close

Вход

Забыли?

вход по аккаунту

код для вставкиСкачать
1
Chapter 2 - Introduction to Java
Applications
Outline
2.1
2.2
2.3
2.4
2.5
2.6
Introduction
A Simple Program: Printing a Line of Text
Another Java Application: Adding Integers
Memory Concepts
Arithmetic
Decision Making: Equality and Relational
Operators
 2000 Prentice Hall, Inc. All rights reserved.
2
2.1
Introduction
• In this chapter
– Introduce examples to illustrate features of Java
– Two program styles - applications and applets
 2000 Prentice Hall, Inc. All rights reserved.
3
2.2
A Simple Program: Printing a Line of
Text
• Application
– Program that executes using the java interpreter
• Sample program
– Show program, then analyze each line
 2000 Prentice Hall, Inc. All rights reserved.
4
Outline
1
// Fig. 2.1: Welcome1.java
2
// A first program in Java
Java program
3
4
public class Welcome1 {
5
public static void main( String args[] )
6
{
7
System.out.println( "Welcome to Java Programming!" );
8
9
}
}
Welcome to Java Programming!
 2000 Prentice Hall, Inc. All rights reserved.
Program Output
5
2.2
A Simple Program: Printing a Line of
Text
1 // Fig. 2.1: Welcome1.java
– // 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 */
2 // A first program in Java
– Another line of comments
– Note: line numbers not part of program, added for reference
 2000 Prentice Hall, Inc. All rights reserved.
6
2.2
A Simple Program: Printing a Line of
Text
3
– Blank line
• Makes program more readable
• Blank lines, spaces, and tabs are whitespace characters
– Ignored by compiler
4 public class Welcome1 {
– 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
 2000 Prentice Hall, Inc. All rights reserved.
7
2.2
A Simple Program: Printing a Line of
Text
4 public class Welcome1 {
– 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
 2000 Prentice Hall, Inc. All rights reserved.
8
2.2
A Simple Program: Printing a Line of
Text
4 public class Welcome1 {
– 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)
5
public static void main( String args[] )
– Part of every Java application
• Applications begin executing at main
– Parenthesis indicate main is a method
– Java applications contain one or more methods
 2000 Prentice Hall, Inc. All rights reserved.
9
2.2
5
A Simple Program: Printing a Line of
Text
public static void main( String args[] )
• 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
6
{
– Left brace begins body of method definition
• Ended by right brace
 2000 Prentice Hall, Inc. All rights reserved.
10
2.2
7
A Simple Program: Printing a Line of
Text
System.out.println( "Welcome to Java Programming!" );
– 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
• Statements must end with semicolon ;
 2000 Prentice Hall, Inc. All rights reserved.
11
2.2
A Simple Program: Printing a Line of
Text
8
}
– Ends method definition
9
}
– Ends class definition
– Can add comments to keep track of ending braces
– Lines 8 and 9 could be rewritten as:
8
9
}
}
// end of method main()
// end of class Welcome1
– Remember, compiler ignores comments
 2000 Prentice Hall, Inc. All rights reserved.
12
2.2
A Simple Program: Printing a Line of
Text
• 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
– Interpreter calls method main
 2000 Prentice Hall, Inc. All rights reserved.
13
Outline
1
// Fig. 2.1: Welcome1.java
2
// A first program in Java
Java program
3
4
public class Welcome1 {
5
public static void main( String args[] )
6
{
7
System.out.println( "Welcome to Java Programming!" );
8
9
}
}
Program Output
 2000 Prentice Hall, Inc. All rights reserved.
14
2.2
A Simple Program: Printing a Line of
Text
• Other methods
– System.out.println
• Prints argument, puts cursor on new line
– System.out.print
• Prints argument, keeps cursor on same line
 2000 Prentice Hall, Inc. All rights reserved.
15
1
// Fig. 2.3: Welcome2.java
2
// Printing a line with multiple statements
1. Comments
3
4
Outline
public class Welcome2 {
5
public static void main( String args[] )
6
{
7
System.out.print( "Welcome to " );
8
System.out.println( "Java Programming!" );
9
2. Blank line
3. Begin class
Welcome2
3.1 Method main
}
10 }
4. Method
System.out.print
System.out.print keeps the cursor on
the same line, so System.out.println
continues on the same line.
