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Jump Start : Java. B.Ramamurthy Copyright 1998 CSE Department. Topics for Discussion. OO Design Principles Java Virtual Machine Java Application Structure Class and objects Methods and Variables Access/Visibility Modifiers Debugging Inheritance

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jump start java

Jump Start : Java

B.Ramamurthy

Copyright 1998

CSE Department

BR

topics for discussion
Topics for Discussion
  • OO Design Principles
  • Java Virtual Machine
  • Java Application Structure
  • Class and objects
  • Methods and Variables
  • Access/Visibility Modifiers
  • Debugging
  • Inheritance
  • Abstract classes, interfaces and implementation
  • JDK Event Model
  • Applets and Appletviewer
  • Summary

BR

object oriented principles
Object-Oriented Principles

OOP

Polymorphism

-- Many forms of

same function

-- Abstract Methods

-- Abstract Classes

Inheritance

-- Hierarchy

-- Reusability

-- Extensibility

-- Expressive power

-- Reflects many

real-world problems

Encapsulation

(class)

-- Information Hiding

-- Interface and

Implementations

-- Standardization

-- Access Control mechanisms

(private /public etc.)

BR

conventional compiled languages

Mac Hardware

Mac Compiler

Mac Compiler

C++source

PC Hardware

C++source

PC Compiler

PC Hardware

C++source

Sun Compiler

Conventional Compiled Languages

BR

java virtual machine
Java Virtual Machine

JVM

Java compiler :javac

Mac Hardware

Byte code

Mac Compiler

C++source

Mac interpreter

JVM

PC Hardware

Byte code

C++source

PC Interpreter

JVM

PC Hardware

Byte code

C++source

Sun Interpreter

BR

run anywhere capability
On any machine when you compile Java source code using javac byte code equivalent to the source is generated.

Byte code is machine-independent. This enables the “run-anywhere” capability.

You invoke java command which will feed the byte code to the machine-dependent interpreter.

“Run-anywhere” Capability

BR

java application program interface java api
Java Application Program Interface (Java API)

(JAVA API)

Package of related

classes : java.awt

Random

java.util

Date

Dictionary

Java.io, java.beans,..

Etc..

package

class

BR

java api a simplistic view
Java API : A Simplistic View

API

packages

classes

methods and data declarations

BR

java api classes
Java API Classes
  • Unlike many other languages, you will referring to the classes in the API.
  • Where is the API?
  • Packages are in:

/util/lang/jdk1.1.6/src/java

  • “cd” to any package of interest:

awt, lang, io, graphics…..

  • You will see the methods and data defined in the classes in the package.

BR

types of programs
Java is fully object-oriented.

Every “function” has to be attached to a class.

You will mainly deal with three types of programs:

class: methods and data (variables + constants) describing a collection (type) of object

application: class that has a main method: represents a our regular program

applet: class that is meant for execution using a appletviewer/browser

Types of Programs

BR

problem solving using java
Problem Solving Using Java

OO Design and Progamming in Java

Write an

applet

class

Identify classes needed

Write an

application

class

Reuse API

classes

Reuse

your classes

Design new

classes

Create and use objects

BR

what is an object
What is an Object?
  • Object-oriented programming supports the view that programs are composed of objects that interact with one another.
  • How would you describe an object?
  • Using its characteristics (has a ----?) and its behaviors (can do ----?)
  • Object must have unique identity (name) : Basketball, Blue ball
  • Consider a ball:
    • Color and diameter are characteristics (Data Declarations)
    • throw, bounce, roll are behaviors (Methods)

BR

classes are blueprints
Classes are Blueprints
  • A class defines the general nature of a collection of objects of the same type.
  • The process creating an object from a class is called instantiation.
  • Every object is an instance of a particular class.
  • There can be many instances of objects from the same class possible with different values for data.

