t h e same be h aviours but its own values

assigned to t h e attributes (e.g. 30mp h , Red, etc)

" § § ¨

rogramming" --©

bstract Types c ombine data and behaviour

--------------------

"Conventional¨

rogramming" --Functions or

rocedures operating on independent data

Encapsulation

Th e data (state) of anobject is private itcannot be accessed

directly.Th e state can only bech anged t h roug h itsbe h aviour, ot h erwiseknown as its publici nterface or contract Th is is calledencapsulat i on

Private Data

Public Interface

"The Doughnut Diagram"Showing that an object hasprivate state and public

behaviour. State can only bechanged by invoking somebehaviour

Encapsulation

M ain benefit of encapsulation± Internal state and processes can be c h anged independently of t h e

public interface± Limits t h e amount of large-scale c h anges required to a system

Aggregation

Aggregation is t h e ability to create new classes out of existingclasses± Treating t h em as building blocks or components

Aggregation allows reuse of existing code± H oly G rail of software engineering

Two forms of aggregationWh ole-Part relations h ips± Car is made of Engine, Ch assis, W h eels

Containment relations

hips± A Sh opping Cart contains several Products

± A List contains several Items

OO Programming Concepts

data ield 1

method n

data ield n

method 1

n ob ect

ehavior

Data Fieldradius = 5

Methodind rea

Circle ob ect

Class Declaration

class Circle {double radius = 1.0;

double findArea(){return radius * radius * 3.14159;

Creating Objects

ob j ectReference = new ClassName();

Example:myCircle = new Circle();

Th e object reference is assigned to t h e objectreference variable.

Declaring/ Creating Objects

in a Single StepClassName ob j ectReference = new ClassName()

Example:Circle myCircle = new Circle();

Diff b t i bl f

Differences between variables of primitive Data types and object types

c: Circle

radius = 1

Primitive type int i = 1 i

Object type Circle c c reference

Created usingnew Circle()

Copying Variables of Primitive DataTypes and Object Types

c1 : Circle

radius = 5

Primitive type assignmenti = j

Before :

After :

Object type assignmentc1 = c2

Before :

After :

c2: Circle

radius = 9

G arbage Collection

Consider, c 1 =c2. After the assignmentstatement c 1 = c2, c 1 points to the sameobject referenced by c2. The object

previously referenced by c 1 is no longer useful. This object is known as garbage.

arbage is automatically collected by JVM.

G arbage Collection, cont

TIP : If you know that an object is no longer needed, you can explicitly assign null to areference variable for the object. The Java

VM will automatically collect the space if theobject is not referenced by any variable.

Constructors

Circle(double r) {radius = r;

Circle() {radius = 1.0;

myCircle = new Circle(5.0);

Constructors are aspecial kind of methods that areinvoked to constructobjects.

Constructors, cont.

A constructor with no parameters is referred to as adef a ul t constructor .

· Constructors must have the same name as the

class itself.· Constructors do not have a return type²noteven void.

· Constructors are invoked using the newoperator when an object is created. Constructorsplay the role of initializing objects.

Vi ibili M difi d

Visibility M odifiers andAccessor M et h ods

By default, t h e class, variable, or data can beaccessed by any class in t h e same package.

public

The class, data, or method is visible to any class in anypackage.

private

The data or methods can be accessed only by the declaringclass.

The get and set methods are used to read and modify privateproperties.

InstanceVariables, and M et h ods

Instance variables belong to a specific instance.

Instance methods are invoked by an instance of the class.

Cl V i bl C t t

Class Variables, Constants,and M et h ods, cont.

To declare class variables, constants, and methods,use the static modifier.

Scope of Variables

Th e scope of instance and class variables is t h eentire class. T h ey can be declared anyw h ere

inside a class.Th e scope of a local variable starts from itsdeclaration and continues to t h e end of t h e blockth at contains t h e variable. A local variable mustbe declared before it can be used.

Th e Keyword t h is

Use t h is to refer to t h e current object.

Use t h is to invoke ot h er constructors of t h e

object.

1d Arrays

Arrays in Java are dynamic; t h ey are allocatedwit h th e new operator.Creating a (1d) array is a two-step process:int[] x; //declare x to be an array of ints

//x h as t h e value of null rig h t nowx = new int[100]; //allocate 100 ints wort h

At t h is point t h e values of x are all zeroes.Assignment is t h en just like C .

1d Arrays

Note t h at in Java, unlike C , t h e compiler will not letyou overrun t h e bounds of an array. G ive it a try.Note t h at you must use t h e new operator to size t h earray. It cannot be done statically as in C . Until t h is isdone, t h e array reference h as a value of null.Th e array comes wit h a field called lengt h wh ich stores t h e number of elements in t h e array. Tryprinting x.lengt h in t h e previous example. T h is is asimple but nice convenience.

Array of Objects

Circle[] circleArray = new Circle[ 10 ];

An array of objects is actually an a rr a y of

reference v a ri abl es. So invokingcircleArray[ 1 ].findArea() involves two levels of referencing as shown in the next figure.circleArray references to the entire array.circleArray[ 1 ] references to a Circle object.

Array of Objects, cont.

Circle[] circleArray = new Circle[ 10 ];

re erence Circle ob ect 0 circleArray[ 0 ]circleArraycircleArray[ 1 ]

circleArray[ 9 ] Circle ob ect 9

Circle ob ect 1

Th ank You

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