Primitive Data types

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Primitive Data types

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Title: Primitive Data types

1
Chapter 2
Primitive Types and Simple I/O

Primitive Data types
Comments
Variables, Tokens, identifiers, Literals
Strings a class
Assignment
Expressions
Java Input and Output

2
Structure

Java Program Structure
A program is made up of one or more classes
A class contains one or more methods
A method contains program statements
A Java application always executes the main
method

3
White Spaces

White Space
Spaces, blank lines, and tabs are collectively
called white space and are used to separate words
and symbols in a program
Extra white space is ignored
A valid Java program can be formatted many
different ways
Programs should be formatted to enhance
readability, using consistent indentation

4
Comments

Comments are an important part of every program.
They provide information thats useful for anyone
who will need to read the program in the future.
Typical uses of comments
To document who wrote the program, when it was
written, what changes have been made to it, and
so on.
To describe the behavior or purpose of a
particular part of the program, such as a
variable or method.
To describe how a particular task was
accomplished, which algorithms were used, or what
tricks were employed to get the program to work.

5
Types of Comments

Single-line comments
// Comment style 1
Multiline comments
/ Comment style 2 /
Doc comments
/ Comment style 3 /
Doc comments are designed to be extracted by a
special program, javadoc.

6
Problems with Multiline Comments

Forgetting to terminate a multiline comment may
cause the compiler to ignore part of a program
System.out.print("My ") / forgot to close
this comment...
System.out.print("cat ")
System.out.print("has ") / so it ends here /
System.out.println("fleas")

7
Single line comment

Many programmers prefer // comments to / /
comments, for several reasons
Ease of use
Safety
Program readability
Ability to comment out portions of a program

8
Tokens

A Java compiler groups the characters in a
program into tokens.
The compiler then puts the tokens into larger
groups (such as statements, methods, and
classes).
Tokens in the JavaRules program
public class JavaRules public static void
main (
String args ) System . out .
println (
"Java rules!" )

9
What is a program variable?

A named location to store data
a container for data
It can hold only one type of data
for example only integers, only floating point
(real) numbers, or only characters

10
Creating Variables

All program variables must be declared before
using them
A variable declaration
associates a name with a storage location in
memory and
specifies the type of data it will store
Type Variable_1, Variable_2,
For example, to create three integer variables to
store
number of baskets,
number of eggs per basket, and
total number of eggs
int numberOfBaskets, eggsPerBasket, totalEggs

11
Declaring Variables

Form of a variable declaration
The type of the variable
The name of the variable
A semicolon
Example
int i // Declares i to be an int variable
Several variables can be declared at a time
int i, j, k
Its often better to declare variables
individually.

12
Initializing Variables

A variable is given a value by using , the
assignment operator
i 0
Initializing a variable means to assign a value
to the variable for the first time.
Variables always need to be initialized before
the first time their value is used.
The Java compiler checks that variables declared
in methods are initialized prior to their first
use.

13
Assigning Initial Values to Variables

Initial values may or may not be assigned when
variables are declared
//These are not initialized when declared
//and have unknown values
int totalEggs, numberOfBaskets, eggsPerBasket
//These are initialized to 0 when declared
int totalEggs 0
int numberOfBaskets 0
int eggsPerBasket 0
Programming tip it is good programming practice
always to initialize variables.

14
Literals

A literal is a token that represents a particular
number or other value.
Examples of int literals
0 297 30303
Examples of double literals
48.0 48. 4.8e1 4.8e1 .48e2 480e-1
The only boolean literals are true and false.
char literals are enclosed within single quotes
'a' 'z' 'A' 'Z' '0' '9' '' '.' ' '

15
Using literals

Literals are often used as initializers
double x 0.0, y 1.0
boolean b true
char ch 'f'

16
Two Main Kinds of Types in Java

primitive data types
the simplest types
cannot decompose into other types
values only, no methods
Examplesint - integerdouble - floating point
(real)char - character

class types
more complex
composed of other types (primitive or class
types)
both data and methods
ExamplesString

17
Identifiers

An identifier is the name of something (e.g. a
variable, object, or method) used in a Java
program.
Syntax rules for identifiers tell what names are
allowed.
Naming conventions are not required by the
compiler but are good practice.