BR

example
Example

objects

Object

References

redRose

class Rose

blueRose

class

BR

instantiation examples
Instantiation : Examples
  • class FordCar ---- defines a class name FordCar
  • FordCar windstar; ---- defines a Object reference windStar
  • windstar = new FordCar(); ---- instantiates a windstar Object
  • class HousePlan1 { color….
  • HousePlan1 blueHouse;
  • blueHouse = new HousePlan1(BLUE);
  • HousePlan1 greenHouse = new HousePlan1(GREEN);

BR

operator new and dot
Operator new and “dot”
  • new operator creates a object and returns a reference to that object.
  • After an object has been instantiated, you can use dot operator to access its methods and data declarations (if you have access permissions).
  • EX: redRose.bloom(); greenHouse.color

BR

elements of a class
Elements of a Class

class

data declarations (variables,

constants)

methods

header

body

header

statements

modifiers,

type, name

variables,

constants

parameters

repetition

others

selection

assignment

BR

class structure
Class Structure

class

variables

constants

methods

BR

defining classes
Defining Classes
  • Syntax:
  • class class_name {
  • data-declarations
  • constructors
  • methods }
  • Constructors are special methods used for instantiating (or creating) objects from a class.
  • Data declarations are implemented using variable and constant declarations.

BR

naming convention
Constants: All characters in uppercase, words in the identifier separated by underscore: EX: MAX_NUM

Variables, objects, methods: First word all lowercase, subsequent words start with uppercase. EX: nextInt, myPen, readInt()

Classes: Start with an uppercase letter. EX: Tree, Car, System , Math

Naming Convention

BR

a complete example
A complete example
  • Problem Statement: You have been hired to assist in an secret encryption project. In this project each message (string) sent out is attached to a randomly generated secret code (integer) between 1 and 999. Design and develop an application program in Java to carry out this project.

BR

identify objects
Identify Objects
  • There are two central objects:
    • Message
    • Secret code
  • Is there any class predefined in JAVA API that can be associated with these objects? Yes ,
    • “string” of java.lang and “Random” of java.util

BR

the random class
The Random class
  • Random class is defined in java.util package.
  • nextInt() method of Random class returns an integer between 0 and MAXINT of the system.

BR

design

Class Random

Class String

An instance of Random

number generator

An instance of string

Input and fill up message.

Generate Random integer

Attach (concatenate)

Design

Output combined message.

For implementation see /projects/bina/java/secretMsg.java

BR

debugging and testing
Compile-time Errors : Usually typos or syntax errors

Run-time Errors : Occurs during execution. Example: divide by zero .

Logic Errors: Software will compile and execute with no problem, but will not produce expected results. (Solution: testing and correction)

See /projects/bina/java/Peets directory for an exercise.

Debugging and Testing

BR

class components
Class Components
  • Class name (starts with uppercase), constants, instance variables, constructors definitions and method definitions.
  • Constants:

public final static double PI = 3.14;

  • Variables:

private double bonus;

public string name;

BR

method invocation call
Method Invocation/Call
  • Syntax:

method_name (values);

object_name.method_name(values);

classname.method_name(values);

Examples:

computeSum(); // call to method from within the class where it is located

YourRose.paintIt(Red);

Math.abs(X);

BR

defining methods
Defining Methods
  • A method is group of (related) statements that carry out a specified function.
  • A method is associated with a particular class and it specifies a behavior or functionality of the class.
  • A method definition specifies the code to be executed when the method is invoked/activated/called.

BR

method definition syntax
Method Definition : Syntax

visibilityreturn_type method_name

(parameter_list)

{

statements

}

BR

return type
Return Type
  • can be void, type or class identifier
  • void indicates that the method called to perform an action in a self-standing way: Example: println
  • type or class specify the value returned using a return statement inside the method.

