1 Chapter 1 Java –Originally for intelligent consumer-electronic devices –Then used for creating...

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Chapter 1

• Java– Originally for intelligent consumer-electronic devices

– Then used for creating Web pages with dynamic content

– Now also used for:• Develop large-scale enterprise applications• Enhance WWW server functionality• Provide applications for consumer devices (cell phones, etc.)

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PDA and Cell Phone

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Java’s History• James Gosling and Sun Microsystems

• Oak

• Java, May 20, 1995, Sun World

• HotJava – The first Java-enabled Web browser

• Early History Website:– http://en.wikipedia.org/wiki/Java_(programming_language)– http://java.sun.com/features/1998/05/birthday.html

http://en.wikipedia.org/wiki/Java_(programming_language)

http://java.sun.com/features/1998/05/birthday.html

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Characteristics of Java• 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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Characteristics of Java• 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

Java is partially modeled on C++, but greatly simplified and improved. Some people refer to Java as "C++--" because it is like C++ but with more functionality and fewer negative aspects.

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Java is inherently object-oriented. Although many object-oriented languages began strictly as procedural languages, Java was designed from the start to be object-oriented. Object-oriented programming (OOP) is a popular programming approach that is replacing traditional procedural programming techniques.

One of the central issues in software development is how to reuse code. Object-oriented programming provides great flexibility, modularity, clarity, and reusability through encapsulation, inheritance, and polymorphism.

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Distributed computing involves several computers working together on a network. Java is designed to make distributed computing easy. Since networking capability is inherently integrated into Java, writing network programs is like sending and receiving data to and from a file.

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You need an interpreter to run Java programs. The programs are compiled into the Java Virtual Machine code called bytecode. The bytecode is machine-independent and can run on any machine that has a Java interpreter, which is part of the Java Virtual Machine (JVM).

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Java compilers can detect many problems that would first show up at execution time in other languages.

Java has eliminated certain types of error-prone programming constructs found in other languages.

Java has a runtime exception-handling feature to provide programming support for robustness.

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Java implements several security mechanisms to protect your system against harm caused by stray programs.

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Write once, run anywhere

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

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Because Java is architecture neutral, Java programs are portable. They can be run on any platform without being recompiled.

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Multithread programming is smoothly integrated in Java, whereas in other languages you have to call procedures specific to the operating system to enable multithreading.

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Java was designed to adapt to an evolving environment. New code can be loaded on the fly without recompilation. There is no need for developers to create, and for users to install, major new software versions. New features can be incorporated transparently as needed.

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JDK Versions• JDK 1.02 (1995)• JDK 1.1 (1996)• JDK 1.2 (1998)• JDK 1.3 (2000)• JDK 1.4 (2002)• JDK 1.5 (2004) a. k. a. JDK 5 or Java SE 5• JDK 1.6(2006) a. k. a. JDK 6 or Java SE 6

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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.

This course uses J2SE to introduce Java programming.

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Java IDE Tools

• Borland JBuilder

• NetBeans Open Source by Sun

• Sun ONE Studio by Sun MicroSystems

• Eclipse Open Source by IBM (preferred in this course)

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Introduction to Classes and objects• Agenda

– What is an object oriented view of programming?

– What is a class?– How to define a class?– Creating and using an object

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What is an object oriented view of programming

Suppose I want to invite my friends Tom, Liz and Rita to meet me for coffee at Tim Horton’s in the Devonshire Mall at 8PM today.

Approach 1:Give a complete and very detailed set of instructions on how to go from the University to Tim Horton’s. In this approach, every one exactly follows my instructions.

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What is an object oriented view of programming

Approach 2:Simply tell my friends “ Pl meet me for coffee at 8 this evening at Tim Horton’s in the Devonshire Mall”

Here each person follows his/her own way of interpreting my message and reacting to it.I don’t know and I don’t care the specific mechanism used by each individual in response to my message.

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A model for this approach

• Community of interacting agents called objects• Each object provides services• All action is initiated by a message to an object

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Object oriented view

• Importance of information hiding • Service oriented view of the world

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Notion of classes and objects

My Cookie cutter

• Every time I use the cookie cutter, I get a cookie ready for baking.•Each cookie looks exactly like the others•View my cookie cutter as a template

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Class - a template for similar objects

Class definition• What are the properties of the class?• What are the capabilities of the class?

Worker class• How to characterise an object?

– name, age, place_working• What can an object do?