18
Syntax Rules for Identifiers

Identifiers
cannot be reserved words (e.g. if, for, etc.
see App. 1)
must contain only letters, digits, and the
underscore character, _.
cannot have a digit for the first character.
is allowed but has special meaning, so do not
use it.
have no official length limit (there is always a
finite limit, but it is very large and big enough
for reasonable names)
are case sensitive!
junk, JUNK, and Junk are three valid and
different identifiers, so be sure to be careful
in your typing!
Note that no spaces or dots are allowed.

19
Examples of Identifiers

numberOfStudents
number_of_students
student1
score2grade
score_to_grade

20
Naming Conventions

Always use meaningful names, e.g. finalExamScore,
instead of something like x, or even just score.
Use only letters and digits.
Capitalize interior words in multi-word names,
e.g. answerLetter.
Names of classes start with an uppercase letter.
every program in Java is a class as well as a
program.
Names of variables, objects, and methods start
with a lowercase letter.

21
Primitive Numeric Data Types

integerwhole number
examples 0, 1, -1, 497, -6902
four data types byte, short, int, long
floating-point numberincludes fractional part
examples 9.99, 3.14159, -5.63, 5.0
Note 5.0 is a floating-point number even though
the fractional part happens to be zero.
two data types float, double

22
The char Data Type

The char data type stores a single printable
character
For example
char answer y
System.out.println(answer)
prints (displays) the letter y

23
Primitive Data Types
24
Which Ones to Know for Now
Display in text is for reference for now stick
to these simple primitive types

int
just whole numbers
may be positive or negative
no decimal point
char
just a single character
uses single quotes
for examplechar letterGrade A

double
real numbers, both positive and negative
has a decimal point (fractional part)
two formats
number with decimal point, e.g. 514.061
e (or scientific, or floating-point) notation,
e.g. 5.14061 e2, which means 5.14061 x 102

25
Changing the Value of a Variable

Usually a variable is changed (assigned a
different value) somewhere in the program
May be calculated from other values
totalEggs numberOfBaskets eggsPerBasket
or read from keyboard input
totalEggs readLineInt()

26
Assignment Statements

most straightforward way to change value of a
variable
Variable Expression
answer 42
is assignment operator
evaluate expression on right-hand side of the
assignment operator
variable on the left-hand side of the assignment
operator gets expression value as new value

27
Assignment Operator

Not the same as the equals sign in algebra.
It means - Assign the value of the expression
on the right side to the variable on the left
side.
Can have the same variable on both sides of the
assignment operator
int count 10// initialize counter to ten
count count - 1// decrement counter
new value of count 10 - 1 9

28
compound assignment operators

The compound assignment operators make it easier
to modify the value of a variable.
A partial list of compound assignment operators
Combines addition and assignment
- Combines subtraction and assignment
Combines multiplication and assignment
/ Combines division and assignment
Combines remainder and assignment

29
CONT

Examples
i 2 // Same as i i 2
i - 2 // Same as i i - 2
i 2 // Same as i i 2
i / 2 // Same as i i / 2
i 2 // Same as i i 2

30
Specialized Assignment Operators

A shorthand notation for performing an operation
on and assigning a new value to a variable
General form var ltopgt expression
equivalent to var var ltopgt (expression)
ltopgt is , -, , /, or
Examples
amount 5
//amount amount 5
amount 1 interestRate
//amount amount (1 interestRate)
Note that the right side is treated as a unit
(put parentheses around the entire expression)

31
Performing Calculations

In general, the right side of an assignment can
be an expression.
A literal is an expression, and so is a variable.
More complicated expressions are built out of
operators and operands.
In the expression 5 / 9, the operands are 5 and
9, and the operator is /.
The operands in an expression can be variables,
literals, or other expressions.

32
OPERATORS

Javas arithmetic operators
Addition
- Subtraction
Multiplication
/ Division
Remainder
Examples
6 2 ? 8
6 - 2 ? 4
6 2 ? 12
6 / 2 ? 3

33
Integer Division

If the result of dividing two integers has a
fractional part, Java throws it away (we say that
it truncates the result).
Examples
1 / 2 ? 0
5 / 3 ? 1

34
double Operands

, -, , and / accept double operands
6.1 2.5 ? 8.6
6.1 - 2.5 ? 3.6
6.1 2.5 ? 15.25
6.1 / 2.5 ? 2.44
int and double operands can be mixed
6.1 2 ? 8.1
6.1 - 2 ? 4.1
6.1 2 ? 12.2
6.1 / 2 ? 3.05

35
Binary Operators

The , -, , and / operators are said to be
binary operators, because they require two
operands.
Theres one other binary arithmetic operator
(remainder).
The operator produces the remainder when the
left operand is divided by the right operand
13 3 ? 1
is normally used with integer operands.
Its a good idea to put a space before and after
each binary operator.