BR

return statement
Return Statement
  • Syntax of return statement:

return; // for void methods

return expression; // for type or class return value

// the expression type and return type should be same

BR

parameter list
Parameter List
  • Parameter list specified in method header provides a mechanism for sending information to a method.
  • It is powerful mechanism for specializing an object.
  • The parameter list that appears in the header of a method
    • specifies the type and name of each parameter and
    • is called formal parameter list.
  • The corresponding parameter list in the method invocation is called an actual parameter list.

BR

parameter list syntax
Parameter list : Syntax
  • Formal parameter list: This is like molds or templates

(parm_type parm_name, parm_type parm_name, ....)

  • Actual parameter list: This is like material that fit into the mold or template specified in the formal list:

(expression, expression....)

BR

method definition review
Method Definition : review

definition

header

body

Visibility

modifiers

parameter list

return type

Name

{ statements }

BR

method definition example
Method Definition : Example
  • Write a method that computes and returns the perimeter of a rectangle class.
  • Analysis:
    • Send to the method: Length and Width
    • Compute inside the method: Perimeter
    • Return from the method: Perimeter

BR

example contd
...Example (contd.)

public int Perimeter (int Length, int Width)

{

int Temp; // local temporary variable

Temp = 2 * (Length + Width); // compute perimeter

return Temp; // return computed value

}

BR

what happens when a method is called
What happens when a method is called?
  • Control is transferred to the method called and execution continues inside the method.
  • Control is transferred back to the caller when a return statement is executed inside the method.

BR

method invocation semantics
Method Invocation : semantics

Operating

System

1. OS to main method

2. Main method execution

3. Invoke Area

4. Transfer control to Area

5. Execute Area method

6. Return control back to

main method

7. Resume executing main

8. Exit to OS

8

1

2

Main method

Rect.Area(….)

3

7

4

8

Area

method

5

6

BR

constructors
Constructors
  • A Constructor is used to create or instantiate an object from the class.
  • Constructor is a special method:
    • It has the same name as the class.
    • It has no return type or return statement.
  • Typically a class has more than one constructor: a default constructor which has no parameters, and other constructors with parameters.

BR

constructors contd
Constructors (contd.)
  • You don’t have to define a constructor if you need only a default constructor.
  • When you want initializing constructors :

1. you must include a default constructor in this case.

2. You will use initializing constructors when you want the object to start with a specific initial state rather than as default state.

3. Example: Car myCar(Red); // initializing constructor for Car class with color as parameter

BR

visibility modifiers
Visibility Modifiers

type

Method/variable name

public

protected

“nothing”

DEFAULT

private

static

“nothing”

DEFAULT

To indicate

class method/

variable

To indicate

object

method/

variable

BR

modifiers contd
private : available only within class

“nothing” specified : DEFAULT: within class and within package

protected : within inherited hierarchy (only to sub classes)

public : available to any class.

..Modifiers (contd.)

BR

classpath
Many times classes needed are available in a repository other than the Java API.

Set the CLASSPATH environment variable to point to such repositories. One such is located in /projects/bina/cs114/structures

setenv CLASSPATH

.:/util/lang/jdk1.1.6/lib/classes.zip:/projects/bina/cs114/structures

(all in a continuous line)

CLASSPATH

BR

classpath44
CLASSPATH
  • Now you may refer to any one of the classes in the structures directory.
  • The source code is available in
  • /projects/bina/cs114/sources
  • Checkout ReadStream and Keyboard classes for nice inputs.

BR

inheritance
Inheritance
  • Inheritance is the act of deriving a new class from an existing one.
  • A primary purpose of inheritance is to reuse existing software.
  • Original class used to derive a new class is called “super” class and the derived class is called “sub” class.
  • Inheritance represents “is a” relationship between the superclass and the subclass.

BR

syntax
Syntax

class subclass extends superclass {

class definition

}

Example:

class Windstar extends FordCar // meaning it inherits from class Fordcar{ ....}

Windstar myCar();

In this example, FordCar is the super-class and Windstar is a sub-class and myCar is an object Windstar class.

