Java02.ppt

(1)

Chapter 2 - Introduction to Java

Applications

Outline

2.1 Introduction

2.2 A Simple Program: Printing a Line of Text

2.3 Another Java Application: Adding Integers

2.4 Memory Concepts

2.5 Arithmetic

2.6 Decision Making: Equality and Relational

(2)

• In this chapter

(3)

2.2 A Simple Program: Printing a Line of

Text

• Application

– Program that executes using the

java

interpreter

• Sample program

(4)

Program Output

3

4 public class Welcome1 {

5 public static void main( String args[] )

6 {

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

8 }

Welcome to Java Programming!

(5)

2.2 A Simple Program: Printing a Line of

Text

–

_//

_{remainder of line is comment}

• Comments ignored

• Document and describe code

– Multiple line comments:

**/* ... */**

**/* This is a multiple**

line comment. It can

**be split over many lines */**

– Another line of comments

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

1 // Fig. 2.1: Welcome1.java

(6)

– Blank line

• Makes program more readable

• Blank lines, spaces, and tabs are

whitespace characters

– Ignored by compiler

– Begins class definition for class

Welcome1

• Every Java program has at least one user-defined class

• Keyword: words reserved for use by Java

–

_class

keyword followed by class name

• Naming classes: capitalize every word

–

_{SampleClassName}

3

(7)

2.2 A Simple Program: Printing a Line of

Text

– Name of class called

identifier

• 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

• 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

(8)

– Saving files

• File name is class name and

_.java

extension

• _{Welcome1.java}

– Left brace

{

• Begins body of every class

• Right brace ends definition (line

₉

)

– Part of every Java application

• Applications begin executing at

main

– Parenthesis indicate

_main

is a method

4 public class Welcome1 {

(9)

2.2 A Simple Program: Printing a Line of

Text

• 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 definition

• Ended by right brace

5 public static void main( String args[] )

(10)

– Instructs computer to perform an action

• Prints string of characters

– String - series characters inside double quotes

• Whitespaces 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

(11)

2.2 A Simple Program: Printing a Line of

Text

– Ends method definition

– Ends class definition

– Can add comments to keep track of ending braces

– Lines 8 and 9 could be rewritten as:

– Remember, compiler ignores comments

8 }

9 }

8 } // end of method main()

(12)

• Compiling a program

– Open a command window, go to directory where program is

stored

– Type

javac Welcome1.java

– If no errors,

Welcome1.class

created

• Has bytecodes that represent application

• Bytecodes passed to Java interpreter

• Executing a program

– Type

java Welcome1

• Interpreter loads

_.class

file for class

_Welcome1

• _.class

extension omitted from command

(13)

Outline

Java program

Program Output

1 // Fig. 2.1: Welcome1.java

2 // A first program in Java

3

6 {

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

8 }

(14)

• Other methods

–

_{System.out.println}

• Prints argument, puts cursor on new line

–

_{System.out.print}

(15)

Outline

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

Program Output

2 // Printing a line with multiple statements

3

6 { 7

7 System.out.print( "Welcome to " );

8 System.out.println( "Java Programming!" );

9 }

10 }

Welcome to Java Programming!

System.out.print

keeps the cursor on

the same line, so

System.out.println

(16)

• Escape characters

– Backslash (

\

)

– Indicates special characters be output

• Backslash combined with character makes escape sequence

• _\n

- newline

_\r

- carriage return

_\"

- double quote

• _\t

- tab

_\\

- backslash

• Usage

– Can use in

_{System.out.println}

or

System.out.print

to create new lines

• _{System.out.println( "Welcome\nto\nJava\}

(17)

Outline

Class

_Welcome1

1. _main

2. System.out.println

(uses

_\n

for newline)

Program Output

2 // Printing multiple lines with a single statement

3

6 { 7

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

8 }

9 }

Welcome to

Java

Programming!