36
Unary operators

Java also has two unary arithmetic operators
Plus
- Minus
Unary operators require just one operand.
The unary and - operators are often used in
conjunction with literals (-3, for example).

37
Round-Off Errors

Calculations involving floating-point numbers can
sometimes produce surprising results.
If d is declared as follows, its value will be
0.09999999999999987 rather than 0.1
double d 1.2 - 1.1
Round-off errors such as this occur because some
numbers (1.2 and 1.1, for example) cant be
stored in double form with complete accuracy.

38
Operator Precedence

Whats the value of 6 2 3?
(6 2) 3, which yields 24?
6 (2 3), which yields 12?
Operator precedence resolves issues such as this.
, /, and take precedence over and -.
Examples
5 2 / 2 ? 5 (2 / 2) ? 6
8 3 - 5 ? (8 3) - 5 ? 19
6 - 1 7 ? 6 - (1 7) ? 1
9 / 4 6 ? (9 / 4) 6 ? 8
6 2 3 ? 6 (2 3) ? 8

39
Associativity

Precedence rules are of no help when it comes to
determining the value of 1 - 2 - 3.
Associativity rules come into play when
precedence rules alone arent enough.
The binary , -, , /, and operators are all
left associative
2 3 - 4 ? (2 3) - 4 ? 1
2 3 / 4 ? (2 3) / 4 ? 1

40
Parentheses in Expressions

Parentheses can be used to override normal
precedence and associativity rules.
Parentheses in the expression (6 2) 3 force
the addition to occur before the multiplication.
Its often a good idea to use parentheses even
when theyre not strictly necessary
(x x) (2 x) - 1
However, dont use too many parentheses
((x) (x)) ((2) (x)) - (1)

41
Returned Value

Expressions return values the number produced by
an expression is returned (return value).
int numberOfBaskets, eggsPerBasket, totalEggs
numberOfBaskets 5
eggsPerBasket 8
totalEggs numberOfBaskets eggsPerBasket
in the last line numberOfBaskets returns the
value 5 and eggsPerBasket returns the value 8
numberOfBaskets eggsPerBasket is an expression
that returns the integer value 40
Similarly, methods return values
readLine() is a method that returns a string read
from the keyboard

42
Assignment Compatibility

Can't put a square peg in a round hole
Can't put a double value into an int variable
Conversion for different types.
Casting converting a value from one type to
another
automatic or implicit casting
explicit casting

43
Casting changing the data type of the returned
value

Casting only changes the type of the returned
value
not the type of the variable
For example
double x
int n 5
x n
n is an integer and x is a double
the value returned by n must be converted to type
double before it is assigned to x

44
Implicit Casting

Casting is done implicitly (automatically) when a
lower type is assigned to a higher type
The data type hierarchy (from lowest to highest)
An int value will automatically be cast to a
double value.
A double value will not automatically be cast to
an int value.

45
Implicit Casting Exampleint to double

double x
int n 5
x n
the value returned by n is cast to a double, then
assigned to x
x contains 5.000 (as accurately as it can be
encoded as a floating point number)
This casting is done automatically because int is
lower than double in the data type hierarchy
The data type of the variable n is unchanged is
still an int

data type hierarchy
46
Data Types in an ExpressionMore Implicit Casting

Some expressions have a mix of data types
All values are automatically advanced (implicitly
cast) to the highest level before the calculation
For example
n and x are automatically cast to type double
before performing the multiplication and division

double a int n 2 float x 5.1 double y
1.33 a (n x)/y
47
Explicit Casting

Explicitly (manually) change the data type of the
value for a single use of the variable
Precede the variable name with the new data type
in parentheses
(ltdata typegt) variableName
For example
the value of x is converted from double to
integer before assigning the value to n

int n double x 2.0 n (int)x
48
Explicit casting is required to assign a higher
type to a lower

ILLEGAL Implicit casting to a lower data type
int n
double x 2.1
n x //illegal in java
It is illegal since x is double, n is an int,
and double is a higher data type than integer
LEGAL Explicit casting to a lower data type int
n
double x 2.1
n (int)x //legal in java
Though not necessary, use an explicit cast where
an implicit one will be done automatically.