BR

representing the relationship
Representing the Relationship

BankClass

has a

has a

has a

Account [ ]

MortgageSVC

BrokerageSVC

is a

is a

Savings

Checking

is a : use inheritance

has a : use composition, or membership

BR

modifers
Modifers
  • Visibility modifiers: private, public, protected
  • Protected modifier is used when the entity needs to be available to the subclass but not to the public.

BR

example words
Example : Words

Main class

Book class

Super class

Uses

is a

Dictionary Class

subclass

BR

example school
Example : School

Student

Main class

uses

Grad Student

BR

abstract class abc and methods
Abstract Class (ABC) and methods
  • An abstract class or method is defined using “abstract” modifier.
  • An abstract method has no definition in the super class.
  • ABC cannot be instantiated.
  • Sole purpose of ABC is to provide an appropriate super class from which classed may inherit.
  • Classes from which objects can be instantiated are called concrete classes.

BR

example52
Example

Food

Abstract super class

Pizza

Hamburger

HotDog

subclasses

BR

interface
An abstract method is one that does not have a definition within the class. Only the prototype is present.

An interface is collection of constants and abstract methods.

Syntax

interface interface_name {

constant -declarations;

abstract methods;

}

Interface

BR

example54
interface EPA {

bool emissionControl();

bool pollutionControl();

}

class NYepa implements EPA {

bool emissionControl () {

details/semantics /statements how to implement it }

class CAepa implements EPA {

bool emissionControl () {….

// different details on implementation….}

Example

BR

inheritance and interfaces
In Java class may inherit (extend) from only one class. (C++ allows multiple inheritance).

But a Java class may implement many interfaces.

For example,

public class Scribble extends Applet implements MouseListner, MouseMotionListner {

Inheritance and Interfaces

BR

types of programs56
Types of Programs
  • Console programs:
    • Control-driven: You start the program and the program code determines the sequence of events.
    • Mostly the actions are predetermined.
  • Windows-based programs:
    • Event-driven: The operation of the program depends on what you do with the controls presented (usually by the GUI)
    • Selecting menu items, pressing buttons, dialog interaction cause actions within the program.

BR

events
Events

1. Actions such as clicking a button, moving a mouse, are recognized and identified by the operating systems(OS).

2. For each action, OS determines which of the many currently running programs should receive the signal (of the action)

3. The signals that the application receives from the OS as result of the actions are called events.

BR

event generation
Event Generation

User Interface

Other

event

sources

Mouse actions, Keystrokes

Operating Systems

JAVA API

Events

Methods and handlers

Methods and handlers

Application 1

Application 2

BR

event handlers
An application responds to the events by executing particular code meant for each type of event.

Not all events need to be handled by an application. For example, a drawing application may be interested in handling only mouse movements.

As a designer of an event-driven application you will write methods to handle the relevant events.

Event Handlers

BR

event handling process
Event Handling Process
  • Source of an event is modeled as an object. Ex: button click’s object is a button
  • Type of the event: ActionEvent, WindowEvent, MouseEvent etc. Ex: An ActionEvent object is passed to the application that contains information about the action.
  • Target of an event: Listener object of the event. Passing the event to a listener means calling the particular method of the listener object.

BR

java event model
Java Event Model
  • Java event model has changed quite significantly between JDK1.0 and JDK 1.1.
  • Many Web Browsers still use JDK1.0.
  • JDK1.0 event model is good for understanding event-driven programming and for simple applications but does not scale well for large applications.
  • It is impossible to take a in-depth look at any one. But we will learn enough to use some basic features.

BR

java class hierarchy
Java Class Hierarchy

java.lang

Object

java.awt

Component

“others”

Label

TextComponent

Button

Container

java.applet package

Window

Panel

Applet

Dialog

Frame

BR

jdk1 1 event model system support
JDK1.1 Event Model : System support

Many Event classes

Low-level classes

Semantic classes

FocusEvent

MouseEvent

Etc.

Etc.

