JAVA

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Lecture 02Introduction to Java

-The First Java Application-

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Outline

• Basic components of Java

• Data type

• Operators

• JBuilder

• Java Application

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What is Java?

• Designed in the early of 1990s by Sun Microsystems

• Provide animation and interactivity on the World Wide Web– Web browsers have provided the

opportunities to run Java applets

• The fastest growing language

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Java Language

• Standard language used for programming, creating applets, servlets, JavaBeans, and enterprise components

• Java is simple

• Java is object-oriented language

• Java is distributed

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Java Language

• Java is interpreted– Need an interpreter to

run Java program– The program are

compiled into Java Virtual Machine (JVM) code called bytecode

Java Source Code

Java compiler

Java Bytecode Code

Java Interpreter

CPU

JVM

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Java Language

• Java is robust– Reliabile

• Detect many problems

• Java is secure• Java is platform-independent• Java is portable

– Can be run on any platform without being recompiled

• Java is multithreaded

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Java Virtual Machine (JVM)

• Interpreter for the Java programming language– a simple platform that all Java applications

run on.

• Comes with Java Development Kit (JDK)– Contains JVM and run-time system– Java 2 SDK

• www.sun.com

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Java Environment

• Editor– Integrated Development Environment (IDE)

• Jbuilder, J++, Forte, Visual Cafe

• Compiler– Translate into bytecode

• For Sun Microsystems- javac (included in SDK)• Class loader produces .class file

• Loading– Applications

• loaded and executed using Java Interpreter java example.class

– Applet• loaded in the browser and could be viewed by applet viewer using the html

file in which the applet is placed.

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The Basics of Java Program

• To write meaningful program, learn the programming language’s special symbols, words, syntax rules, and semantic rules– Syntax rules: tell you which statements are legal, or accepted by the

programming language– Semantic rules: determine the meaning of the instructions

• Programming language– A set of rules, symbols, and special words

Public class Welcome{ public static void main (String [ ] args) { System.out.println (“Welcome to Java Class”); }}

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Data

• Categorize data as variable or constant

• Constant– Data cannot be changed

• Variable– Data might be changed– Variable are named memory locations that

your program can use to store values

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Data Types

Primitive data types

integral Floating-point Boolean

char byte short int long float double

The fundamental data types in Java

A data type that deals with integers, or numbers without a decimal part

A data type that deals with decimal numbers

A data type that deals with logicalvalues

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The Integral Data Type

Type Minimum Value Maximum Value Size in Bytes

byte - 128 127 1 (8 bits)

short - 32,768 32,767 2

Int - 2,147,483,648 2,147,483,647 4

long - 9,223,372,036,854,775,808 9,223,372,036,854,775,807 8

char 0 65535 2

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The boolean Data Type

• Boolean logic is based on true-false comparisons

• Boolean variable– A variable that holds a Boolean value (true or

false)

Operator Name Example

! Not&& and true && true true| | or false | | false false

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Floating-point Data Types

Type Minimum Value Maximum Value Size in Bytes

Float - 3.4 * 1038 3.4 * 1038 4

double - 1.7 * 10308 1.7 * 10308 8

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Arithmetic Statements

Operator Description Example

+ addition 45+2, the result is 47

- subtraction 45-2, the result is 43

* multiplication 45*2, the result is 90

/ addition 45/2, the result is 22 (not 22.5)

% modulus (remainder) 45%2, the result is 1

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Order of Precedence

• When more than one arithmetic operator is used in an expression, Java uses operator precedence rules to determine the order in which operations are performed to evaluate the expression– Higher level of precedence: *, /, %– Lower level of precedence: +, -

• Expression exercise

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Cast Operator

• Implicit type coercion– When a value of one data type is automatically changed to

another data type– Generate unexpected results

• E.g.3 / 2 + 5.0 = 6.0

• To avoid implicit type coercion, Java provides for explicit type of conversion through use of a cast operator (also called the type conversion or type casting)(dataType) expression

e.g.(int) (7.9) 7

(double) (15) / 2 (double) (15 / 2)

7.5

7.0

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Declaration and Assignment• Variable declaration

– To use a variable, you declare it by telling the compiler the name of the variable as well as what type of data it represents

datatype variableName e.g.

int x; //Declare x to be an integer variable• After a variable is declared, you can assign a value to it by using an

assignment statementvariable = expression;e.g.

x = 1; //Assign 1 to variable x x = y + 1 ; //assign the addition of y and 1 to x– You can declare variable and initialize it in one step

int x;x = 1 int x = 1;

