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Introductory Concepts in Programming
1.1 Representing Data
Data representation
Computers have changed a lot from the mainframes (big irons) of the 1950 to laptops and
mobile devices, which are prevalent today. Not only have they changed in size, but advances
in technology have greatly enhanced user experience through better graphical user interface
and interaction design. One thing that has not changed though is the way data is represented
in computers.
Traditionally, computer data is represented using two states—0 and 1—in a binary (or base 2)
system. Each of these is known as a bit—a contraction of binary digit. Computers are binary
systems which use sequences of 0s and 1s to represent values. A sequence of 8 bits is known
as a byte, from where we get the following units of data representation: kilobyte, megabyte,
gigabyte, terabyte, etc. The value of each of these can be represented as a power of 2 (since
it is a binary number) as follows:
Unit
kilobyte
megabyte
gigabyte
terabyte
Number of Bytes
210 = 1024
220 (over 1 million)
230 (over 1 billion)
240 (over 1 trillion)
Symbol
KB
MB
GB
TB
The Java programming language also uses hexadecimal (base 16) and octal (base 8) numbers.
For more details on number systems, see Mathematics 1.
Types and identifiers
The Java programming language uses variables to represent values. A variable name refers to
the specific location in memory where the value is stored. This could be visualised as
follows, if we assign the value 3 to the variable Total.
int Total = 3;
Total
3
Every variable has a data type, which determines the values that could be stored in it. Data
types in Java fall into two main categories: primitive and class. Class types will be discussed
in week 6.
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Introductory Concepts in Programming
There are eight primitive (or built-in) data types in Java.




Four of these are used to represent integers—byte, short, int, long
Two are used for real (floating point) numbers—double, float
One is the character data type—char
The final one is used for representing a boolean values—boolean
A variable must declared as a specific data type before it is used.
For example, the code below prints out the number of sides in a regular polygon. The name
of the variable is sides and its data type is int. The highlighted line shows how to make a
declaration in Java; comments are included after the pair of forward slashes (//).
public class PolygonSides
{
public static void main (String[] args)
{
int sides; // declaration of the variable sides
sides = 7; // initialization of variable sides
System.out.println ("A heptagon has " + sides + " sides");
sides = 10; // sides is assiged a new value
System.out.println ("A decagon has " + sides + " sides");
sides = 12; // sides is assiged a another value
System.out.println ("A dodecagon has " + sides + " sides");
}
}
Here is what the output looks like once the code is compiled and run.
A heptagon has 7 sides
A decagon has 10 sides
A dodecagon has 12 sides
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Introductory Concepts in Programming
Variables of different types require different memory sizes. The data type byte uses one byte
(eight bits) of storage.
0
1
1
1
1
1
1
1
It is important to determine at the start what data type is needed to ensure that the values of
the variables will fit correctly at all stages of the program. The table below shows the sizes
and ranges of the primitive data types.
Type
byte
short
int
long
float
Storage size
(in bytes)
1
2
4
8
4
double
8
boolean
char
1
2
Range
-27 to 27 (-128 to 127)
-215 to 215 (-32,768 to 32,767)
-231 to 231
-263 to 263
a single-precision 32-bit IEEE 754 floating point
(-3.4x1038 to 3.4x1038 approximately)
a double-precision 64-bit IEEE 754 floating point
(-1.8x10308 to 1.8x10308 approximately)
true or false
Unicode character set (includes ASCII)
Information on when each of these data types should be used can be found here.
Final variables
The keyword final is used to ensure that the quantity of a variable cannot be changed. Java
programmers traditionally use all upper case with words separated by an underscore for
naming constants.
For example, we can assume that the income tax rates are fixed values, and so it makes sense
to define these as constants.
final double BASIC_RATE = 0.2; // initialization and declaration of the constant
BASIC_RATE
final double HIGHER_RATE = 0.4; // initialization and declaration of the constant
HIGHER_RATE
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