– change the place of work– describe the object as a string

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Creating an object• First step is to define the class if needed• Use the class as a template to create objects• All objects of the same class has the same

set of properties and capabilities

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Class Hierarchies and inheritance

Notion of superclass and subclasses

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Defining a classpublic class worker { String Name; // Example of a variable int age; String placeWorking; public void ChangeJob(String NewPlace) // Method {placeWorking = NewPlace; } // Other methods as needed public worker(String WorkerName) // Constructor {Name = WorkerName; }}

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How to use the services of a class• A service => a method • An object of the class must exist for nonstatic

methods

worker w;

w = new worker( “Jim”);//use of constructor

• Use service => invoke right method

w. ChangeJob(“Ford, Windsor”);

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Thinking About Objects

• Objects– Reusable software components that model

real-world items– Look all around you

• People, animals, plants, cars, etc.

– Attributes• Size, shape, color, weight, etc.

– Behaviors• Babies cry, crawl, sleep, etc.

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Java Class Libraries• Classes

– Include methods that perform tasks• Return information after task completion

– Include variables defining properties of class– Used to build Java programs

• Java contains class libraries– Known as Java APIs (Application Programming

Interfaces)– http://java.sun.com/j2se/1.5.0/docs/api/

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Basics of a Typical Java Environment

• Java programs normally undergo five phases– Edit

• Programmer writes program (and stores program on disk)– Compile

• Compiler creates bytecodes from program– Load

• Class loader stores bytecodes in memory– Verify

• Verifier ensures bytecodes do not violate security requirements

– Execute• Interpreter translates bytecodes into machine language

33Typical Java environment.

PrimaryMemory

.

.

.

.

.

.

Disk

Disk

Disk

Editor

Compiler

Class Loader

Program is created in an editor and stored on disk in a file ending with .java.

Compiler creates bytecodes and stores them on disk in a file ending with .class.

Class loader reads .class files containing bytecodes from disk and puts those bytecodes in memory.

Phase 1

Phase 2

Phase 3

PrimaryMemory

.

.

.

.

.

.

Bytecode Verifier Bytecode verifier

confirms that all bytecodes are valid and do not violate Java’s security restrictions.

Phase 4

PrimaryMemory

.

.

.

.

.

.

InterpreterInterpreter reads bytecodes and translates them into a language that the computer can understand, possibly storing data values as the program executes.

Phase 5

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Compiling Java Source CodeYou can port a source program to any machine with appropriate compilers. The source program must be recompiled, however, because the object program can only run on a specific machine. Nowadays computers are networked to work together. Java was designed to run object programs on any platform. With Java, you write the program once, and compile the source program into a special type of object code, known as bytecode. The bytecode can then run on any computer with a Java Virtual Machine, as shown below. Java Virtual Machine is a software that interprets Java bytecode.

Java Bytecode

Java Virtual Machine

Any Computer

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Thinking About Objects (60-322)

• Object-oriented design (OOD)– Models real-world objects– Models communication among objects– Encapsulates attributes and operations (behaviors)

• Information hiding• Communication through well-defined interfaces

• Object-oriented language– Programming in object oriented languages is called

object-oriented programming (OOP)– Java

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Thinking About Objects

• Object-Oriented Analysis and Design (OOA/D)– Essential for large programs– Analyze program requirements, then develop

solution– UML

• Unified Modeling Language

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• A First Program in Java: Printing a Line of Text

• Modifying Our First Java Program

• Displaying Text in a Dialog Box

•Another Java Application: Adding Integers

•Memory Concepts

•Arithmetic

•Decision Making: Equality and Relational Operators

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A Simple Java Program

//This program prints Welcome to Java! public class Welcome { public static void main(String[] args) { System.out.println("Welcome to Java!"); }}

Listing 1.1

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Creating and Editing Using NotePadTo use NotePad, type

notepad Welcome.java from the DOS prompt.

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Creating and Editing Using WordPad

To use WordPad, type write Welcome.java

from the DOS prompt.

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Creating, Compiling, and Running Programs

Source Code

Create/Modify Source Code

Compile Source Code i.e., javac Welcome.java

Bytecode

Run Byteode i.e., java Welcome

Result

If compilation errors

If runtime errors or incorrect result

public class Welcome { public static void main(String[] args) { System.out.println("Welcome to Java!"); } }

… Method Welcome() 0 aload_0 … Method void main(java.lang.String[]) 0 getstatic #2 … 3 ldc #3 <String "Welcome to Java!"> 5 invokevirtual #4 … 8 return

Saved on the disk

stored on the disk

Source code (developed by the programmer)

Byte code (generated by the compiler for JVM to read and interpret, not for you to understand)

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Comments

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 the same line. When it sees /*, it scans for the next */ and ignores any text between /* and */.