49
Truncation When Casting a doubleto an Integer

Casting a double to integer does not round it
truncates
the fractional part is lost (thrown away)
For example
the value of n is now 2 (truncated value of x)
the cast is required
This behavior is useful for some calculations
(Case Study Vending Machine Change)

int n double x 2.99999 n (int)x
50
Characters as Integers

Characters are actually stored as integers
according to a special code
each printable character (letter, number,
punctuation mark, space, and tab) is assigned a
different integer code
the codes are different for upper and lower case
for example 97 may be the integer value for a
and 65 for A
ASCII (Appendix 3) and Unicode are common
character codes
Unicode ASCII codes plus additional ones for
languages with an alphabet other than English
Java uses Unicode

51
ASCII/Unicode
52
Casting a char to an int

Casting a char value to int produces the
ASCII/Unicode value
For example, what would the following display?
char answer y
System.out.println(answer)
System.out.println((int)answer)
Answer the letter y on one line followed by
the ASCII code for y (lower case) on the next
line
gtygt89gt

53
GOTCHA Imprecision of Floating Point Numbers

Computers store numbers using a fixed number of
bits, so not every real (floating point) number
can be encoded precisely
an infinite number of bits would be required to
precisely represent any real number
For example, if a computer can represent up to 10
decimal digits, the number 2.5 may be stored as
2.499999999 if that is the closest it can come to
2.5
Integers, on the other hand, are encoded
precisely
if the value 2 is assigned to an int variable,
its value is precisely 2
This is important in programming situations you
will see later in the course

54
More onArithmetic Operators

addition (), subtraction (-), multiplication
(), division (/)
can be performed with numbers of any integer
type, floating-point type, or combination of
types
result will be the highest type that is in the
expression
Example
result will be int if both amount and adjustment
are int
result will be float if amount is int and
adjustment is float

amount - adjustment
55
Truncation When DoingInteger Division

No truncation occurs if at least one of the
values in a division is type float or double (all
values are promoted to the highest data type).
Truncation occurs if all the values in a division
are integers.
For example
int a 4, b 5, c
double x 1.5, y
y b/x//value returned by b is cast to double
//value of y is approximately 3.33333
c b/a//all values are ints so the division
//truncates the value of c is 1!

56
Vending Machine Change
java program 2

int amount, originalAmount,
quarters, dimes, nickels, pennies
. . . // code that gets amount from user not
shown
originalAmount amount
quarters amount/25
amount amount25
dimes amount/10
amount amount10
nickels amount/5
amount amount5
pennies amount

If amount is 90 then there 90/25 will be 3, so
there are three quarters.
If amount is 90 then the remainder of 90/25 will
be 15, so 15 cents change is made up of other
coins.
57
Increment and Decrement Operators

Shorthand notation for common arithmetic
operations on variables used for counting
Some counters count up, some count down, but they
are integer variables
The counter can be incremented (or decremented)
before or after using its current value
int count
count preincrement count count count 1
before using it
count postincrement count count count 1
after using it
--count predecrement count count count -1
before using it
count-- postdecrement count count count -1
after using it

58
Increment and Decrement Operator Examples

common code
int n 3
int m 4
int result
What will be the value of m and result after each
of these executes?
(a) result n m//preincrement m
(b) result n m//postincrement m
(c) result n --m//predecrement m
(d) result n m--//postdecrement m

59
The String Class

A string is a sequence of characters
The String class is used to store strings
The String class has methods to operate on
strings
String constant one or more characters in double
quotes
Examples
char charVariable a//single quotes
String stringVariable "a"//double quotes
String sentence "Hello, world"

60
String Variables

Declare a String variable
String greeting
Assign a value to the variable
greeting "Hello!"
Use the variable as a String argument in a
method
System.out.println(greeting)
causes the string Hello! to be displayed on
the screen

61
Concatenating (Appending) Strings

Stringing together strings - the operator for
Strings
String name "Mondo"
String greeting "Hi, there!"
System.out.println(greeting name "Welcome")
causes the following to display on the screen
gtHi, there!MondoWelcome
gt
Note that you have to remember to include spaces
if you want it to look right
System.out.println(greeting " " name
" Welcome")
causes the following to display on the screen
gtHi, there! Mondo Welcome
gt

62
Escape Characters

How do you print characters that have special
meaning?
For example, how do you print the following
string?
The word "hard"
Would this do it?
System.out.println("The word "hard"")
No, it would give a compiler error - it sees the
string The word between the first set of double
quotes and is confused by what comes after
Use the backslash character, \, to escape the
special meaning of the internal double quotes
System.out.println("The word \"hard\"") //this
works

63
More Escape Characters

Use the following escape characters to include
the character listed in a quoted string
\" Double quote.
\' Single quote.
\\ Backslash.
\n New line. Go to the beginning of the next
line.
\r carriage return. Go to the beginning of the
current line.
\t Tab. White space up to the next tab stop.

