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Chapter 3 Decision Structures
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Fall 2007
Variable Scope
The Conditional Operator
The switch Statement
DecimalFormat Class
printf
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Variable scope
Scope: a variable is only known within the block where it is declared.
Local variable: a variable declared within a method. It is not known
outside the method, and it can only be used from the point where it
is declared to the end of the method.
Aside: Java different kinds of variables are instance, static, final, local
See VariableScope.java
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Conditional Operator
General Syntax for a Conditional Expression
expression1 ? expression2 : expression3 ;
Consider:
If sale is less than or equal to 300
Then reward points are dollar value / 25
Otherwise reward points are 50 plus (dollar value / 25)
Java:
sale <= 300 ? rewardPoints = dollars / 25 : rewardPoints = 50 + dollars / 25 ;
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Conditional Expression
Can also appear in an assignment statement
Java:
rewardPoints = sale <= 300 ? dollars / 25 : 50 + dollars/25 ;
See Checkpoint exercises on page 148
e.g. rewrite
if (x > y)
z = 1;
else
z = 20;
as a conditional expression
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switch statement – an n-way decision structure
Value of integer expression is
…
=case1
case1 actions
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=case2
=casen
case2 actions
…
casen actions
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switch
• As a UML activity diagram
[expression=case1]
Action 1
[expression=case2]
Action 2
Notice use of break
in the cases
…
[expression=casen]
Action n
SwitchDemo.java
NoBreaks.java
PetFood.java
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DecimalFormat Class
• Used to control the way floating-point numbers are formatted
Required to make the class available to the compiler:
import java.text.DecimalFormat;
Must instantiate a DecimalFormat object with a pattern:
DecimalFormat formatter = new DecimalFormat(“#0.00”);
Send this object the format() message & it returns a string
System.out.println(formatter.format(number));
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DecimalFormat Class
• Formatting patterns
# used to indicate leading zero is suppressed
0 used to indicate a digit position
. used to specify decimal point
% used to have value multiplied by 100 and obtain a percent sign
, used to obtain a digits separator
• Format1.java
• Format2.java
• Format3.java
• Format4.java
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printf
• This method uses a string with embedded format specifiers and
other arguments that replace the specifiers in the output
System.out.printf(“Hours worked is %d , pay rate is
%d , and gross is %d”, hours, ratePay, grossPay);
Examples
%d
%6d
%f
%8f
%.2f
%8.2f
%12s
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for an integer value
… in 6 positions
for a floating-point value
… in 8 positions
… with 2 decimal places
… in 8 positions with 2 decimal places
for a string in 12 positions
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