4.1 Method
System.out.println
5. end main,
Welcome2
Welcome to Java Programming!
 2000 Prentice Hall, Inc. All rights reserved.
Program Output
16
2.2
A Simple Program: Printing a Line of
Text
• 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\nProgramming!" );
 2000 Prentice Hall, Inc. All rights reserved.
17
1
// Fig. 2.4: Welcome3.java
2
// Printing multiple lines with a single statement
3
4
Class Welcome1
public class Welcome3 {
5
public static void main( String args[] )
6
{
7
System.out.println( "Welcome\nto\nJava\nProgramming!" );
8
9
Outline
}
1. main
2.
System.out.println
(uses \n for newline)
}
Welcome
to
Java
Programming!
Program Output
Notice how a new line is output for each \n
escape sequence.
 2000 Prentice Hall, Inc. All rights reserved.
18
2.2
A Simple Program: Printing a Line of
Text
• 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
• Package has classes for using Graphical User Interfaces (GUIs)
 2000 Prentice Hall, Inc. All rights reserved.
19
2.2
A Simple Program: Printing a Line of
Text
• Sample GUI
 2000 Prentice Hall, Inc. All rights reserved.
20
2.2
A Simple Program: Printing a Line of
Text
• Upcoming program
– Application that uses dialog boxes
– Explanation will come afterwards
 2000 Prentice Hall, Inc. All rights reserved.
21
Outline
1
// Fig. 2.6: Welcome4.java
2
// Printing multiple lines in a dialog box
3
import javax.swing.JOptionPane;
// import class JOptionPane
Java program using
dialog box
4
5
public class Welcome4 {
6
public static void main( String args[] )
7
{
8
JOptionPane.showMessageDialog(
9
null, "Welcome\nto\nJava\nProgramming!" );
10
11
12
System.exit( 0 );
// terminate the program
}
13 }
Program Output
 2000 Prentice Hall, Inc. All rights reserved.
22
2.2
A Simple Program: Printing a Line of
Text
– Lines 1-2: comments as before
3 import javax.swing.JOptionPane;
// import class JOptionPane
– import statements
• Locate the classes we use
• Tells compiler to load JOptionPane from javax.swing
package
4
5 public class Welcome4 {
6
public static void main( String args[] )
7
{
– Lines 4-7: Blank line, begin class Welcome4 and main
 2000 Prentice Hall, Inc. All rights reserved.
23
2.2
A Simple Program: Printing a Line of
Text
8
JOptionPane.showMessageDialog(
9
null, "Welcome\nto\nJava\nProgramming!" );
– 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
 2000 Prentice Hall, Inc. All rights reserved.
24
2.2
8
9
A Simple Program: Printing a Line of
Text
JOptionPane.showMessageDialog(
null, "Welcome\nto\nJava\nProgramming!" );
– Executing lines 8 and 9 displays the dialog box
• Automatically includes an OK button
– Hides or dismisses dialog box
• Title bar has string Message
 2000 Prentice Hall, Inc. All rights reserved.
25
2.2
11
A Simple Program: Printing a Line of
Text
System.exit( 0 );
// terminate the program
– 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
 2000 Prentice Hall, Inc. All rights reserved.
26
Outline
1
// Fig. 2.6: Welcome4.java
2
// Printing multiple lines in a dialog box
3
import javax.swing.JOptionPane;
// import class JOptionPane
1. import statement
4
5
public class Welcome4 {
6
public static void main( String args[] )
7
{
8
JOptionPane.showMessageDialog(
9
null, "Welcome\nto\nJava\nProgramming!" );
10
11
12
System.exit( 0 );