ActionEvent

TextEvent

: methods

Listener Interfaces

BR

jdk1 1 event model application support
Instantiate objects from component classes such as buttons and checkboxes which will serve as source of the events.

Implement listener interfaces.

Define methods to handle events of interest.

Example: /projects/bina/java/Doodle.java

JDK1.1 Event Model : Application Support

BR

implementing listeners
Implementing Listeners
  • An application (class) becomes a listener when it implements the listener interface.
  • Java 1.1 event model provides a variety of listeners.
  • EX:
    • If an application is interested in button click, it will have to implement ActionListener interface.
    • actionPerformed() method of the ActionListener will have be implemented by the application.
    • When the button click event occurs, actionPerformed() method will be called with event object as argument.

BR

example doodle java
Example: Doodle.java

1. Include “implements” at the header of the class : This application uses MouseListener and MouseMotionListener interfaces

2. Invoke “ methods” to add/activate required listeners. Here we have three:

a) mouse click

b) mouse motion

c) window close

3. Set up board : use paint() and fillRect() methods.

BR

example contd67
Example (contd.)

4. Implement all the methods (even if it is empty) of the interface (why?)

5. This application needs only MousePressed and MouseReleased for line color selection and MouseDragged for drawing lines.

6. Finally an adapter class that handles window closing.

BR

applet
Applet
  • An applet is a Java program that operates within a browser and hence can appear in a web page.
  • It can be fetched from a remote computer and run on the local computer.
  • Applet’s enabling technologies:
    • Java Virtual Machine (JVM)
    • Interpretation rather than compilation

BR

requirements
Requirements
  • Besides the classes, an applet requires a .html file and a tool for viewing the applet (Ex: appletviewer of JDK)
  • An applet extends Applet class of java.applet package.

import java.applet.Applet;

  • An applet should have at least an init() or a paint() method (just as an application should have main() method)

BR

applet class
Applet Class
  • Applet class has four important methods:

init()

start()

stop()

destroy()

Besides these paint() method of component class is commonly used for drawing graphics in an applet window.

BR

applet runtime structure

Applet

file

.java

Applet Runtime Structure

html

file

.html

includes

javac

appletviewer

Java runtime

JVM

Applet

file

.class

BR

example 1 using only paint
//file : MyApplet.java

import java.applet.Applet; // step 1

import java.awt.Graphics;

public class MyApplet extends Applet //step 2

{

public void paint( Graphics g) // step 3

{ g.drawString(“To climb a ladder”, 20, 90); }

}

Example 1: Using only paint()

BR

example 1 interpretation
Example 1: interpretation

step 1: import Applet class

step 2: inherit from Applet class

step 3: Overload /redefine/customize paint() method

BR

html file
html file

1. Create a html file:

<APPLET code=“MyApplet.class” width=300 height=200>

<\APPLET>

2. Save as MyApplet.html

3. To create class: javac MyApplet.java

4. To execute: appletviewer MyApplet.html

BR

example 1 execution semantics
Example 1: execution semantics

Java run-time system will look for init(), then start() methods.

Default definitions of init() and start() are executed.

Then user-applet-defined paint() method is executed.

Rest of the control follows default methods.

BR

more on applets
More on Applets

1. main() is replaced by init() or paint() methods

2. An applet cannot access local programs or files

3. Very useful in creation of active/dynamic web pages

4. Implements “behavior streaming”

5. Popular component of Java language.

BR

on line information
On-line Information

file:/util/lang/jdk1.1.6/api/API_users_guide.html

has user’s manual for Java API

/util/lang/jdk1.1.6/demo

has many demo programs

and of course, Sun’s home page has the JDK distribution.

BR

summary
Summary
  • We studied Java classes and objects, object-oriented programming, applications and applets.
  • A good understanding of the event-model is necessary to write event-driven programs.
  • Graphics and awt features (buttons, panels etc.) will tremendously enhance the capabilities and presentation of the event driven programs.

BR

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