Notice how a new line is output for each

\n

(18)

• Display

– Most Java applications use windows or a dialog box

• We have used command window

– Class

JOptionPane

allows us to use dialog boxes

• Packages

– Set of predefined classes for us to use

– Groups of related classes called

packages

• Group of all packages known as Java class library or Java

applications programming interface (Java API)

–

_JOptionPane

_{is in the}

_javax.swing

_package

(19)

2.2 A Simple Program: Printing a Line of

Text

(20)

• Upcoming program

(21)

Outline

Java program using

dialog box

Program Output

2 // Printing multiple lines in a dialog box

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

4

7 {

8 JOptionPane.showMessageDialog(

9 null, "Welcome\nto\nJava\nProgramming!" );

10

11 System.exit( 0 ); // terminate the program

12 }

(22)

– Lines 1-2: comments as before

–

_import

statements

• Locate the classes we use

• Tells compiler to load

_JOptionPane

from

_javax.swing

package

– Lines 4-7: Blank line, begin class

Welcome4

and

main

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

4

5 public class Welcome4 {

6 public static void main( String args[] )

(23)

2.2 A Simple Program: Printing a Line of

Text

– Call method

_{showMessageDialog}

of class

JOptionPane

• Requires two arguments

• Multiple arguments separated by commas (

,

)

• For now, first argument always

null

• Second argument is string to display

–

_{showMessageDialog}

is a

static

method of class

JOptionPane

• _static

methods called using class name, dot (

_.

) then

method name

– All statements end with

_;

• A single statement can span multiple lines

• Cannot split statement in middle of identifier or string

8 JOptionPane.showMessageDialog(

(24)

– Executing lines 8 and 9 displays the dialog box

• Automatically includes an

OK

button

– Hides or dismisses dialog box

• Title bar has string

Message

8 JOptionPane.showMessageDialog(

(25)

2.2 A Simple Program: Printing a Line of

Text

– Calls

static

method

exit

of class

System

• Terminates application

– Use with any application displaying a GUI

• Because method is

static

, needs class name and dot (

_.

)

• Identifiers starting with capital letters usually class names

– Argument of

0 means application ended successfully

• Non-zero usually means an error occurred

– Class

System

part of package

java.lang

• No

import

statement needed

• _java.lang

automatically imported in every Java program

– Lines 12-13: Braces to end

_Welcome4

and

main

(26)

2. Class

_Welcome4

2.1 _main

2.2 showMessageDialog

2.3 _System.exit

Program Output

7 {

9 null, "Welcome\nto\nJava\nProgramming!" );

10

12 }

(27)

2.3 Another Java Application: Adding

Integers

• Upcoming program

– Use input dialogs to input two values from user

(28)

input dialogs

6 public class Addition {

8 {

9 String firstNumber, // first string entered by user

10 secondNumber; // second string entered by user

11 int number1, // first number to add

12 number2, // second number to add

13 sum; // sum of number1 and number2

14

15 // read in first number from user as a string

16 firstNumber =

17 JOptionPane.showInputDialog( "Enter first integer" );

18

19 // read in second number from user as a string

20 secondNumber =

21 JOptionPane.showInputDialog( "Enter second integer" );

22

23 // convert numbers from type String to type int

24 number1 = Integer.parseInt( firstNumber );

25 number2 = Integer.parseInt( secondNumber );

26

(29)

Outline

Program Output

32 null, "The sum is " + sum, "Results",

33 JOptionPane.PLAIN_MESSAGE );

34

36 }

(30)

– Location of

JOptionPane

for use in the program

– Begins

public

class

Addition

• Recall that file name must be

_{Addition.java}

– Lines 7-8:

main

– Declaration

• _firstNumber

and

_secondNumber

are variables

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

6 public class Addition {

9 String firstNumber, // first string entered by user

(31)

2.3 Another Java Application: Adding

Integers

– Variables

• Location in memory that stores a value

– Declare with name and data type before use

• _firstNumber

and

_secondNumber

are of data type

String

(package

java.lang

)

– Hold strings

• Variable name: any valid identifier

• Declarations end with semicolons

_;

– Can declare multiple variables of the same type at a time

– Use comma separated list

– Can add comments to describe purpose of variables

9 String firstNumber, // first string entered by user

(32)

– Declares variables

number1

,

number2

, and

sum

of type

int

• _int

holds integer values (whole numbers): i.e.,

0 ,

-4

,

97 • Data types

_float

and

_double

can hold decimal numbers

• Data type

_char

can hold a single character

• Primitive data types - more Chapter 4

12 number2, // second number to add

(33)

2.3 Another Java Application: Adding

Integers

– Reads

String

from the user, representing the first number

to be added

• Method

JOptionPane.showInputDialog

displays the

following:

• Message called a prompt - directs user to perform an action

• Argument appears as prompt text

• If wrong type of data entered (non-integer), error occurs

15 // read in first number from user as a string

16 firstNumber =

(34)

– Result of call to

showInputDialog

given to

firstNumber

using assignment operator

=

• Assignment statement

• ₌

binary operator - takes two

operands

– Expression on right evaluated and assigned to variable on

left

• Read as:

firstNumber

gets value of

JOptionPane.showInputDialog( "Enter first

integer" )

