introduction to java 113 th february 2004sifei he © 2004 introduction to java for cs381, ee4.web by...

13th February 2004 Sifei HE © 2004

Introduction to JavaIntroduction to Java

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Introduction to Java for Introduction to Java for CS381, EE4.WebCS381, EE4.Web

By Sifei [email protected] of ComputingSchool of EPSFebruary 2004



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Abstract Class & methodAbstract Class & methodpublic abstract class MyFirstAbstractClass {

int anInstanceVariable;public abstract int aMethodMyNonAbstractSubclassesMustImplement();public void doSomething() {// a normal method}

}public class AConcreteSubClass extends MyFirstAbstractClass {

public int aMethodMyNonAbstractSubclassesMustImplement() {// we *must* implement this method}

}

• Some attempted uses of these classes:Object a = new MyFirstAbstractClass(); // illegal, is abstract

Object c = new AConcreteSubClass(); // OK, a concrete subclass



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Programming flow controlProgramming flow control

• The while and do..while Statements • The for Statement • The if .. else if Statement • The switch … case Statement • The break statement (in loops and switch only)

• The continue statement (in loops only)

• The return statement (in method definitions only)



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The “Circle” classThe “Circle” classpublic class Circle { // A class field public static final double PI= 3.14159; // A useful constant // A class method: just compute a value based on the arguments public static double radiansToDegrees(double rads) { return rads * 180 / PI; } // An instance field public double r; // The radius of the circle

// Two instance methods: they operate on the instance fields of an object public double area() { // Compute the area of the circle return PI * r * r; } public double circumference() { // Compute the circumference return 2 * PI * r; }}



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The “main” methodThe “main” methodpublic class Circle { public static final double PI= 3.14159; // A useful constant public double r; // The radius of the circle public Circle(double r) {this.r = r;} // Constructor public Circle() {} // Nothing Happen Here. public double area() { // Compute the area of the circle return PI * r * r; } public double circumference() { // Compute the circumference return 2 * PI * r; } //……… find them in the previous slides

}

public static void main(String[] args) { int input = Integer.parseInt(args[0]); Circle c = new Circle(); c.r = input; double result = c.circumference(); System.out.println(result); }



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Put “main” in a new class?Put “main” in a new class?public class MakeCircle { public static void main(String[] args) {

int input = Integer.parseInt(args[0]);

Circle c = new Circle(); c.r = input; double circum = c.circumference(); System.out.println(circum); double a = c.area(); System.out.println(a); }}



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An AssociationAn Association

MakeCircle

main

Circle

radius

circumferencearea

creates instances of



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The Circle exampleThe Circle exampleC:\>cd Java

C:\Java>javac Circle.java

C:\Java>javac MakeCircle.java

C:\Java>java MakeCircle 4

25.1327250.26544

C:\Java>java MakeCircle 5

31.415978.53975

C:\Java contains Circle.java and MakeCircle.java

C:\Java now also contains Circle.class and MakeCircle.class



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The Members of a ClassThe Members of a Class

• Class fields– public static final double PI = 3.1416;

• Class methods– public static double radiansToDegrees(double rads) {…}

• Instance fields– public double radius;

• Instance methods– public double circumference() {…}



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Class FieldsClass Fields

public static final double PI = 3.14159

• A field of type double• Named PI (capitalise constants)• Assigned a value of 3.14159• The static modifier tags this as a Class Field

– Associated with the class in which it is defined

• The final modifier means it cannot be changed



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Class Fields…Class Fields…

• There is only one copy of PI• Any instance of Class can refer to this

field as PI• PI is essentially a Global VariableBUT• Methods that are not part of Circle

access this as Circle.PI– No name collisions



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Class MethodsClass Methods

public static double radiansToDegrees(double rads) {

return rads * 180 / PI; }

• Single parameter of type double and returns a value of type double

• Is essentially a “global method”// how many degrees is 2.0 radians?double d = Circle.radiansToDegrees(2.0);



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Instance FieldsInstance Fields

public double r;• Each Circle object can have a have a radius

independent of other Circle objects• Outside a class, a reference to an instance field

must be pre-pended by a reference to the object that contains itCircle c = new Circle();c.r = 2.0;Circle d = new Circle();d.r = c.r;

Are they the same object?