2. Class Welcome4
2.1 main
2.2
showMessageDialog
// terminate the program
}
2.3 System.exit
13 }
Program Output
 2000 Prentice Hall, Inc. All rights reserved.
27
2.3
Another Java Application: Adding
Integers
• Upcoming program
– Use input dialogs to input two values from user
– Use message dialog to display sum of the two values
 2000 Prentice Hall, Inc. All rights reserved.
28
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
Outline
// Fig. 2.8: Addition.java
// An addition program
import javax.swing.JOptionPane;
public class Addition {
public static void main(
{
String firstNumber,
secondNumber;
int number1,
number2,
sum;
// import class JOptionPane
String args[] )
//
//
//
//
//
first string entered by user
second string entered by user
first number to add
second number to add
sum of number1 and number2
// read in first number from user as a string
firstNumber =
JOptionPane.showInputDialog( "Enter first integer" );
// read in second number from user as a string
secondNumber =
JOptionPane.showInputDialog( "Enter second integer" );
// convert numbers from type String to type int
number1 = Integer.parseInt( firstNumber );
number2 = Integer.parseInt( secondNumber );
// add the numbers
sum = number1 + number2;
// display the results
 2000 Prentice Hall, Inc. All rights reserved.
Java Program using
input dialogs
29
31
32
33
34
35
36
37 }
JOptionPane.showMessageDialog(
null, "The sum is " + sum, "Results",
Outline
JOptionPane.PLAIN_MESSAGE );
System.exit( 0 );
// terminate the program
}
Program Output
 2000 Prentice Hall, Inc. All rights reserved.
30
2.3
Another Java Application: Adding
Integers
4 import javax.swing.JOptionPane;
// import class JOptionPane
– Location of JOptionPane for use in the program
6 public class Addition {
– Begins public class Addition
• Recall that file name must be Addition.java
– Lines 7-8: main
9
10
String firstNumber,
secondNumber;
// first string entered by user
// second string entered by user
– Declaration
• firstNumber and secondNumber are variables
 2000 Prentice Hall, Inc. All rights reserved.
31
2.3
9
Another Java Application: Adding
Integers
String firstNumber,
10
secondNumber;
// first string entered by user
// second string entered by user
– 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
 2000 Prentice Hall, Inc. All rights reserved.
32
2.3
Another Java Application: Adding
Integers
11
int number1,
// first number to add
12
number2,
// second number to add
13
sum;
// sum of number1 and number2
– 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
 2000 Prentice Hall, Inc. All rights reserved.
33
2.3
15
16
17
Another Java Application: Adding
Integers
// read in first number from user as a string
firstNumber =
JOptionPane.showInputDialog( "Enter first integer" );
– 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
 2000 Prentice Hall, Inc. All rights reserved.
34
2.3
15
16
17
Another Java Application: Adding
Integers
// read in first number from user as a string
firstNumber =
JOptionPane.showInputDialog( "Enter first integer" );
– 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" )
 2000 Prentice Hall, Inc. All rights reserved.
35
2.3
19
20
21
Another Java Application: Adding
Integers
// read in second number from user as a string
secondNumber =
JOptionPane.showInputDialog( "Enter second integer" );
– Similar to previous statement
• Assigns variable secondNumber to second integer input
23
24
25
// convert numbers from type String to type int
number1 = Integer.parseInt( firstNumber );
number2 = Integer.parseInt( secondNumber );
– 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
 2000 Prentice Hall, Inc. All rights reserved.
36
2.3
27
28
Another Java Application: Adding
Integers
// add the numbers
sum = number1 + number2;
– 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
 2000 Prentice Hall, Inc. All rights reserved.
37
2.3
31
Another Java Application: Adding
Integers
JOptionPane.showMessageDialog(
32
null, "The sum is " + sum, "Results",
33
JOptionPane.PLAIN_MESSAGE );
– 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
 2000 Prentice Hall, Inc. All rights reserved.
38
2.3
31
Another Java Application: Adding
Integers
JOptionPane.showMessageDialog(
32
null, "The sum is " + sum, "Results",
33
JOptionPane.PLAIN_MESSAGE );
– 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
– JOptionPane.QUESTION_MESSAGE
 2000 Prentice Hall, Inc. All rights reserved.
39
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
Outline
// Fig. 2.8: Addition.java
// An addition program
import javax.swing.JOptionPane;
public class Addition {
public static void main(
{
String firstNumber,
secondNumber;
int number1,
number2,
sum;
// import class JOptionPane
1. import
Declare variables: name and data type.