15 // read in first number from user as a string

16 firstNumber =

(35)

2.3 Another Java Application: Adding

Integers

– Similar to previous statement

• Assigns variable

_secondNumber

to second integer input

– Method

Integer.parseInt

• Converts

String

argument into an integer (type

int

)

– Class

_Integer

in

_java.lang

• Integer returned by

_{Integer.parseInt}

is assigned to

variable

_number1

(line 24)

– Remember that

number1

was declared as type

_int

• Line 25 similar

19 // read in second number from user as a string

20 secondNumber =

21 JOptionPane.showInputDialog( "Enter second integer" );

23 // convert numbers from type String to type int

24 number1 = Integer.parseInt( firstNumber );

(36)

– Assignment statement

• Calculates sum of

_number1

and

_number2

(right hand side)

• Uses assignment operator

=

to assign result to variable

_sum

• Read as:

_sum

gets the value of

_{number1 + number2}

27 // add the numbers

(37)

2.3 Another Java Application: Adding

Integers

– Use

showMessageDialog

to display results

–

_{"The sum is " + sum}

• Uses the operator + to "add" the string literal

_{"The sum}

is"

and

_sum

• Concatenation of a

String

and another data type

– Results in a new string

• If

sum

contains

117 , then

"The sum is " + sum

results

in the new string

"The sum is 117"

• Note the space in

_{"The sum is "}

• More on strings in Chapter 10

31 JOptionPane.showMessageDialog(

32 null, "The sum is " + sum, "Results",

(38)

– Different version of

showMessageDialog

• Requires four arguments (instead of two as before)

• First argument:

null

for now

• Second: string to display

• Third: string in title bar

• Fourth: type of message dialog

–

_{JOptionPane.PLAIN_MESSAGE}

- no icon

–

_{JOptionPane.ERROR_MESSAGE}

–

_{JOptionPane.INFORMATION_MESSAGE}

–

_{JOptionPane.WARNING_MESSAGE}

31 JOptionPane.showMessageDialog(

32 null, "The sum is " + sum, "Results",

(39)

Outline

1. _import

2. class

_Addition

2.1 Declare variables

(name and data type)

3. showInputDialog

4. _parseInt

5. Add numbers, put

result in

_sum

1 // Fig. 2.8: Addition.java

2 // An addition program

3

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

5

6 public class Addition {

8 { 9

10 secondNumber; // second string entered by user

11 int number1, // first number to add

12 number2, // second number to add

13 sum; // sum of number1 and number2

14

15 // read in first number from user as a string 16

17 JOptionPane.showInputDialog( "Enter first integer" );

18

19 // read in second number from user as a string

21 JOptionPane.showInputDialog( "Enter second integer" );

22

23 // convert numbers from type String to type int 24

26

27 // add the numbers 28

28 sum = number1 + number2;

Declare variables: name and data type.

Input first integer as a

String

, assign

to

firstNumber

.

Convert strings to integers.

(40)

showMessageDialog

7. System.exit

Program Output

36 }

(41)

2.4 Memory Concepts

• Variables

– Every variable has a name, a type, a size and a value

• Name corresponds to

location

in memory

– When new value is placed into a variable, replaces (and

destroys) previous value

– Reading variables from memory does not change them

• Visual representation

(42)

• Arithmetic calculations used in most programs

– Usage

• _*

for multiplication

• _/

for division

• ₊

,

-• No operator for exponentiation (more in Chapter 5)

– Integer division truncates remainder

7 / 5

evaluates to

₁

(43)

2.5 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}

(44)

Operator(s)

Operation(s)

Order of evaluation (precedence)

()

Parentheses

Evaluated first. If the parentheses

are nested, the expression in the

innermost pair is evaluated first. If

there are several pairs of parentheses

“on the same level” (i.e., not

nested), they are evaluated left to

right.

*, /, or %

Multiplication

Division

Modulus

Evaluated second. If there are

several, they are

evaluated left to right.

