02 Variables
1
March 13
02 Variables
CE00858

1:
Fundamental Programming Techniques
02 Variables
2
March 13
Objectives
In this session, we will:
•
look at integers and doubles
•
declare and initialise variables in Java applications
•
perform arithmetic operations on numbers
•
output values
•
discuss downloading the JDK and Netbeans
02 Variables
3
March 13
Handling data
•
most programs need to handle data
•
to solve a problem, we need to think about:
•
what data is used
•
what results are required
•
how we convert the data into the results
•
program must store data in memory and access it later
•
memory is like boxes that can contain values:
20
a
3.141
fred
02 Variables
4
March 13
Numbers
•
most programs use numbers
•
may be whole numbers or decimal point numbers
•
whole numbers:
integer
•
days in a week
•
legs on a dog
•
pages in a book
•
decimal point numbers:
double
•
distance between Stafford and Bristol in kilometres
•
average exam mark for a group of students
•
person’s reaction time in seconds
02 Variables
5
March 13
Declaring variables
•
values stored in variables may change as program runs
•
variables have:
•
a name
•
a type
•
a value
•
must declare variable before use
int count;
double average;
count
average
02 Variables
6
March 13
Variable names
•
all names are case sensitive
•
rules for variable names:
•
may be any length
•
must start with a letter _ or $
•
must not contain spaces, tabs or special characters
•
must not be keywords (main, void, public, etc)
•
conventions for variable names:
•
first letter should be lowercase
•
each new word starts with a capital
•
should be meaningful
02 Variables
7
March 13
Initialisation
•
giving a variable an initial value
•
good practice to initialise variables
•
error raised if variable doesn't have a value before it is
used
int count = 10;
double average = 2.5;
10
count
2.5
average
02 Variables
8
March 13
Assignment
•
storing data in an existing variable
•
evaluates right hand side of equals sign and
overwrites
contents of variable on left hand side
count = 46;
average = 9.75;
46
count
9.75
average
02 Variables
9
March 13
Arithmetic operators
•
Java supports the standard arithmetic operators:
•
all expressions must be given in full
Operator
Meaning
*
multiply
/
divide
%
remainder
+
add

subtract
energy = mass * speedOfLight * speedOfLight; //e = mc
2
02 Variables
10
March 13
Operator precedence
•
operators have precedence which determines the order in
which they are evaluated:
•
operators with same order of precedence are evaluated
from left to right
•
brackets can override the order of operation
Operator
Precedence
* / %
carried out before + and

+

carried out after *, / and %
average = (a + b) / 2;
02 Variables
11
March 13
Displaying variables
•
contents of variables can be output using calls to
System.out.print and System.out.println
•
can output more than one thing using concatenator, +:
System.out.println("Count is: " + count + " items");
System.out.print("Answer is: " + ans);
02 Variables
12
March 13
Analysis of calculations
•
need to consider what data:
•
type
•
value
•
where it comes from
•
input by user
•
hard

coded in program
•
calculated by program
•
what operations
•
calculations required
•
output needed
02 Variables
13
March 13
Calculation example
–
Rectangle
•
problem:
•
calculate and output area of rectangle given width of 10 and
height of 20
•
analysis
:
•
what data?
•
width: integer set to 10
•
height: integer set to 20
•
area: integer calculated in program
•
what operations are performed?
•
area = width
* height
•
output area
02 Variables
14
March 13
//calculate and output area of rectangle
public class Rectangle
{
public static void main (String[] args)
{
int width = 10;
int height = 20;
//calculate area
int area = width * height;
//output area
System.out.println ("The area is " + area);
}
}
Rectangle.java
02 Variables
15
March 13
Integers and doubles
•
double variable can store integer values:
•
integer variable cannot store double values:
double average;
int count = 7;
average = count;
double average;
int count = 7;
count = average;
integer is promoted to a double
potential loss of accuracy
fails to compile
02 Variables
16
March 13
Division
•
if both operands are
integers
and result is
integer
, integer
division takes place:
•
if both results are
integer
and result is
double
, integer division
takes place and result is promoted to double:
•
if either result is
double
, result must be
double
and real
division takes place:
double ave = (1 + 2 + 3 + 4) / 4.0;
//ave = 2.5
double ave = (1 + 2 + 3 + 4) / 4;
//ave = 2.0
int ave = (1 + 2 + 3 + 4) / 4;
//ave = 2
02 Variables
17
March 13
Java Development Kit & NetBeans
•
we will be using Java Development Kit (JDK)
•
Java SE
•
versions are available for Windows and Linux
•
NetBeans is an integrated development for developing
Java application
•
JDK and NetBeans are available from Sun
•
visit the Sun Java website:
http://java.sun.com
•
download instructions are available from the FPT web site
02 Variables
18
March 13
Summary
In this session we have:
•
looked at when we use integers and doubles
•
declared, initialised and used variables in Java applications
•
performed calculations using numbers
•
output results of calculations
•
considered downloading the JDK and Netbeans
In the next session we will:
•
look at more calculations
•
input data into programs
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