String args[] )
2. class Addition
// first string entered by user
// second string entered by user
// first number to add
// second number to add
//
sumfirst
of integer
number1asand
number2 assign
Input
a String,
2.1 Declare variables
(name and data type)
to firstNumber.
3. showInputDialog
// read in first number from user as a string
firstNumber =
JOptionPane.showInputDialog( "Enter first integer" );
// read in second number from user as a string
secondNumber =
Convert
strings
JOptionPane.showInputDialog( "Enter second
integer"
);
// convert numbers from type String to type int
number1 = Integer.parseInt( firstNumber );
Add, place
number2 = Integer.parseInt( secondNumber
); result
// add the numbers
sum = number1 + number2;
// display the results
 2000 Prentice Hall, Inc. All rights reserved.
in sum.
4. parseInt
5. Add numbers, put
to integers.
result in sum
40
31
32
33
34
35
36
37 }
JOptionPane.showMessageDialog(
null, "The sum is " + sum, "Results",
Outline
JOptionPane.PLAIN_MESSAGE );
System.exit( 0 );
// terminate the program
}
6.
showMessageDialog
7. System.exit
Program Output
 2000 Prentice Hall, Inc. All rights reserved.
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
number1
 2000 Prentice Hall, Inc. All rights reserved.
45
42
2.5
Arithmetic
• 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
– Modulus operator % returns the remainder
7 % 5 evaluates to 2
 2000 Prentice Hall, Inc. All rights reserved.
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
• Use: (a + b + c ) / 3
 2000 Prentice Hall, Inc. All rights reserved.
44
2.5
Arithmetic
Operator(s)
Operation(s)
Order of evaluation (precedence)
()
Parentheses
*, /, or %
Multiplication
Division
Modulus
Addition
Subtraction
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.
Evaluated second. If there are
several, they are
evaluated left to right.
Evaluated last. If there are several,
they are
evaluated left to right.
+ or -
 2000 Prentice Hall, Inc. All rights reserved.
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
• No semicolon needed after condition
 2000 Prentice Hall, Inc. All rights reserved.
46
2.6
Decision Making: Equality and
Relational Operators
Standard algebraic equality Java equality
operator or
or relational
relational operator
operator
Example
of Java
condition
Meaning of
Java condition
x is greater than y
x is less than y
Relational operators
>
>
x>y
<
<
x<y
_
>
>=
x >= y
_
<
<=
x <= y
x is greater than or equal to
y
x is less than or equal to y
==
!=
x == y
x != y
x is equal to y
x is not equal to y
Equality operators
=
=
• Upcoming program uses if structures
– Discussion afterwards
 2000 Prentice Hall, Inc. All rights reserved.
47
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
);
23
24
25
26
27
28
29
30
// Fig. 2.17: Comparison.java
// Using if statements, relational operators
// and equality operators
import javax.swing.JOptionPane;
public class Comparison {
public static void main(
{
String firstNumber,
secondNumber,
result;
int number1,
number2;
String args[] )
//
//
//
//
//
first string entered by user
second string entered by user
a string containing the output
first number to compare
second number to compare
// read first number from user as a string
firstNumber =
JOptionPane.showInputDialog( "Enter first integer:" );
// read second number from user as a string
secondNumber =
JOptionPane.showInputDialog( "Enter second integer:"
// convert numbers from type String to type int
number1 = Integer.parseInt( firstNumber );
number2 = Integer.parseInt( secondNumber );
// initialize result to the empty string
result = "";
 2000 Prentice Hall, Inc. All rights reserved.
Outline
48
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56 }
if ( number1 == number2 )
result = result + number1 + " == " + number2;
if ( number1 != number2 )
result = result + number1 + " != " + number2;
if ( number1 < number2 )
result = result + "\n" + number1 + " < " + number2;
if ( number1 > number2 )
result = result + "\n" + number1 + " > " + number2;
if ( number1 <= number2 )
result = result + "\n" + number1 + " <= " + number2;
if ( number1 >= number2 )
result = result + "\n" + number1 + " >= " + number2;
// Display results
JOptionPane.showMessageDialog(
null, result, "Comparison Results",
JOptionPane.INFORMATION_MESSAGE );
System.exit( 0 );