64
Screen Output print and println

Sometimes you want to print part of a line and
not go to the next line when you print again
Two methods, one that goes to a new line and one
that does not
System.out.println()//ends with a new line
System.out.print()//stays on the same line
For example
System.out.print("This will all ")
System.out.println("appear on one line")
System.out.print() works similar to the
operator
System.out.println("This will all "
"appear on one line, too")

65
Keyboard Input Gotchas
Note the two variations for reading each type of
number

readLine variation
reads a whole line
asks the user to reenter if it is not the right
format
Try to use these
ExamplesreadLineInt()readLineDouble()

read variation
reads just the number
aborts the program if it is not the right format
Avoid using these for now
ExamplesreadInt()readDouble()

66
User-Friendly Input

Print a prompt so that the user knows what kind
of information is expected.
Echo the information that the user typed in so
that it can be verified.
System.out.println("Enter the number of
trolls")
int trolls readLineInt()
System.out.println(trolls " trolls")

Prints prompt
Sample output with user input in italic
Echoes user input
Enter the number of trolls 38 38 trolls
67
Named Constants

Named constantusing a name instead of a value
Example use MORTGAGE_INTEREST_RATE instead of
8.5
Advantages of using named constants
Easier to understand program because reader can
tell how the value is being used
Easier to modify program because value can be
changed in one place (the definition) instead of
being changed everywhere in the program.
Avoids mistake of changing same value used for a
different purpose

68
Defining Named Constants
public static final double PI 3.14159

publicno restrictions on where this name can be
used
staticmust be included, but explanation has to
wait
finalthe program is not allowed to change the
value
The remainder of the definition is similar to a
variable declaration and gives the type, name,
and initial value.
A declaration like this is usually at the
beginning of the file and is not inside the main
method definition.

69
Adding Constants to the FtoC Program

FtoC2.java
// Converts a Fahrenheit temperature to Celsius
public class FtoC2
public static void main(String args)
final double FREEZING_POINT 32.0
final double DEGREE_RATIO 5.0 / 9.0
double fahrenheit 98.6
double celsius
(fahrenheit - FREEZING_POINT)
DEGREE_RATIO
System.out.print("Celsius equivalent ")
System.out.println(celsius)

70
2.9 Methods

A method is a series of statements that can be
executed as a unit.
A method does nothing until it is activated, or
called.
To call a method, we write the name of the
method, followed by a pair of parentheses.
The methods arguments (if any) go inside the
parentheses.
A call of the println method
System.out.println("Java rules!")

71
Methods in the Math Class

The Math class contains a number of methods for
performing mathematical calculations.
These methods are called by writing Math.name,
where name is the name of the method.
The methods in the Math class return a value when
they have completed execution.

72
The pow and sqrt Methods

The pow method raises a number to a power
Math.pow(2.0, 3.0) ? 8.0
Math.pow(-2.0, 3.0) ? 8.0
Math.pow(2.0, -1.0) ? 0.5
The sqrt method computes the square root of a
number
Math.sqrt(2.0) ? 1.4142135623730951
Math.sqrt(4.0) ? 2.0
Both pow and sqrt return values of type double.

73
The round Method

The round method rounds a double value to the
nearest integer
Math.round(4.1) ? 4
Math.round(4.5) ? 5
Math.round(4.9) ? 5
Math.round(5.5) ? 6
Math.round(-4.1) ? 4
Math.round(-4.5) ? 4
Math.round(-4.9) ? 5
Math.round(-5.5) ? 5
round returns a long value rather than an int
value.

