Dale Roberts

Introduction to JavaIntroduction to Java

- First Program- First Program

Dale Roberts, LecturerComputer Science, IUPUIE-mail: droberts@cs.iupui.edu

Department of Computer and Information Science,School of Science, IUPUI

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Java Program StructureJava Program Structure

In the Java programming language:In the Java programming language:A program is made up of one or more A program is made up of one or more classesclasses

A class contains one or more A class contains one or more methodsmethods

A method contains program A method contains program statementsstatements

Program statements can reference local or instance Program statements can reference local or instance variables. There is no concept of global variable.variables. There is no concept of global variable.

These terms will be explored in detail These terms will be explored in detail throughout the coursethroughout the course

A Java application always contains a method A Java application always contains a method called called mainmain

Dale Roberts

© 2004 Pearson Addison-Wesley. All rights reserved 1-3

Java Program StructureJava Program Structure

public class MyProgram

{

}

// comments about the class

class header

class body

Comments can be placed almost anywhere

Dale Roberts

© 2004 Pearson Addison-Wesley. All rights reserved 1-4

Java Program StructureJava Program Structure

public class MyProgram

{

}

// comments about the class

public static void main (String[] args)

{

}

// comments about the method

method headermethod body

Dale Roberts

© 2004 Pearson Addison-Wesley. All rights reserved 1-5

CommentsComments

Comments in a program are called Comments in a program are called inline documentationinline documentation

They should be included to explain the purpose of the They should be included to explain the purpose of the program and describe processing stepsprogram and describe processing steps

They do not affect how a program worksThey do not affect how a program works

Java comments can take three forms:Java comments can take three forms:

// this comment runs to the end of the line

/* this comment runs to the terminating symbol, even across line breaks */

/** this is a javadoc comment */

Dale Roberts

© 2004 Pearson Addison-Wesley. All rights reserved 1-6

IdentifiersIdentifiers

IdentifiersIdentifiers are the words a programmer uses in a are the words a programmer uses in a programprogram

An identifier can be made up of letters, digits, the An identifier can be made up of letters, digits, the underscore character ( _ ), and the dollar signunderscore character ( _ ), and the dollar sign

Identifiers cannot begin with a digitIdentifiers cannot begin with a digit

Java is Java is case sensitivecase sensitive - - Total, total, Total, total, andand TOTAL TOTAL are different identifiersare different identifiers

By convention, programmers use different case styles By convention, programmers use different case styles for different types of identifiers, such asfor different types of identifiers, such as

title case title case for class names - for class names - LincolnLincoln

upper caseupper case for constants - for constants - MAXIMUMMAXIMUM

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First Program in Java: Printing a Line of TextFirst Program in Java: Printing a Line of Text

ApplicationApplicationExecutes when you use the Executes when you use the javajava command to launch the command to launch the Java Virtual Machine (JVM)Java Virtual Machine (JVM)

Sample programSample programDisplays a line of textDisplays a line of text

Illustrates several important Java language featuresIllustrates several important Java language features

Dale Roberts

OutlineOutline

1 // Fig. 2.1: Welcome1.java 2 // Text-printing program. 3 4 public class Welcome1 5 { 6 // main method begins execution of Java application 7 public static void main( String args[] ) 8 { 9 System.out.println( "Welcome to Java Programming!" ); 10 11 } // end method main 12 13 } // end clazss Welcome1

Welcome to Java Programming!

Welcome1.javaWelcome1.java

8

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First Program in Java: Printing a Line of Text (Cont.)First Program in Java: Printing a Line of Text (Cont.)

Comments start with: Comments start with: ////Comments ignored during program executionComments ignored during program executionDocument and describe codeDocument and describe codeProvides code readabilityProvides code readability

Traditional comments: Traditional comments: /* ... *//* ... *//* This is a traditional/* This is a traditional comment. It can be comment. It can be split over many lines */ split over many lines */

Another line of commentsAnother line of commentsNote: line numbers not part of program, added for referenceNote: line numbers not part of program, added for reference

1 // Fig. 2.1: Welcome1.java

2 // Text-printing program.

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First Program in Java: Printing a Line of Text (Cont.)First Program in Java: Printing a Line of Text (Cont.)