}
 2000 Prentice Hall, Inc. All rights reserved.
Outline
49
Outline
Program Output
 2000 Prentice Hall, Inc. All rights reserved.
50
2.6
Decision Making: Equality and
Relational Operators
– Lines 1-9: Comments, import JOptionPane, begin
class Comparison and main
10
11
12
13
14
String firstNumber,
secondNumber,
result;
int number1,
number2;
//
//
//
//
//
first string entered by user
second string entered by user
a string containing the output
first number to compare
second number to compare
– Declare variables
17
18
21
22
firstNumber =
JOptionPane.showInputDialog( "Enter first integer:" );
secondNumber =
JOptionPane.showInputDialog( "Enter second integer:" );
– Input data from user and assign to variables
 2000 Prentice Hall, Inc. All rights reserved.
51
2.6
25
26
Decision Making: Equality and
Relational Operators
number1 = Integer.parseInt( firstNumber );
number2 = Integer.parseInt( secondNumber );
– Convert Strings to ints and assign to variables
29
result = "";
– Initialize result with empty string
31
32
if ( number1 == number2 )
result = result + number1 + " == " + number2;
– 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
 2000 Prentice Hall, Inc. All rights reserved.
52
2.6
Decision Making: Equality and
Relational Operators
• Other if structures
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
if ( number1 != number2 )
result = result + number1 + " != " + number2;
if ( number1 < number2 )
result = result + "\n" + number1 + " < " + number2;
if ( number1 > number2 )
result = result + "\n" + number1 + " > " + number2;
if ( number1 <= number2 )
result = result + "\n" + number1 + " <= " + number2;
if ( number1 >= number2 )
result = result + "\n" + number1 + " >= " + number2;
– Lines 50-52: result displayed in a dialog box using
showMessageDialog
 2000 Prentice Hall, Inc. All rights reserved.
53
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
);
23
24
25
26
27
28
29
30
// Fig. 2.17: Comparison.java
// Using if statements, relational operators
// and equality operators
1. import
import javax.swing.JOptionPane;
public class Comparison {
public static void main(
{
String firstNumber,
secondNumber,
result;
int number1,
number2;
String args[] )
//
//
//
//
//
Outline
first string entered by user
second string entered by user
a string containing the output
first number to compare
second number to compare
2. Class Comparison
2.1 main
2.2 Declarations
// read first number from user as a string
firstNumber =
JOptionPane.showInputDialog( "Enter first integer:" );
2.3 Input data
(showInputDialog)
// read second number from user as a string
secondNumber =
JOptionPane.showInputDialog( "Enter second integer:"
2.4 parseInt
// convert numbers from type String to type int
number1 = Integer.parseInt( firstNumber );
number2 = Integer.parseInt( secondNumber );
// initialize result to the empty string
result = "";
 2000 Prentice Hall, Inc. All rights reserved.
2.5 Initialize result
54
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56 }
Outline
if ( number1 == number2 )
result = result + number1 + " == " + number2;
Test for equality, greater than, less
if ( number1 != number2 )
result = result + number1 + " != " + number2;
than, etc. Create new
string,
assign
3. if
statements
to result.
if ( number1 < number2 )
result = result + "\n" + number1 + " < " + number2;
if ( number1 > number2 )
result = result + "\n" + number1 + " > " + number2;
4.
showMessageDialog
if ( number1 <= number2 )
result = result + "\n" + number1 + " <= " + number2;
if ( number1 >= number2 )
result = result + "\n" + number1 + " >= " + number2;
// Display results
JOptionPane.showMessageDialog(
null, result, "Comparison Results",
JOptionPane.INFORMATION_MESSAGE );
System.exit( 0 );
}
Notice use of
JOptionPane.INFORMATION_MESSAGE
 2000 Prentice Hall, Inc. All rights reserved.
55
Outline
Program Output
 2000 Prentice Hall, Inc. All rights reserved.
1/--страниц
Пожаловаться на содержимое документа