Transcript Java Programming, Second Edition Chapter Five Input and Selection

Java Programming,
Second Edition
Chapter Five
Input and Selection
In this chapter, you will:










Accept keyboard input
Use the JOptionPane class for GUI input and output
Draw flowcharts
Make decisions with the if and if…else structures
Use compound statements in an if or if…else structure
Nest if and if…else statements
Use AND and OR operators
Use the switch statement
Use the conditional and NOT operators
Understand precedence
Accepting Keyboard Input
 Run time - When the program is executing
 A program that accepts values at run time is
interactive because it exchanges communications
with the user
 Providing values during run time requires input,
usually through the keyboard
 The in object has access to a method named read()
that retrieves data from the keyboard
Accepting Keyboard Input
 An exception is an error situation
 There are many different error situations
 For example:
 Keyboard issues
 The user might enter the wrong data type
Accepting Keyboard Input
 Let the compiler handle the problem by
throwing the exception, or passing the
error, to the operating system
 Use throws Exception after the main()
method header
Accepting Keyboard Input
 Prompt- Message requesting user
input
 For example:
 The string “Please enter a character”
 You are not required to supply a prompt but
it is helpful for the user if you do and the
user will be more likely to enter an
appropriate response
Using the JOptionPane Class for
GUI Input and Output
 Swing components- The classes found in the
javax.swing package define GUI elements and
provide alternatives to the System.in.read() and
System.out.println() methods
 Swing classes are part of the Java Foundation
Classes, or JFC
 To access the Swing components import the
javax.swing package using javax.swing.*;
Using the JOptionPane Class for
GUI Input and Output
 JOptionPane - Used to create standard
dialog boxes
 Three standard dialog boxes
 InputDialog - prompts the user for text input
 MessageDialog - displays a user message
 ConfirmDialog - asks the user a question,
with buttons for Yes, No, and Cancel
responses
Input Dialog Boxes
 showInputDialog() method- Creates
an input dialog box
 Asks a question and uses a text field for
entering a response
 Two components
 The parent component
 The string component- contains a string or icon to
be displayed
Input Dialog Boxes
showInputDialog() method with four
arguments
 Parent component
 String component (prompt)
 The title to be displayed in the title bar
 A class variable describing the type of dialog
box
 For example:
 ERROR_MESSAGE
 INFORMATION_ MESSAGE
 QUESTION_MESSAGE
Message Dialog Boxes
 Message Dialog Boxes- Uses a simple
window to display information
 Created with the
showMessageDialog() method
 Parent component
 String component
Message Dialog Boxes
showMessageDialog() method with four
arguments
 Parent component
 String component
 The title to be displayed in the title bar
 A class variable describing the type of dialog box
 For example:
 ERROR_MESSAGE
 INFORMATION_ MESSAGE
 QUESTION_MESSAGE
Confirm Dialog Boxes
 showConfirmDialog() method- To
create a confirm dialog box which displays
the options Yes, No, and Cancel
Confirm Dialog Boxes
A confirm dialog box with 5 components:
 Parent component
 String component
 The title to be displayed in the title bar
 An integer that indicates which option button will
be shown
 An integer that describes the kind of dialog box
using the class variables ERROR_MESSAGE,
INFORMATION_MESSAGE, PLAIN_MESSAGE,
QUESTION_MESSAGE, or
WARNING_MESSAGE
Drawing Flowcharts
 Pseudocode- Programmers use a list of
tasks that must be accomplished to help
them plan a program’s logic
 Flowchart- The programmer writes the
steps in diagram form as a series of
shapes connected by arrows
Making Decisions with the if and
if…else Structures
 Making a decision involves choosing between
alternate courses of action based on some value
within a program
 The value the decision is based on is always
Boolean-true or false
 You can use if or if…else statements to make a
decision
if and if…else statements
 Single alternative- You only perform an action
based on one alternative
 Dual alternative- Requires two options for a
course of action
 Provides the mechanism for performing one action
when a Boolean expression evaluates as true and if it
evaluates to false a different action occurs
Using Compound Statements in an
if or if…else Structure
 To execute more than one statement that
depends on the evaluation of a Boolean
expression, use a pair of curly braces to
place the dependent statements within a
block
Nesting if and if…else
statements
 Nesting if and if…else statements- Statements
with an if inside another if
 Nested if statements are useful when two
conditions must be met before some action can
occur
Using AND and OR Operators
 AND operator- Used to determine whether
two expressions are both true
 Written as &&
 OR operator- Only one of two conditions is
true
 Written as ||
if (itemsSold > 3 && totalValue > 1000)
bonus=50;
Using the Switch Statement
 Switch statement- To test a single variable
against a series of exact integer or character
values
 The switch statement uses four keywords
 switch - starts the structure and is followed
immediately by a test expression enclosed in
parentheses
 case - is followed by one of the possible values for
the test expression and a colon
 break - optionally terminates a switch structure at the
end of each case
 default - optionally is used prior to any action that
should occur if the test variable does not match any
case
Using the Conditional and NOT
Operators
 Conditional operator- Requires three
expressions separated with a question
mark and a colon
 Is used as an abbreviated version of the
if…else structure
(testExpression) ? true Result : false Result
Using the Conditional and NOT
Operators
 NOT operator- To negate the result of any
Boolean expression
 Written as the exclamation point (!)
boolean oldEnough = (age > 25);
if (!oldEnough)
System.out.println(“Too young!”);
Understanding Precedence
 Operations have higher and lower
precedences
 The order in which you use operators
makes a difference
 You can always use parentheses to
change precedence or make your
intentions clear