Java Names and
Variables
GEEN163
Introduction to
Computer Programming
“What’s in a name? That which we
call a rose by any other name
would smell as sweet.”
William Shakespeare
Clickers
Clicking will be required on
Monday, August 22
Instead of using a clicker, you can
purchase the Android or iPhone app
Textbook
“Java Illuminated”, 4th edition,
by Anderson and Franceschi,
ISBN 9781284045314
The 3rd edition is compatible
Available at the Bookstore or online
Additional students interested in selling their textbook
are listed in the announcement on BlackBoard
MyCodeLab
• Do 25 out of the 68 possible questions in
sections 2.1 and 2.2 of MyCodeLab on the
jblearning.turingscraft.com website
• You will earn 4 points for each correct answer
up to a maximum of 100 points
• You can retry incorrect answers
• Due by midnight on Thursday, August 25
Java Source Code
• What you type is called the source code of
your program
• The source code is not directly run by the
computer
• The source code must be compiled into an
executable form before the computer can
execute it
Traditional Java Programs
Source Code
Compiler
Bytecode
Libraries
Bytecodes
Java Virtual
Machine
Errors
• When programming you will make mistakes
• There are three types of programming errors
– Compile errors – When you compile your
program, the compiler might detect an error (i.e.
missing semicolon)
– Run time errors – An error can occur when your
program is running (i.e. division by zero)
– Logic errors – Your program might not produce
the correct results
Software Development Process
• Requirements – high level description of what
the system is supposed to do
• Specifications – detailed description
• Design – how will the system do this
• Coding – writing the program
• Integration – putting the pieces together
• Testing – make sure it works correctly
• Maintenance – updates and corrections
Class Programs
•
•
•
•
•
Requirements – faculty will define briefly
Specifications – faculty will define
Design – student responsibility
Coding – student responsibility
Testing – informal student responsibility
Programming Assignments
• Programming assignments will define a
program you are to create
• The assignment will explicitly define what the
program is supposed to do
• Sample data and the expected results for that
data is often provided
• Your program should work with any data, not
just the sample data
Example Assignment
• Write a program to input two numbers and
display the average of the numbers
• Sample input
2 6
• Sample output
Average is 4.0
General Algorithm
•
•
•
•
•
•
Tell the user to enter the first number
Read a number into num1
Tell the user to enter the second number
Read a number into num2
average is (num1 + num2) / 2.0
Display the result
Java Program
/* Simple program to average two numbers.
written by Ken Williams */
public class Average {
public static void main(String[] unused) {
double first, second;
// input numbers
double avg;
// result average
java.util.Scanner keyboard = new
java.util.Scanner(System.in);
System.out.print("Enter a number ");
first = keyboard.nextDouble();
System.out.print("Enter another number ");
second = keyboard.nextDouble();
avg = (first + second) / 2.0;
System.out.println("Average is " + avg);
}
}
Steps in the Right Order
• In computer programming, you must write the
steps in the correct order
• The program is executed sequentially
• You must read or calculate a data value before
you use it
• You cannot display the answer until you
calculate it
Put the steps in order to bake a cake
1.
2.
3.
4.
5.
6.
7.
8.
Bake for 30 minutes
Mix the ingredients in the mixing bowl
Pour batter from mixing bowl to cooking pan
Preheat oven
Put all ingredients to the mixing bowl
Put the cooking pan in the oven
Remove the cooked cake from the pan
Remove the cooking pan from the oven
Possible Solution
4.
5.
2.
3.
6.
1.
8.
7.
Preheat oven
Put all ingredients to the mixing bowl
Mix the ingredients in the mixing bowl
Pour batter from mixing bowl to cooking pan
Put the cooking pan in the oven
Bake for 30 minutes
Remove the cooking pan from the oven
Remove the cooked cake from the pan
Writing and Running a Program
Writing a program
Done first
Written by you
Left alone once
complete
Data values are
unknown
Running a program
Run after it is written
Run by anyone
May be run many times
May use any data
values
Structure of a Java Program
import package.class;
public class ProgramName {
public static void main(String[] rabbit) {
// your simple program here
}
}
Comments
• Programs are written for both computers and
humans to read
• Comments are notes for humans that the
compiler ignores
• There are two formats for comments
// the rest of the line is a comment
/* everything is ignored
until */
• Javadoc comments start with /** more to come */
Programming Requirement
All programs in GEEN163 MUST start with a
comment that gives:
• Your name
• Brief description of the program
• There is a 15 point penalty for no comments
• Exams do not require comments
import
• If you use existing objects in your program, you
need to tell Java where to find them
• import tells Java to look in this package for
classes you are using
• The import statement allows you to use the
short name of a class
• import is not required. You can always use the
full name of an object.
import example
import
javax.swing.JOptionPane;
… other parts of the program ...
JOptionPane.showMessageDialog(…);
or without import
javax.swing.JOptionPane.showMessageDialog(…);
Class statement
•
•
•
•
Java is an Object-Oriented language
Objects are defined by classes
All Java programs are an object
Much more on objects to come later
main method
• Methods are functions or programs that can
be executed
• The method with the name “main” is always
executed first
• main has an array of String parameters that
can be used to pass command line arguments
– We will not be using these parameters
Names
• Names or identifiers in Java (such as program
names, class names, variable names, etc.) can
be as long as you like
• Names can contain letters, numbers
underscores (_), and dollar signs ($)
• Spaces are not allowed in a name
• Names cannot start with a number
• Java is case sensitive, upper and lower case
letters are different
Java Reserved Words
abstract
assert
boolean
break
byte
case
catch
char
class
const
continue
default
do
double
else
enum
extends
final
finally
float
for
goto
if
implements
import
instanceof
int
interface
long
native
new
package
private
protected
public
return
short
static
strictfp
super
switch
synchronized
this
throw
throws
transient
try
void
volatile
while
• Reserved words cannot be used for
program names
Meaningful Names
• Names should make sense to humans
• Variable names such as:
i
X
v027
v028
• are not very meaningful
• The variable name should describe the data
balance
numWidgets
xCoordinate
Why Animals?
