Download Chapter 3:Decision Structures

Chapter 3:Decision
Structures
Chapter 3:Decision Structures
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3.1 The if Statement
3.2 The if-else Statement
3.3 The if-else-if Statement
3.4 Nested if Statements
3.5 Logical Operators
3.6 Comparing String Objects
3.7 More About Variable Declaration and Scope
3.9 The switch Statement
3.10 Creating Objects with the DecimalFormat Class
3.12 Common Errors to Avoid
Variable Scope
 In Java, a local variable does not have to
be declared at the beginning of the
method.
 The scope of a local variable begins at the
point it is declared and terminates at the
end of the method.
 When a program enters a section of code
where a variable has scope, that variable
has come into scope, which means the
variable is visible to the program.
VariableScope.java
// Get the user's first name.
String firstName;
firstName = JOptionPane.showInputDialog(
"Enter your " + "first name.");
String lastName;
lastName = JOptionPane.showInputDialog(
"Enter your " + "last name.");
JOptionPane.showMessageDialog(null, "Hello " +
firstName + " " + lastName);
The Conditional Operator
 The conditional operator is a ternary
(three operand) operator.
 The conditional operator allows a
programmer to write a simple if-else
type statement.
 The format of the operators is:
expression1 ? expression2 : expression3
 The conditional operator can also
return a value.
The Conditional Operator
 The conditional operator can be used
as a shortened if-else statement:
x > y ? z = 10 : z = 5;
 This line is functionally equivalent to:
if(x > )
z = 10;
else
z = 5;
The Conditional Operator
 Many times, the conditional operator
is used to supply a value.
number = x > y ? 10 : 5;
 This is functionally equivalent to:
if(x > y)
number = 10;
else
number = 5;
ConsultantCharges.java
double hours,
// To hold the hours worked
charges; // To hold the charges
String input;
// To hold user input
input = JOptionPane.showInputDialog(
"How many hours were worked? ");
hours = Double.parseDouble(input);
hours = hours < 5 ? 5 : hours;
charges = 50.0 * hours;
JOptionPane.showMessageDialog(null,
"The charges are $" + charges);
The switch Statement
 The if-else statements allow the
programmer to make true / false branches.
 The switch statement allows the
programmer to use an ordinal value to
determine how a program will branch.
 The switch statement can evaluate an
integer type or character type variable and
make decisions based on the value.
The switch Statement
 The switch statement takes the
form:
switch (SwitchExpression)
{
case CaseExpression:
// place one or more statements here
break;
case CaseExpression:
// place one or more statements here
break;
default:
// place one or more statements here
}
The switch Statement
 The switch statement takes an ordinal
value (byte, short, int, long, char) as the
SwitchExpression.
switch (SwitchExpression)
{
…
}
 The switch statement will evaluate the
expression.
 If there is an associated case statement
that matches that value, program execution
will be transferred to that case statement.
The switch Statement
 Each case statement will have a
corresponding CaseExpression that
must be unique.
case CaseExpression:
// place one or more statements here
break;
 If the SwitchExpression matches the
CaseExpression, the Java statements
between the colon and the break
statement will be executed.
The switch Case
 The break statement ends the case
statement.
 The break statement is optional.
 If a case does not contain a break,
then program execution continues
into the next case.
NoBreaks.java
switch (number)
{
case 1:
System.out.println("You entered 1.");
case 2:
System.out.println("You entered 2.");
case 3:
System.out.println("You entered 3.");
default:
System.out.println("That's not 1, 2, or 3!");
}
PetFood.java
switch(foodGrade)
{
case 'a': case 'A':
System.out.println("30 cents per lb.");
break;
case 'b': case 'B':
System.out.println("20 cents per lb.");
break;
case 'c': case 'C':
System.out.println("15 cents per lb.");
break;
default:
System.out.println("Invalid choice.");
}
The switch Case
 The default case is optional and will
be executed if no CaseExpression
matches the SwitchExpression.
SwitchDemo.java
// Determine the number entered.
switch (number)
{
case 1:
System.out.println("You entered 1.");
break;
case 2:
System.out.println("You entered 2.");
break;
case 3:
System.out.println("You entered 3.");
break;
default:
System.out.println("That's not 1, 2, or 3!");
}
The DecimalFormat Class
 When printing out double and float values,
the full fractional value will be printed.
 The DecimalFormat class can be used to
format these values.
 In order to use the DecimalFormat class, the
Java import statement must be used.
 At the top of the program the statement
import java.text.DecimalFormat; must be
used.
Format1.java
double number1 = 0.166666666666667;
double number2 = 1.666666666666667;
double number3 = 16.666666666666667;
double number4 = 166.666666666666667;
DecimalFormat formatter = new
DecimalFormat("#0.00");
// Display the formatted variable contents.
System.out.println(formatter.format(number1));
System.out.println(formatter.format(number2));
System.out.println(formatter.format(number3));
System.out.println(formatter.format(number4));
Format2.java
double number1 = 0.166666666666667;
double number2 = 1.666666666666667;
double number3 = 16.666666666666667;
double number4 = 166.666666666666667;
// Create a DecimalFormat object.
DecimalFormat formatter = new
DecimalFormat("000.00");
// Display the formatted variable contents.
System.out.println(formatter.format(number1));
System.out.println(formatter.format(number2));
System.out.println(formatter.format(number3));
System.out.println(formatter.format(number4));
Format3.java
double number1 = 123.899;
double number2 = 1233.899;
double number3 = 12345.899;
double number4 = 123456.899;
double number5 = 1234567.899;
DecimalFormat formatter = new
DecimalFormat("#,##0.00");
System.out.println(formatter.format(number1));
System.out.println(formatter.format(number2));
System.out.println(formatter.format(number3));
System.out.println(formatter.format(number4));
System.out.println(formatter.format(number5));
Format4.java
double number1 = 0.12;
double number2 = 0.05;
DecimalFormat formatter = new DecimalFormat("#0%");
// Display the formatted variable contents.
System.out.println(formatter.format(number1));
System.out.println(formatter.format(number2));