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Instance MethodsInstance Methods

• Instance methods operate on instances of a Class, and not on the Class itself

• E.g.– area()– circumference()

• If an instance method is used from outside the Class itself, it must be pre-pended by a reference to the instance to be operated on:– Circle c = new Circle();– c.r = 2.0;– double a = c.area();



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Implementing InheritanceImplementing Inheritance

Circle

radius

circumferencearea

PlaneCircle

cxcy

isInside



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““PlaneCircle” as a subclassPlaneCircle” as a subclasspublic class PlaneCircle extends Circle {

}

public double cx, cy;

public PlaneCircle(double r, double x, double y) { super(r); this.cx = x; this.cy = y;}

// automatically inherit fields and methods of Circle

PlaneCircle

cxcy

isInside

public boolean isInside(double x, double y) { …}



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Subclass ConstructorsSubclass Constructors

• In this case, the word “super”:– Invokes the constructor method of the superclass– Must only be used in this way within a constructor

method– Must apear within the first statement of the constructor

method

public PlaneCircle(double r, double x, double y) { super(r); // invoke constructor of superclass this.cx = x; // initialise instance field cx this.cy = y; // initialise instance field cy}



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Making more circlesMaking more circles

• PlaneCircle pc = new PlaneCircle(1.0, 0.0, 0.0);// Create a unit circle at the origin

• double a = pc.area( );// Calculate it’s area by invoking an inherited method

• boolean test = pc.isInside(1.5, 1.5); // Test if the point (1.5, 1.5) is inside the PlaneCircle pc or not

• What other methods might we want in PlaneCircle?



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More robust ClassesMore robust Classes

• Declare fields as Private or Protected– Protected fields can be accessed by subclasses or

members of the same package

• Declare public “get” and “set” methods– with appropriate checks on the “set” methods

• E.g.public void setRadius(double radius) { checkRadius(radius);

this.r = radius; }



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public class Circle { public static final double PI = 3.14159; // a constant protected double radius; protected void checkRadius(double radius) { if (radius < 0.0) { throw new IllegalArgumentException("radius must

not be negative"); } } public Circle(double radius) {

checkRadius(radius); this.radius = radius; } public double getRadius() {return radius;} public void setRadius(double radius) {

checkRadius(radius); this.radius = radius;

} public double area() {return PI*radius*radius; } public double circumference() {return 2*PI*radius; }}



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Java ExceptionsJava Exceptions• http://java.sun.com/docs/books/tutorial/essential/exceptions/

• The term exception is shorthand for the phrase "exceptional event."

• Definition: An exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions.

• Separating Error Handling Code from "Regular" Code .

• Some kinds-of Exception– Exception– ClassNotFoundException– IllegalAccessException– RuntimeException– Illegal ArgumentException– …



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Catching ExceptionsCatching Exceptions

• Use the following construction:try {

// code that may throw exceptions}

catch (AnException e1) { // Code to handle this kind of exception

}

finally {// Code that will always execute

}



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Trying to succeedTrying to succeed

try {

// Normally this block of code will be

// executed successfully.

// However, if an exception is thrown

// execution will stop and the interpreter

// will attempt to find an appropriate

// “catch” statement.

}



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Catching ExceptionsCatching Exceptions

• There may be several “catch” blocks for different kinds of exceptions

catch (AnException e1) { // This block will be invoked if an // AnException kind of exception is // thrown, or a sub-type of AnException.// The block can refer to the exception// object by its name “e1”

}



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And FinallyAnd Finally

finally {

// This block contains code that will always be run

// even if control leaves the “try” block

// because of a return, continue or

// break statement.

// It will , however, be skipped if there is a

// System.exit( ) clause in the “try” block



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Try…Catch…FinallyTry…Catch…Finally

try {

checkRadius(radius);

}

catch (IllegalArgumentException ex) {

System.out.println(ex.getMessage());

}

finally{

System.out.println("Finally Here");

}



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Java AppletsJava Applets

• Java applications are executed from the command line

• Applets are small applications that can be embedded in html pages

• They can then be downloaded via the world wide web and invoked in a browser



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Template for an AppletTemplate for an Applet

• Subclass one of the Java foundation classes:– java.applet.Applet;– javax.swing.JApplet;

• It’s behaviour is invoked in a “paint” method, not a “main” method

// File: MyApplet.javaimport java.awt.*;import java.applet.Applet;public class MyApplet extends Applet { public void paint(Graphics g) {

// Do something interesting}

}



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Viewing Area of AppletViewing Area of Applet

y

x(0,0)

height

width

These dimensions are set in the HTML file



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Contract for the Contract for the paintpaint method method

• Dimensions of the region on the Web page assigned to the applet are set within an <applet> tag in an HTML file. (width & height)

• The paint( ) method defines the appearance of the applet within this rectangular region