74
Using the Result of a Method Call

The value returned by a method can be saved in a
variable for later use
double y Math.abs(x)
Another option is to use the result returned by a
method directly, without first saving it in a
variable. For example, the statements
double y Math.abs(x)
double z Math.sqrt(y)
can be combined into a single statement
double z Math.sqrt(Math.abs(x))

75
Using the Result of a Method Call

Values returned by methods can also be used as
operands in expressions.
Example (finding the roots of a quadratic
equation)
double root1
(-b Math.sqrt(b b - 4 a c)) / (2
a)
double root2
(-b - Math.sqrt(b b - 4 a c)) / (2
a)
Because the square root of b2 4ac is used
twice, it would be more efficient to save it in a
variable
double discriminant Math.sqrt(b b - 4 a
c)
double root1 (-b discriminant) / (2 a)
double root2 (-b - discriminant) / (2 a)

76
Using the Result of a Method Call

The value returned by a method can be printed
without first being saved in a variable
System.out.println(Math.sqrt(2.0))

77
Obtaining Input from the User

The SimpleIO class (not part of standard Java)
simplifies the task of obtaining input.
The SimpleIO.prompt method is first used to
prompt the user
SimpleIO.prompt("Enter Fahrenheit temperature
")
Next, the SimpleIO.readLine method is used to
read the users input
String userInput SimpleIO.readLine()

78
Obtaining Input from the User

If the users input represents a number, it will
need to be converted to numeric form.
The Convert class (not part of standard Java)
provides a toDouble method that converts a string
to a double value
double fahrenheit Convert.toDouble(userInput)
To convert a string to an integer, the
Integer.parseInt method is used instead
int n Integer.parseInt(userInput)

79
Packages

Java allows classes to be grouped into larger
units known as packages.
The package that contains SimpleIO and Convert is
named jpb (Java Programming From the Beginning).
Java comes with a large number of standard
packages.

80
Import Declarations

Accessing the classes that belong to a package is
done by using an import declaration
import package-name .
Import declarations go at the beginning of a
program. A program that needs SimpleIO or Convert
would begin with the line
import jpb.

81
Application Programming Interfaces

The packages that come with Java belong to the
Java Application Programming Interface (API).
In general, an API consists of code that someone
else has written but that we can use in our
programs.
Typically, an API allows an application
programmer to access a lower level of software.
In particular, an API often provides access to
the capabilities of a particular operating system
or windowing system.

82
Application Programming Interfaces

The packages that come with Java belong to the
Java Application Programming Interface (API).
In general, an API consists of code that someone
else has written but that we can use in our
programs.
Typically, an API allows an application
programmer to access a lower level of software.
In particular, an API often provides access to
the capabilities of a particular operating system
or windowing system.

83
Program Converting from Fahrenheitto Celsius
(Revisited)

FtoC3.java
// Converts a Fahrenheit temperature entered by
the user to
// Celsius
import jpb.
public class FtoC3
public static void main(String args)
final double FREEZING_POINT 32.0
final double DEGREE_RATIO 5.0 / 9.0
SimpleIO.prompt("Enter Fahrenheit
temperature ")
String userInput SimpleIO.readLine()
double fahrenheit Convert.toDouble(userInput
)
double celsius
(fahrenheit - FREEZING_POINT)
DEGREE_RATIO
System.out.println("Celsius equivalent "
celsius)

84
Output

Welcome to the CSc 2310 average calculation
program.
Enter Program 1 score 20
Enter Program 2 score 19
Enter Program 3 score 15
Enter Program 4 score 18.5
Enter Program 5 score 20
Enter Program 6 score 20
Enter Program 7 score 18
Enter Program 8 score 20
Enter Quiz 1 score 9
Enter Quiz 2 score 10
Enter Quiz 3 score 5.5
Enter Quiz 4 score 8
Enter Quiz 5 score 9.5
Enter Test 1 score 78
Enter Test 2 score 92

85
Design of the CourseAverage Program

1. Print the introductory message ("Welcome to
the CSc 2310 average calculation program").
2. Prompt the user to enter eight program scores.
3. Compute the program average from the eight
scores.
4. Prompt the user to enter five quiz scores.
5. Compute the quiz average from the five scores.
6. Prompt the user to enter scores on the tests
and final exam.
7. Compute the course average from the program
average, quiz average, test scores, and final
exam score.
8. Round the course average to the nearest
integer and display it.