+ or -

Addition

(45)

2.6 Decision Making: Equality and

Relational Operators

• _if

control structure

– Simple version in this section, more detail later

– If a condition is true, then the body of the

if

statement

executed

• ₀

interpreted as false, non-zero is true

– Control always resumes after the

if

structure

– Conditions for

_if

structures can be formed using equality or

relational operators (next slide)

if ( condition )

statement executed if condition

true

(46)

• Upcoming program uses

_if

structures

– Discussion afterwards

operator or

relational operator

or relational

operator

of Java

condition

Java condition

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 operators

=

==

x == y

x

is equal to

y

!=

x != y

x

is not equal to

y

<_ >_

(47)

Outline

1 // Fig. 2.17: Comparison.java

2 // Using if statements, relational operators

3 // and equality operators

4

5 import javax.swing.JOptionPane;

6

7 public class Comparison {

9 {

11 secondNumber, // second string entered by user

12 result; // a string containing the output

13 int number1, // first number to compare

14 number2; // second number to compare

15

16 // read first number from user as a string

18 JOptionPane.showInputDialog( "Enter first integer:" );

19

20 // read second number from user as a string

22 JOptionPane.showInputDialog( "Enter second integer:" );

23

27

(48)

36

37 if ( number1 < number2 )

38 result = result + "\n" + number1 + " < " + number2;

39

40 if ( number1 > number2 )

41 result = result + "\n" + number1 + " > " + number2;

42

43 if ( number1 <= number2 )

44 result = result + "\n" + number1 + " <= " + number2;

45

46 if ( number1 >= number2 )

47 result = result + "\n" + number1 + " >= " + number2;

48

49 // Display results

51 null, result, "Comparison Results",

52 JOptionPane.INFORMATION_MESSAGE );

53

54 System.exit( 0 );

55 }

(49)

Outline

(50)

– Lines 1-9: Comments,

import

JOptionPane,

begin

class

Comparison

and

main

– Declare variables

– Input data from user and assign to variables

10 String firstNumber, // first string entered by user

11 secondNumber, // second string entered by user

12 result; // a string containing the output

13 int number1, // first number to compare

14 number2; // second number to compare

17 firstNumber =

18 JOptionPane.showInputDialog( "Enter first integer:" );

21 secondNumber =

(51)

2.6 Decision Making: Equality and

Relational Operators

– Convert

String

s to

int

s and assign to variables

– Initialize

result

with empty string

–

_if

_{structure to test for equality using (}

₌₌

₎

• If variables equal (condition true)

–

_result

concatenated using

₊

operator

–

_{result = result + other strings}

– Right side evaluated first, new string assigned to

_result

• If variables not equal, statement skipped

25 number1 = Integer.parseInt( firstNumber );

26 number2 = Integer.parseInt( secondNumber );

29 result = "";

31 if ( number1 == number2 )

(52)

• Other

_if

structures

– Lines 50-52:

result

displayed in a dialog box using

showMessageDialog

34 if ( number1 != number2 )

35 result = result + number1 + " != " + number2;

36

37 if ( number1 < number2 )

38 result = result + "\n" + number1 + " < " + number2;

39

40 if ( number1 > number2 )

41 result = result + "\n" + number1 + " > " + number2;

42

43 if ( number1 <= number2 )

44 result = result + "\n" + number1 + " <= " + number2;

45

46 if ( number1 >= number2 )

47 result = result + "\n" + number1 + " >= " + number2;

(53)

Outline

1. _import

2. Class

_Comparison

2.1 _main

2.2 Declarations

2.3 Input data

(

_{showInputDialog}

)

2.4 parseInt

2.5 Initialize

_result

1 // Fig. 2.17: Comparison.java

2 // Using if statements, relational operators

3 // and equality operators

4

5 import javax.swing.JOptionPane;

6

7 public class Comparison {

9 {

11 secondNumber, // second string entered by user

12 result; // a string containing the output

13 int number1, // first number to compare

14 number2; // second number to compare

15

16 // read first number from user as a string

18 JOptionPane.showInputDialog( "Enter first integer:" );

19

20 // read second number from user as a string

22 JOptionPane.showInputDialog( "Enter second integer:" );

23

27

(54)

4. showMessageDialog

36

37 if ( number1 < number2 )

38 result = result + "\n" + number1 + " < " + number2;

39

40 if ( number1 > number2 )

41 result = result + "\n" + number1 + " > " + number2;

42

43 if ( number1 <= number2 )

44 result = result + "\n" + number1 + " <= " + number2;

45

46 if ( number1 >= number2 )

47 result = result + "\n" + number1 + " >= " + number2;

48

49 // Display results

51 null, result, "Comparison Results", 52

52 JOptionPane.INFORMATION_MESSAGE );

53

54 System.exit( 0 );

55 }

56 }

Notice use of

JOptionPane.INFORMATION_MESSAGE

(55)