2003 prentice hall, inc. all rights reserved. customized by sana odeh for the use of this class. 1...

2003 Prentice Hall, Inc. All rights reserved. Customized by Sana Odeh for the use of this class.

1

Introduction to Computers and Programming in JAVA Section 1

Professor: Sana` [email protected] Website: http://www.cs.nyu.edu/courses/fall07/V22.0002-001

Office Hours: Mon. 3:30 pm - 4:30 pm in room 418 in WWH


2Announcement

Lab Session

Wed. 1/12

• Class will be held at the ITS Lab at 14 Washington Place.

• We will meet in class first, then will go to the lab together. Make sure you are on time!


3

Introduction

• Introductions to Java Programming basics– Introduce a basic Java program

– We will use Jcreator

– to create/edit/compile/ debug and execute/run our Java programs


4

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

• Application– Program that executes using the java interpreter

• Sample program– Show program, then analyze each line

2003 Prentice Hall, Inc.All rights reserved.

Outline5

Welcome1.java

Program Output

1 // first Java program called Welcome1.java2 // 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 main12 13 } // end class Welcome1

Welcome to Java Programming!


6Lets look at the first and second line of the previous

programThis is a comment

– Comments start with: //• Comments ignored during program execution

• Improves code readability

• Document and describe code

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

– Another line of comments

– Note: line numbers not part of program, added for reference

1 // first Java program called Welcome1.java

2 // Text-printing program.


7

– Blank line• Makes program more readable• Blank lines, spaces, and tabs are white-space characters

– Ignored by compiler

– Begins class declaration for class Welcome1

– Public & class are reserved key words and have to be included in every program

• Reserved Keyword: words reserved for use by Java.– class keyword followed by class name ( in this case it is

Welcome1) is referred to as an identifier. – An identifier is user defined– Welcome1 is the name of the program and should be same as the

file name Welcome1.java• Naming classes: capitalize every word

– SampleClassName

3

4 public class Welcome1 {

Lets look at the following lines of the previous program


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Reserved Words

•Reserved words or keywords are words that have a specific meaning to the compiler and cannot be used for other purposes in the program. •For example, when the compiler sees the word class, it understands that the word after class is the name for the class. •Other reserved words in Example 1 are public, static, and void. Their use will be introduced later in the book.


9

Lets look at the following lines of the previous program

– Name of class called identifier ( in this case its Welcome1 )• Series of characters consisting of letters, digits,

underscores ( _ ) and dollar signs ( $ )

• Does not begin with a digit, has no spaces

• Examples: Welcome1, $value, _value, button7– 7button is invalid

• Java is case sensitive (capitalization matters)

– a1 and A1 are different

– For chapters 2 to 7, use public keyword• Certain details not important now

• Mimic certain features, discussions later



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2.2 A Simple Program: Printing a Line of Text

– Saving files• File name must be class name with .java extension

• Welcome1.java

– Left brace {• Begins body of every class

• Right brace ends declarations (line 13)

– Part of every Java application• Applications begin executing at main

– Parenthesis indicate main is a method (ch. 6)

– Java applications contain one or more methods


7 public static void main( String args[] )


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main Method

•The main method provides the control of program flow. •The Java interpreter executes the application by invoking the main method.

•The main method looks like this:

public static void main(String[] args) {

Statements;

}


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Blocks

A pair of braces in a program forms a block that groups components of a program.

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

Class block

Method block


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Statements

•A statement represents an action or a sequence of actions. •The statement System.out.println("Welcome to Java!"); •in the program in Example 1.1 is a statement to display the greeting "Welcome to Java!" Every statement in Java ends with a semicolon (;).


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Classes

•The class is the essential Java construct. •A class is a template or blueprint for objects. •To program in Java, you must understand classes and be able to write and use them.• The mystery of the class will continue to be unveiled throughout this book.• For now, though, understand that a program is defined by using one or more classes.


15


• Exactly one method must be called main

– Methods can perform tasks and return information• void means main returns no information

• For now, mimic main's first line

– Left brace begins body of method declaration• Ended by right brace } (line 11)

7 public static void main( String args[] )

8 {


16


– Instructs computer to perform an action• Prints string of characters

– String - series characters inside double quotes

• White-spaces in strings are not ignored by compiler

– System.out• Standard output object

• Print to command window (i.e., MS-DOS prompt)

– Method System.out.println • Displays line of text

• Argument inside parenthesis

– This line known as a statement• Statements must end with semicolon ;

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


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– Ends method declaration

– Ends class declaration

– Can add comments to keep track of ending braces

– Lines 8 and 9 could be rewritten as:

– Remember, compiler ignores comments

– Comments can start on same line after code

11 } // end method main

13 } // end class Welcome1


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Compiling a Java program

• Compiling a program– OpenJCreator

• See website for instructions• http://www.cs.nyu

.edu/courses/spring07/V22.0002-001/software.htm

– Create a new project, then a new java file/class and will save it.