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Example

int firstNumber;

int secondNumber;

int finalNumber;

firstNumber = 7;

secondNumber = 8;

finalNumber = firstNumber + secondNumber

firstNumber ?

secondNumber ?

finalNumber ?

firstNumber 7

secondNumber 8

finalNumber 15

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Increment and Decrement Operators

• Increment operator– Increases the value of a variable by 1

• E.g.int count = 0; count = count + 1;

– Pre-increment: ++ variable (++x)• First the value of x is incremented by 1, and then the new value of x is used to evaluate

the expression– E.g. x = 5; y = ++ x; (after execution, x = 6 and y = 6)

– Post-increment: variable ++ (x ++)• First the current value of x is used in the expression, and then the value of x is

incremented by 1– E.g.

x = 5; y = x ++; (after execution, x = 6 and y = 5)

• Decrement operator– Decreases the value of a variable by 1

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String and the Operator +

• The operator + can be used to concatenate two strings as well as a string and a numeric value or a character– E.g.

String str;

str = “Sunny”;

str = str + “Day”; (results in “Sunny Day”)

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Relational Operators

• Relational operator

Operator Name Example Answer

< less than 1 < 2 true<= less than or equal to 1 <=2 true> greater than 1 > 2 false>= greater than or equal to 1 >= 2 false= = equal to 1 = = 2 false!= not equal to 1 != 2 true

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Constant

• A constant represents permanent data that never changesfinal datatype CONSTANTNAME = VALUE;

In java, the world final means that the constant cannot be changed.

e.g.final double PI = 3.14159;

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Programming Style and Documentation

• Appropriate Comments– Every program has the following block comment

appear at the top of the source code file:/* Programmer: Jaeki Song

Course: ISQS 6337

File Name: Assign1XXXX.java

Description: A brief description of the program

*/

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Programming Style and Documentation

• Proper indentation and spacing– Clear and easy to read

e.g.:

public class Test

{

public static void main(String args[])

{

System.out.println(“Example”);

}

}

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Programming Errors

• Syntax error– Result from errors in cod construction

• E.g.: mistyping, omitting some necessary punctuation, using an opening brace without a corresponding closing brace

• Logical error– Occur when a program does not perform the way it was

intended to

• Run-time error– Cause a program to terminate abnormally

• E.g.– Input error: the user enters an unexpected input value that the

program cannot handle– Division by zero

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Formatting Output

• Escape characters

Code Concept Result

\t Horizontal tab Moves insertion point eight spaces to the right

\b Backspace Moves insertion point one space to the left

\n New lineMoves insertion point down one line and to the leftmargin

\r Carriage return Moves insertion point to the left margin

\” Double quote Used to print a double quote character

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Java Naming Conventions

• Standards were originally proposed by Sun and have been widely adopted by the Java programming community

• Packages– The prefix of a unique package name is always written in all-

lowercase ASCII letters and should be one of the top-level domain names

– Use dots to separate the parts• E.g.: com.sun.eng, com.objectcentral.javatools

• Classes– Class (and interface) names should be nouns descriptive of the

purpose of the class– Names are in mixed case, beginning with a capital and with the

first letter of each internal word capitalized– Use complete words and avoid abbreviations

• E.g.: Point, Shape, MovieEditor, ClientList

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Java Naming Conventions• Methods

– Methods should be verbs descriptive of the purpose of the method– Names are mixed case with the first letter lowercase and the first letter

of each internal word capitalized – There are prefix conventions for general types of methods, such as

using get and set for getters and setters• E.g.: getOrigin, findSmallest, drawGraph, saveMoney

• Variables– Except when used as constants, all variables are named using mixed

case with a lowercase first letter, and with intenral words starting with capital letters

– Use one-letter variable names only for temporary variables• E.g.: myMovie, editedMovie, backgroundColor

• Constants– Names should be all uppercase with words seperated by underscores

(“_”)• e.g.: MAX_SIZE, TERM_LIMIT

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Creating First Application

• The Java 2 Platform, Standard Edition

• JBuilder7

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JBuilder: Interface

Main menuProjecttoolbar

Projectpane

Structurepane

ContentpaneFile tab

File view tab

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Example

/* Assignment 1 Printing on the screen Programmer: Jaeki Song Student ID : 999-99-9999 Date : September 2001 Program Name: Address */

public class Address{

public static void main(String[] args) //method header{ System.out.println(“ Jaeki Song”); System.out.println(“ 1234 89th Street”);

System.out.println(“ Lubbock, TX, 79413”); }

}

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Documentation

• Comments– Block comment

/* ….. */

– Line comment: //– e.g.