Java Conventions
While the Java language allows you to use upper
and lower case letters as you please, tradition
dictates you follow some rules:
• Variables and method names start with a lower
case letter
• Class names start with an Upper Case letter
• Constants are all UPPER CASE
• When you combine two English words, upper
case the first letter of the second word (i.e.
firstPlace, mySchool, whoCares)
Which of these are valid names?
a) dog
e) DOG
b) m/h
f) 25or6to4
c) main
g) 1stTime
d) double h) first name
i) a_1
j) studentNumber
k) Main
l) ______
Which of these are valid names?
a) dog
e) DOG
b) m/h
f) 25or6to4
c) main
g) 1stTime
d) double h) first name
i) a_1
j) studentNumber
k) Main
l) ______
Variables
• Variables hold data. They represent a memory
location.
• Variables have a:
– Name: so you can identify them in Java
– Type: the format of the data
– Value: assigned at run time and often changes
• You must set a variable's value before you can
use it
Memory Locations
owner
sum
price
“Fred”
47
665.95
• When you create a variable in Java,
it reserves memory to hold the data
Simple Data Types
• int – integer whole numbers
• double – numbers with decimal points
• char – single characters
• boolean – true or false
Different sized data
• float – a decimal with 7 digit accuracy
• double – a decimal with 16 digit accuracy
•
•
•
•
byte – a 1 byte int for numbers < 128
short – a 2 byte int for number < 16384
int – a 4 byte int for numbers < 2 billion
long – a 8 byte int for numbers < 9 x1018
A Not-So-Simple Data Type
• String – A string of characters
• A String can contain any character on the
keyboard (and more)
• String is a Java class. (Note the first letter is
capitalized.) We will talk more about classes and
complex data types later.
Java is Strongly Typed
• In Java you must specify the type of the data
to be stored in a variable
• In some other programming languages, you
can put any kind of data in any variable
• Java carefully restricts the type of data that
can be stored in a variable
• Strong typing helps prevent subtle errors
Constants
•
•
•
•
•
int
1 2 3 -6 123456
double -1.234 0.000123 1.23e-4
String "inside double quotes"
boolean true false
char
'x' // a single character in single quotes
Declaring Variables
• All variables must be declared before they are
used in the program
• A variable is declared by writing the data type
followed by the variable name
• More than one variable may be declared on
the same line as the same type by separating
the variable names by commas
Example Declarations
double
int
String
boolean
int
interestRate;
numPenguins;
myName;
doit;
first, second;
Assigning Values
• You can set a variable to a value during
execution by putting the name of variable, an
equals sign followed by a value and semicolon
numPenguins = 6;
first = 3;
interestRate = 4.75;
• The type of the variable and the value must
match.
Compile Time vs Run Time
• You can set a variable to a value, such as
dog = 47 + 5;
// set to an equation
or
dog = keyboard.nextInt(); // read from keyboard
• When you write a program, you do not know
what values the user of the program will enter
Not All Are Equal
• The equals sign is used to set a variable to a
value. It does not mean equal in the
mathematical sense.
int cat, dog;
cat = 3;
dog = 5;
cat = dog;
// cat has the value 5
• An old computer language used the arrow character
to indicate assignment.
cat ← 3;
dog ← 5;
cat ← dog ; // not Java
Sequential Execution
• Java programs are executed sequentially one
line at a time
int
cat, dog;
dog = 5;
cat = dog;
// cat has the value 5
dog = 7;
• cat still has the value 5 while dog now
has the value 7
Moving Data
cat
dog
?
?
• When you first create a variable, it
has an undefined value
Moving Data
cat
dog
?
5
dog = 5;
5
Moving Data
cat
dog
5
5
cat = dog;
Moving Data
cat
dog
5
7
dog = 7;
7
Variable Initialization
• When you declare a variable, you can give it
an initial value.
• If you don’t give a variable an initial value, it
will have some unknown random value.
• In the declaration after the variable name, put
an equals sign followed by the value.
• The type of the variable and the value must
match.
Example Initializations
double
int
String
boolean
int
int
interestRate = 0.075;
numPenguins = 7;
myName = "Ken";
doit = false;
first = 1, second = 2;
bad = "This is wrong";
Which are valid statements?
double
int
String
double
boolean
char
String
cow = 47.5;
goat = 14.2;
right = "wrong";
bull = 2.94e14;
bird = "owl";
pony = 'x';
horse = 'x';
Which are valid statements?
double
int
String
double
boolean
char
String
cow = 47.5;
goat = 14.2;
no decimal point
right = "wrong";
bull = 2.94e14;
bird = "owl";
only true or false
pony = 'x';
horse = 'x';
use double quotes
Clickers
Clicking will be required on
Monday, August 22
Instead of using a clicker, you can
purchase the Android or iPhone app
Textbook
“Java Illuminated”, 4th edition,
by Anderson and Franceschi,
ISBN 9781284045314
The 3rd edition is compatible
Available at the Bookstore or online
Additional students interested in selling their textbook
are listed in the announcement on BlackBoard
MyCodeLab
• Do 25 out of the 68 possible questions in
sections 2.1 and 2.2 of MyCodeLab on the
jblearning.turingscraft.com website
• You will earn 4 points for each correct answer
up to a maximum of 100 points
• You can retry incorrect answers
• Due by midnight on Thursday, August 25