– Type the program and compile it

– If no errors, Welcome1.class created• Has bytecodes that represent application• Bytecodes passed to Java interpreter

– If no errors, we will run or execute our program to get the output.


19

Executing program.. Lets take a look how to do this using JCreator

• Executing a program– Type java Welcome1

• Interpreter loads .class file for class Welcome1• .class extension omitted from command

– Interpreter calls method main


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Lets Modify Our First Java Program

• Modify previous example to print same contents using different code


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Modifying Our First Java Program

• Modifying programs– Welcome2.java produces same output as Welcome1.java

(Fig. 2.1)

– Using different code

– Line 9 displays “Welcome to ” with cursor remaining on printed line

– Line 10 displays “Java Programming! ” on same line with cursor on next line

9 System.out.print( "Welcome to " ); 10 System.out.println( "Java Programming!" );


22

Methods

•What is System.out.println() ? – It is a method: a collection of statements that

performs a sequence of operations to display a message on the console.

•It can be used even without fully understanding the details of how it works. It is used by invoking a statement with a string argument. •The string argument is enclosed within parentheses. •In this case, the argument is "Welcome to Java!" You can call the same println method with a different argument to print a different message.


Outline23

Welcome2.java

1. Comments

2. Blank line

3. Begin class Welcome2

3.1 Method main

4. Method System.out.print

4.1 Method System.out.println

5. end main, Welcome2

Welcome to Java Programming!

1 // Fig. 2.3: Welcome2.java2 // Printing a line of text with multiple statements.3 4 public class Welcome2 {5 6 // main method begins execution of Java application7 public static void main( String args[] )8 {9 System.out.print( "Welcome to " ); 10 System.out.println( "Java Programming!" );11 12 } // end method main13 14 } // end class Welcome2

System.out.print keeps the cursor on the same line, so System.out.println continues on the same line.


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2.3 Modifying Our First Java Program

• Newline characters (\n)– Interpreted as “special characters” by methods System.out.print and System.out.println

– Indicates cursor should be on next line

– Welcome3.java (Fig. 2.4)

– Line breaks at \n

• Usage– Can use in System.out.println or System.out.print to create new lines• System.out.println( "Welcome\nto\nJava\nProgramming!" );

9 System.out.println( "Welcome\nto\nJava\nProgramming!" );


Outline25

Welcome3.java

1. main

2. System.out.println (uses \n for new line)

Program Output

1 // Fig. 2.4: Welcome3.java2 // Printing multiple lines of text with a single statement.3 4 public class Welcome3 {5 6 // main method begins execution of Java application7 public static void main( String args[] )8 {9 System.out.println( "Welcome\nto\nJava\nProgramming!" );10 11 } // end method main12 13 } // end class Welcome3

WelcometoJavaProgramming!

Notice how a new line is output for each \n escape sequence.


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2.3 Modifying Our First Java Program

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"

Fig. 2.5 Some common escape sequences.


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Find the ErrorSample 1

1 Printing multiple lines of text with a single statement.2 3 4 public class Welcome3 {5 6 // main method begins execution of Java application7 public static void main( String args[] )8 {9 System.out.println( "Welcome\nto\nJava\nProgramming!" );10 11 } // end method main12 13 } // end class Welcome3


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Find the ErrorSample 2

1 //Printing multiple lines of text with a single statement.2 3 4 public Welcome3 {5 6 // main method begins execution of Java application7 public static void main( String args[] )8 {9 System.out.println( "Welcome\nto\nJava\nProgramming!" );10 11 } // end method main12 13 } // end class Welcome3


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Find the Errors (7)

1 //Printing multiple lines of text with a single statement.2 3 4 Public class 1firstprogram {5 6 // main method begins execution of Java application7 public static void main( String args[] 8 {9 System.out.print( "Welcome\nto\nJava\nProgramming! )10 11 } // end method main12 13 // end class Welcome3


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Anatomy of a Java Program

• Comments• Reserved words• Statements• Blocks• Classes• Methods• The main method

2003 prentice hall, inc. all rights reserved. customized by sana odeh for the use of this class. 1...

Documents

class welcome1 public

program reserved keyword

basic java program

program execution

class keyword

program output

class declaration

class website