86
CourseAverage.java

CourseAverage.java
// Program name CourseAverage
// Author K. N. King
// Written 1998-04-05
// Modified 1999-01-27
//
// Prompts the user to enter eight program scores
(0-20), five
// quiz scores (0-10), two test scores (0-100),
and a final
// exam score (0-100). Scores may contain digits
after the
// decimal point. Input is not checked for
validity. Displays
// the course average, computed using the
following formula
//
// Programs 30
// Quizzes 10
// Test 1 15
// Test 2 15
// Final exam 30
//

87
CourseAverage.java

import jpb.
public class CourseAverage
public static void main(String args)
// Print the introductory message
System.out.println("Welcome to the CSc 2310
average "
"calculation program.\n")
// Prompt the user to enter eight program
scores
SimpleIO.prompt("Enter Program 1 score ")
String userInput SimpleIO.readLine()
double program1 Convert.toDouble(userInput)
SimpleIO.prompt("Enter Program 2 score ")
userInput SimpleIO.readLine()
double program2 Convert.toDouble(userInput)
SimpleIO.prompt("Enter Program 3 score ")
userInput SimpleIO.readLine()
double program3 Convert.toDouble(userInput)

88
Cont

SimpleIO.prompt("Enter Program 4 score ")
userInput SimpleIO.readLine()
double program4 Convert.toDouble(userInput)
SimpleIO.prompt("Enter Program 5 score ")
userInput SimpleIO.readLine()
double program5 Convert.toDouble(userInput)
SimpleIO.prompt("Enter Program 6 score ")
userInput SimpleIO.readLine()
double program6 Convert.toDouble(userInput)
SimpleIO.prompt("Enter Program 7 score ")
userInput SimpleIO.readLine()
double program7 Convert.toDouble(userInput)
SimpleIO.prompt("Enter Program 8 score ")
userInput SimpleIO.readLine()
double program8 Convert.toDouble(userInput)

89
Style Issues

Style issues raised by the CourseAverage program
Comment blocks
Blank lines
Short comments
Long lines

90
Improving the Program

The values of most variables in the CourseAverage
program are used only once.
When a variables value is used only once, the
variable can often be eliminated by substituting
the value thats assigned to it. The lines
String userInput SimpleIO.readLine()
double program1 Convert.toDouble(userInput)
could be replaced by
double program1
Convert.toDouble(SimpleIO.readLine())

91
Improving the Program

The number of variables in CourseAverage can be
reduced by keeping a running total of all
scores entered so far, rather than storing each
score in a separate variable
// Prompt the user to enter eight program
scores.
SimpleIO.prompt("Enter Program 1 score ")
String userInput SimpleIO.readLine()
double programTotal Convert.toDouble(userInput)
SimpleIO.prompt("Enter Program 2 score ")
userInput SimpleIO.readLine()
programTotal Convert.toDouble(userInput)
...
// Compute the program average from the eight
scores.
double programAverage programTotal / 8

92
2.12 Debugging

Debugging is the process of finding bugs in a
program and fixing them.
Types of errors
Compile-time errors
Run-time errors (called exceptions in Java)
Incorrect behavior

93
Fixing Compile-Time Errors

Strategies for fixing compile-time errors
Read error messages carefully. Example
Buggy.java8 Undefined variable i
System.out.println(i)
Buggy.java10 Variable j may not have been
initialized
System.out.println(j)
Pay attention to line numbers.
Fix the first error.

94
Fixing Compile-Time Errors

Dont trust the compiler (completely). The error
isnt always on the line reported by the
compiler. Also, the error reported by the
compiler may not accurately indicate the nature
of the error. Example
System.out.print("Value of i ")
System.out.println(i)
A semicolon is missing at the end of the first
statement, but the compiler reports a different
error
Buggy.java8 Invalid type expression.
System.out.print("Value of i ")
Buggy.java9 Invalid declaration.
System.out.println(i)

95
Fixing Run-Time Errors

When a run-time error occurs, a message will be
displayed on the screen. Example
Exception in thread "main"
java.lang.NumberFormatException foo
at java.lang.Integer.parseInt(Compiled
Code)
at java.lang.Integer.parseInt(Integer.java
458)
at Buggy.main(Buggy.java11)
Once we know what the nature of the error is and
where the error occurred, we can work backwards
to determine what caused the error.

96
Fixing Behavioral Errors

Errors of behavior are the hardest problems to
fix, because the problem probably lies either in
the original algorithm or in the translation of
the algorithm into a Java program.
Other than simply checking and rechecking the
algorithm and the program, there are two
approaches to locating the source of a behavioral
problem, depending on whether a debugger is
available.

97
SummaryPart 1