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Learn about control structures
 Examine relational and logical operators
 Explore how to form and evaluate
logical (Boolean) expressions
 Learn how to use the selection control
structures if, if…else, and switch in a
program

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
Three methods of processing a program
› In sequence
› Branching
› Looping
Branch: altering the flow of program
execution by making a selection or
choice
 Loop: altering the flow of program
execution by repetition of statement(s)

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
Relational Operator
› Allows you to make comparisons in a
program
› Binary operator
Condition is represented by a logical
expression in Java
 Logical expression: expression that has a
value of either true or false

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Can be used with integral and floatingpoint data types
 Can be used with the char data type
 Unicode Collating Sequence

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
class String
› Method compareTo
› Method equals

Given string str1 and str2
an integer  0 if string str1  str2

str1.compareTo(str2) 0 if string str1 is equal to string str2
an integer  0 if string str1  str2

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String
String
String
String
String
str1
str2
str3
str4
str5
=
=
=
=
=
"Hello";
"Hi";
"Air";
"Bill";
"Bigger";
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
Definition: a process in which the
computer evaluates a logical expression
from left to right and stops as soon as the
value of the expression is known
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One-Way Selection
 Two-Way Selection
 Compound (Block of) Statements
 Multiple Selections (Nested if)
 Conditional Operator
 switch Structures

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
Syntax:
if (expression)
statement
Expression referred to as decision maker
 Statement referred to as action
statement

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Example 4-11
//Program to determine the absolute value of an integer
import javax.swing.JOptionPane;
public class AbsoluteValue
{
public static void main(String[] args)
{
int number;
int temp;
String numString;
numString =
JOptionPane.showInputDialog
("Enter an integer:");
//Line 1
number = Integer.parseInt(numString); //Line 2
temp = number;
//Line 3
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if (number < 0)
number = -number;
//Line 4
//Line 5
JOptionPane.showMessageDialog(null,
"The absolute value of " + temp
+ " is " + number,
"Absolute Value",
JOptionPane.INFORMATION_MESSAGE);
//Line 6
System.exit(0);
}
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Syntax:
if (expression)
statement1
else
statement2
 else statement must be paired with an
if

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Example 4-14
if (hours > 40.0)
wages = 40.0 * rate +
1.5 * rate * (hours - 40.0);
else
wages = hours * rate;
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Example 4-15
if (hours > 40.0);
//Line 1
wages = 40.0 * rate +
1.5 * rate * (hours - 40.0); //Line 2
else
//Line 3
wages = hours * rate;
//Line 4
•Because a semicolon follows the closing parenthesis of the if
statement (Line 1), the else statement stands alone
•The semicolon at the end of the if statement (see Line 1) ends the
if statement, so the statement at Line 2 separates the else clause
from the if statement; that is, else is by itself
•Since there is no separate else statement in Java, this code
generates a syntax error
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
Syntax
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if (age > 18)
{
System.out.println("Eligible to vote.");
System.out.println("No longer a minor.");
}
else
{
System.out.println("Not eligible to vote.");
System.out.println("Still a minor.");
}
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 Syntax
if (expression1)
statement1
else if
(expression2)
statement2
else
statement3
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 Else
associated
with most recent
incomplete if
 Multiple if
statements can be
used in place of
if…else statements
 May take longer to
evaluate
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Ternary operator
 Syntax

expression1 ?

expression2
:
expression3
If expression1 = true, then the result of the
condition is expression 2; otherwise, the
result of the condition is expression3
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• In Java, switch, case, break, and
default are reserved words
• In a switch structure, the expression is
evaluated first
• The value of the expression is then used to
perform the actions specified in the statements
that follow the reserved word case
• The expression is usually an identifier
• The value of the identifier or the expression can
be only integral, that is, an integer
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Integral
values also include values of
type char
The expression is sometimes called the
selector; its value determines which
statements are selected for execution
A particular case value must appear
only once
One or more statements may follow a
case label, so you do not need to use
braces to turn multiple statements into a
single compound statement
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The
break statement may or may not
appear after each statements1,
statements2, ..., statementsn
A switch structure may or may not have
the default label
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Example 4-24
switch (grade)
{
case 'A':
System.out.println("The grade is A.");
break;
case 'B':
System.out.println("The grade is B.");
break;
case 'C':
System.out.println("The grade is C.");
break;
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case 'D':
System.out.println("The grade is D.");
break;
case 'F':
System.out.println("The grade is F.");
break;
default:
System.out.println("The grade is invalid.");
}
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Input: customer’s account number,
customer code, number of premium
channels to which customer subscribes,
number of basic service connections (in
case of business customers)
 Output: customer’s account number
and the billing amount

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
Solution:
› Prompt user for information
› Use switch statements based on customer’s
type
› Use an if statement nested within switch
statement to determine amount due by
each customer
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
Control structures are used to process
programs
Logical expressions and order of
precedence of operators are used in
expressions
Compare strings
If statements
if…else statements
switch structures

Proper syntax for using control statements
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