Saving filesSaving filesFile name must be class name with File name must be class name with .java.java extension extension

Welcome1.javaWelcome1.java

Left brace Left brace {{Begins body of every classBegins body of every class

Right brace ends declarations (line 13)Right brace ends declarations (line 13)

4 public class Welcome1

5 {

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First Program in Java: Printing a Line of Text (Cont.)First Program in Java: Printing a Line of Text (Cont.)

Part of every Java applicationPart of every Java applicationApplications begin executing at Applications begin executing at mainmain

Parentheses indicate Parentheses indicate mainmain is a method (Ch. 3 and 6) is a method (Ch. 3 and 6)Java applications contain one or more methodsJava applications contain one or more methods

Exactly one method must be called Exactly one method must be called mainmain

Methods can perform tasks and return informationMethods can perform tasks and return informationvoidvoid means means mainmain returns no information returns no informationFor now, mimic For now, mimic mainmain's first line's first line

Left brace begins body of method declarationLeft brace begins body of method declarationEnded by right brace Ended by right brace }} (line 11) (line 11)

7 public static void main( String args[] )

8 {

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2.2 First Program in Java: Printing a Line of Text (Cont.)2.2 First Program in Java: Printing a Line of Text (Cont.)

Instructs computer to perform an actionInstructs computer to perform an actionPrints string of characters Prints string of characters

String – series of characters inside double quotesString – series of characters inside double quotesWhite-spaces in strings are not ignored by compilerWhite-spaces in strings are not ignored by compiler

System.outSystem.outStandard output objectStandard output objectPrint to command window (i.e., MS-DOS prompt)Print to command window (i.e., MS-DOS prompt)

Method Method System.out.printlnSystem.out.println Displays line of textDisplays line of text

This line known as a statementThis line known as a statementStatements must end with semicolon Statements must end with semicolon ;;

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

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First Program in Java: Printing a Line of Text (Cont.)First Program in Java: Printing a Line of Text (Cont.)

Ends method declarationEnds method declaration

Ends class declarationEnds class declaration

Can add comments to keep track of ending bracesCan add comments to keep track of ending braces

11 } // end method main

13 } // end class Welcome1

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First Program in Java: Printing a Line of Text (Cont.)First Program in Java: Printing a Line of Text (Cont.)

Compiling a programCompiling a programOpen a command prompt window, go to directory where Open a command prompt window, go to directory where program is storedprogram is stored

Type Type javacjavac Welcome1.javaWelcome1.java

If no syntax errors, If no syntax errors, Welcome1.classWelcome1.class created createdHas bytecodes that represent applicationHas bytecodes that represent application

Bytecodes passed to JVMBytecodes passed to JVM

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First Program in Java: Printing a Line of Text (Cont.)First Program in Java: Printing a Line of Text (Cont.)

Executing a programExecuting a programTypeType javajava Welcome1Welcome1

Launches JVMLaunches JVM

JVM loads JVM loads .class.class file for class file for class Welcome1Welcome1

.class.class extension omitted from command extension omitted from command

JVM calls method JVM calls method mainmain

Dale Roberts

Executing Welcome1 in a Microsoft Windows XP Executing Welcome1 in a Microsoft Windows XP Command Prompt window.Command Prompt window.

16

You type this command to execute the application

The program outputs

Welcome to Java Programming!

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Displaying Text with Displaying Text with printfprintf

System.out.printfSystem.out.printfFeature added in Java SE 5.0Feature added in Java SE 5.0

Displays formatted dataDisplays formatted data

Format stringFormat stringFixed textFixed text

Format specifier – placeholder for a valueFormat specifier – placeholder for a value

Format specifier Format specifier %s%s – – placeholder for a stringplaceholder for a string

9 System.out.printf( "%s\n%s\n", 10 "Welcome to", "Java Programming!" );

Dale Roberts

AcknowledgementsAcknowledgementshttp://java.sun.com/docs/books/tutorial/getStarted/TOC.html

Pearson Education, Lewis and Loftus.

Deitel, Java How to Program

http://www.cs.wustl.edu/~plezbert/contcom/thesis/node6.html

http://www.cs.usfca.edu/~parrt/course/652/lectures-Spring-2004/language.impl.overview.pdf

http://ei.cs.vt.edu/~history/Youmans.Java.html