• The paint( ) method is invoked when the applet is initially loaded in the browser (or applet viewer)



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HelloFromVenus.javaHelloFromVenus.java

import java.awt.*;

import java.applet.Applet;

public class HelloFromVenus extends Applet {

public void paint(Graphics g) {

Dimension d = getSize( );

g.setColor(Color.black);

g.fillRect(0, 0, d.width, d.height);

}

}



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painting the textpainting the textpublic void paint(Graphics g) {




// set font style and size:-

g.setFont(new Font(“Sans-serif”, Font.BOLD, 24));

// change colour on paint brush:-

g.setColor(new Color(255, 215, 0));

g.drawString(“Hello from Venus!”, 40, 25);

}



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finally add the imagefinally add the imagepublic void paint(Graphics g) {




g.setFont(new Font(“Sans-serif”, Font.BOLD, 24);

g.setColor(255, 215, 0);

g.drawString(“Hello from Venus!”, 40, 25);

g.drawImage(getImage(getCodeBase( ), “Venus.gif”), 20, 60, this);

}



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Shifting graphical objects in AppletShifting graphical objects in Applet

• To translate a graphical object, use:– translate(xpos, ypos);

• To rotate a graphical object, use:– rotate(angle_in_radians);



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Shifting graphical objects in AppletShifting graphical objects in Applet

public void paint(Graphics gg) {int ii;Graphics2D g = (Graphics2D) gg;g.setFont( new Font("Arial", Font.BOLD, sizeV) );g.setColor(Color.blue)g.translate(200, 200);for(ii = 1; ii <= 16; ii++){ g.rotate(Math.PI/8.0); g.drawString(textV, 20, 0);}



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Embedding Applets in HTMLEmbedding Applets in HTML

<HTML><HEAD> <TITLE>HelloFromVenus Applet</TITLE></HEAD><BODY BGCOLOR=BLACK TEXT=white><CENTER> <H2>Here is the <EM>Hello From Venus</EM> Applet</H2> <APPLET CODE="HelloFromVenus.class" WIDTH=300 HEIGHT=350> </APPLET></CENTER> Venus photo courtesy of NASA.</BODY>

</HTML>



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Parameter passingParameter passing

• To make applets reusable in different contexts, we need to be able to pass parameters from the html files

• Can do this by using:– <param name> tag in html, and– getParameter(String) method in the

applet



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Parameter passingParameter passing

• In HTML file:<applet code=bytecode-filename width=pixels height=pixels><param name=String value=String>

</applet>

• In Applet:String input = getParameter(String);

• Example:– HTML: <param name=“text” value=“Web Technology”>– String textV = getParameter(“text”);



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Initialisation of an appletInitialisation of an applet

• Initialisation of an applet is not done in the Constructor method, but in an init( ) method

• An applet context interprets the <applet> tag in an html file

• The applet context first constructs an instance of the applet, then interprets the remaining parameters in the <applet> tag

• The init( ) method is invoked after the information in the <applet> tag has been processed

• Why?



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Animated appletsAnimated applets

• Create a thread for an instance of the applet• paint( ) the applet into the applet

context

• Each time the relevant graphical properties of the applet are changed, repaint( ) the applet



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Applet LifecycleApplet Lifecycle

• init() is invoked when the applet is loaded

• start() is invoked when the page containing the applet has been entered

• stop() is invoked when the page is left• destroy() is invoked when the page is

discarded



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Using Scripts to Invoke a MethodUsing Scripts to Invoke a Method

• Give a name to the <applet> tag or ID to the <object> tag.

• Within the JavaScript, access the method by using the document object, then the applet name (OJECT ID), then the method name itself.

• Method must be declared as public.• Update the area by calling the repaint()

method.



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Callbacks & EventsCallbacks & Events

• Callbacks are used in procedural libraries when they need to handle asynchronous events

• The alternative is for the client to continuously poll the library for events– This is inefficient, especially if a large number of

library components are registered with a client

• But the use of callback means a client may observe “intermediate” states of a library– “Classically” a library’s operations would run to

completion before returning control



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Call SequenceCall Sequence

Client aUser Library

Client installs callback

Third party calls library

Library invokes callback“I need to tell you something!”

“What’s happened?”