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Discuss four small Java programs

Welcome1.java

Welcome2.java

Welcome3.java

Welcome4.java

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Displaying Text in a Message Dialog Box

you can use the showMessageDialog method in the JOptionPane class. JOptionPane is one of the many predefined classes in the Java system, which can be reused rather than “reinventing the wheel.”

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The showMessageDialog Method

JOptionPane.showMessageDialog(null, "Welcome to Java!", “Display Message", JOptionPane.INFORMATION_MESSAGE));

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Two Ways to Invoke the MethodThere are several ways to use the showMessageDialog method. For the time being, all you need to know are two ways to invoke it.

One is to use a statement as shown in the example:JOptionPane.showMessageDialog(null, x,

y, JOptionPane.INFORMATION_MESSAGE));

where x is a string for the text to be displayed, and y is a string for the title of the message dialog box.

The other is to use a statement like this:JOptionPane.showMessageDialog(null, x);

where x is a string for the text to be displayed.

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The exit Method

Prior to JDK 1.5, you have to invoke System.exit() to terminate the program if the program uses JOptionPane dialog boxes. Since JDK 1.5, it is not necessary.

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Escape characters– Backslash ( \ )– Indicates special characters be outputEscape sequence

Description

\n Newline. Position the screen cursor at the beginning of the next line.

\t Horizontal tab. Move the screen cursor to the next tab stop. \r Carriage return. Position the screen cursor at the beginning of

the current line; do not advance to the next line. Any characters output after the carriage return overwrite the characters previously output on that line.

\\ Backslash. Used to print a backslash character. \" Double quote. Used to print a double-quote character. For

example, System.out.println( "\"in quotes\"" ); displays "in quotes"

Some common escape sequences.

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Another Java Application: Adding IntegersAdditi

on.java

Message dialog type Icon Description

JOptionPane.ERROR_MESSAGE

Displays a dialog that indicates an error to the user.

JOptionPane.INFORMATION_MESSAGE

Displays a dialog with an informational message to the user. The user can simply dismiss the dialog.

JOptionPane.WARNING_MESSAGE

Displays a dialog that warns the user of a potential problem.

JOptionPane.QUESTION_MESSAGE

Displays a dialog that poses a question to the user. This dialog normally requires a response, such as clicking on a Yes or a No button.

JOptionPane.PLAIN_MESSAGE no icon Displays a dialog that simply contains a

message, with no icon. JOptionPane constants for message dialogs.

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Memory Concepts

• Visual Representation– Sum = 0; number1 = 1; number2 = 2;

– Sum = number1 + number2; after execution of statement

sum 0

sum 3

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Arithmetic• Arithmetic calculations used in most rograms

– Usage • * for multiplication • / for division• +, -• No operator for exponentiation (more in Chapter 5)

– Integer division truncates remainder7 / 5 evaluates to 1

– Remainder operator % returns the remainder 7 % 5 evaluates to 2

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Arithmetic• Operator precedence

– Some arithmetic operators act before others (i.e., multiplication before addition)

• Use parenthesis when needed

– Example: Find the average of three variables a, b and c

• Do not use: a + b + c / 3 • Use: ( a + b + c ) / 3

– Follows PEMDAS • Parentheses, Exponents, Multiplication, Division, Addition,

Subtraction

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ArithmeticOperator(s) Operation(s) Order of evaluation (precedence) * /

%

Multiplication Division Remainder

Evaluated first. If there are several of this type of operator, they are evaluated from left to right.

+ -

Addition Subtraction

Evaluated next. If there are several of this type of operator, they are evaluated from left to right.

Precedence of arithmetic operators.

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Decision Making: Equality and Relational Operators

Standard algebraic equality or relational operator

J ava equality or relational operator

Example of J ava condition

Meaning of J ava condition

Equality operators = == x == y x is equal to y != x != y x is not equal to y Relational operators > > x > y x is greater than y < < x < y x is less than y >= x >= y x is greater than or equal to y <= x <= y x is less than or equal to y

Equality and relational operators.

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• Precedence of operators

Operators Associativity Type * / % left to right multiplicative + - left to right additive < <= > >= left to right relational == != left to right equality = right to left assignment Precedence and associativity of the operators discussed so far.

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