/* Assignment 1 Printing on the screen Programmer: Jaeki Song Student ID : 999-99-9999

Date : September 2001 Program Name: Address */

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Java Class

• Java program consists of pieces called classes– Existing classes in Java Class Libraries

• Known as Java APIs (Applications Programming Interfaces)

• Classes consists of pieces called methods– Perform tasks and return information

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Java Class

• A single class resides in a single Java source file with extension .java

• public class Address { …. }• The source code is Address.java.

– The name of the class is the name of the file

• Class name and file name must match

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Creating Class

• When you create a class, first you must assign a name to the class, and then you must determine what data and methods will be part of the class– An optional class modifier

• public, final, or abstract

– They keyword class– Any legal identifier you choose for the name of your classe.g.public class Employee{ ………..}

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Access Control

• The goal of class creator (those who create class) is to build a class that exposes only what’s necessary to the client programmer (the class consumers who use the class in their application) and keeps everything else hidden– Class creator can change the hidden portion without

worrying about the impact to anyone else– Class creator can change the internal working of the

class without worrying about how it will affect the client programmer

• Java use three boundaries– public, private, and protected

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Access Modifier: Class

• Class access modifiers– public, final, or abstract

• public classes are accessible by all objects– public classes can be extended or used as a basis for

any other class

• In the class, allowable field modifiers are public, private, protected, and final

– Private means that no other classes can access a field’s value and only methods of the same class are allowed to set or get

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Main Method

• The class which contains the main() method is the class that starts running the program

• Every Java application (not applet) has a main class

public class Address{ public static void main(String[] args) { … }}

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Access Modifier

• Specifies the circumstances in which the class can be accessed– E.g.: public class Address

{

…….

}

• Public indicates that this code can be access by all objects and can be extended or used as a basis for another class

• The contents of the class must be enclosed in braces { }

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Methods and Method Header

public static void main(String[] args) //method header{

……

}

The methodas accessible to all classes

This method is for class

Three parts• return value

• method name

• arguments lists

Void means that this methoddoes not return a value when it is called

Method name is main. Mainmethod is the usual starting point for all stand-alone Java program

Piece of data. args is an identifier for any string or character argument

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Body Code

{ System.out.println(“ Jaeki Song”); System.out.println(“ 1234 89th Street”); System.out.println(“ Lubbock, TX, 79413”); }

Out is the object that represents the default display

System is the name of the class (program-defined class)

Println is the name of a method thattakes a string argument. It returns its value to theSystem.out device

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Using Java Swing Class

• Refers to the new library of GUI– A component set that makes up all the objects of GUI

• Displays output using windows or dialog boxes– Input Dialog and Output Dialog

• Use packages– Predefined classes grouped into categories of related

classes called packages (sometimes called java class libraries or java applications programming interface (API))

– JOptionPane• Defined in a package called javax.swing

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Output Dialog

• showMessageDialog ( null, “string”);– A method of class JOptionPane– Two arguments

• Syntax

JOptionPane.showMessageDialog(null, “string”);

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Example: Output Dialog

import javax.swing.JOptionPane; //import class JOptionPane

public class Address{

public static void main(String[] args) //method header{ JOptionPane.showMessageDialog(

null, " Jaeki Song\n1234 89th Street\n Lubbock, TX, 79413"); System.exit(0); //terminate program }}

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Output

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Input Dialog

• Uses predefined dialog box from class JOptionPane called input dialog– Allows the user to input a value for use in the

program– Syntax

JOptionPane.showInputDialog(“ Enter first integer”);

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Example: Add Integer

import javax.swing.JoptionPane;

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

String number1, number2; //first and second string entered by user int numInt1, numInt2, sum; number1 = JOptionPane.showInputDialog(“Enter first number”); number2 = JOptionPane.showInputDialog(“Enter second number”); numInt1 = Integer.parseInt(number1); numInt2 = Integer.parseInt(number2); sum = numInt1 + numInt2; JOptionPane.showMessageDialog(null, “The sum is “ + sum, “Results”,

JOptionPane.PLAIN_MESSAGE); System.exit(0); }}

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Output

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Example: Convert Length

• Write a program that takes as input given lengths expressed in feet and inches. The program should then convert and output the lengths in centimeters using GUI. Assume that the given lengths in feet and inches are integers