“He’s changed a name!” Callback queries library

“That’s cool” Callback returns

Library returns



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Java EventsJava Events

• An “Event” encapsulates information about some state change in its source– Pressing a button, Entering text in a text field …– Moving a slider on a scroll bar, …

• Events need not be generated by users:– Expiration of a timer, Arrival of incoming data, …

• The firing of an event is a way of one object telling one or more other recipients that something interesting has happened– The sender fires an event– A recipient is called a listener and handles the event



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Java Event ModelJava Event Model

Event SourceEvent Source

Event ListenerEvent Listener

Register Event Listener

Fire Event

EventObject

EventObject



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Java Event ObjectsJava Event Objects

• Encapsulates information specific to an instance of an event

• E.g. a “mouse click” event may contain:– The position of the mouse pointer– Which mouse button was clicked (and how

many times)

• The event object is passed as a parameter to the event notification method



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Event SourcesEvent Sources

• Objects that fire events

• Implement methods that allow listeners to:– Register their interest in the events they

generate;– Unregister their interest in the events they

generate.

• Multicast event delivery enables an event to be fired to a number of event-listeners



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Event ListenersEvent Listeners

• These are objects that need to be notified when a certain event occurs

• Event notifications are made through method invocations in the listening object– The event object is passed as a parameter– The event source must know which listener

object(s) to call

• This information is contained in an event-listener interface



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Reuse the Applet ExampleReuse the Applet Exampleimport java.applet.*;import java.awt.*;import java.awt.event.*;public class simpleApplet extends Applet implements

ActionListener {private Button clickButton;private int buttonCount;

public void init() {clickButton = new Button("Click here");add(clickButton);buttonCount = 0;clickButton.addActionListener(this);

}public void actionPerformed(ActionEvent ae) {

buttonCount++;showStatus("Click total = " + buttonCount);

}}



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Event SummaryEvent Summary

EventObject

source

getSource()toString()

EventListener

notification(evt)

EventSource

addListener()removeListener()

fires passed to

registers 0..*

invokes notifications in 0..*



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Event SummaryEvent Summary

• The components of a software product must interact to achieve a goal

• In general, it is hard to identify a single component that has overall control

• So a general model is for components to interact by triggering “Events” that other components register an interest in



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PackagesPackages

• Packages create a grouping for related interfaces and classes.

• Packages can have types and members that are available only within the package.

• Packages make both small and large scale development easy to manage.

• Avoid potential name collision.



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PackagesPackages• Here’s a simple example of the creation of a

package in a Java source file:package myFirstPackage;public class MyPublicClass extends ItsSuperclass {

//. . .}

• Package statement must be the first thing in that file (except for comments and white space, of course).

• Another class in package myFirstPackagepackage myFirstPackage.mySecondPackage;public class MySecondPublicClass extends ItsSuperclass {

//. . .}



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PackagesPackages

• Each Java class should be located in a separate source file, the grouping of classes provided by a hierarchy of packages is analogous to the grouping of files into a hierarchy of directories on your file system.

./myFirstPackage/MyPublicClass.class

./myFirstPackage/mySecondPackage/MySecondPublicClass.class

• Both the compiler and the interpreter expect (and enforce) the hierarchy.



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PackagesPackages

• Import packages & useimport myFirstPackage.mySecondPackage.*

import myFirstPackage.mySecondPackage.MySecondPublicClass

MySecondPublicClass mspc = new MySecondPublicClass();

• All import statements must appear after any package statement but before any class definitions. Thus, they are “stuck” at the top of your source file.

• * does NOT mean import recrusively



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Java API:: java.lang.StringJava API:: java.lang.String

• Java strings are standard objects with built-in support.• String objects are read-only• Java also provides StringBuffer class for mutable

strings• Two basic method: length and charAt• indexOf(…) and lastIndexOf(…)String str=“Hello World”;int len = str.length;char c = str.charAt(1);int idx = str.indexOf(“ll”);int lidx = str.lastIndexOf(“l”);



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Java API:: java.lang.StringJava API:: java.lang.String

• String Comparisons– regionMatches(…)– compareTo(…)– compareToIgnoreCase(…)– contentEquals(StringBuffer sb)– equals(…)– equalsIgnoreCase(…)

• Simply use + operator to concatenate strings.

• Use StringBuffer.



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Java API:: java.lang.StringBufferJava API:: java.lang.StringBuffer

• String str = str1+str2+…+str100;• String str = new StringBuffer().append(str1)………append(str100).toString();

• insert(…)• replace(…)• setCharAt(…)



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Thank You!Thank You!

• Questions???

• Comments!!!

introduction to java 113 th february 2004sifei he © 2004 introduction to java for cs381, ee4.web by...

Documents

public class makecircle

public double circumference

circle public circledouble

public class aconcretesubclass

constructor public circle

new circle c

object public double

parseintargs0 circle