Transcript Data

Data
Tonga Institute of Higher
Education
Variables

Programs need to remember values.
 Example:
A program that keeps track of sales
needs to remember data:

A loaf of bread was sold to Sione Latu on 14/02/04
for T$1.00.
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Customer Name: Sione Latu
Date Sold: 02/14/04
Item Sold: Loaf of Bread
Item Cost: T$1.00
Variable – Place in the computer’s
memory where information is stored.
Value Types vs. Reference Types

A variable stores a value or object.

Value Types only store values
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int only stores:
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boolean only stores:
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1
2
5
True
False
Reference Types only store references to an object

Object - A self-contained entity that contains data and procedures to
manipulate the data.
 A reference is a location in memory
 The location is the starting point of an object
Primitive Data Types
Primitives
- The basic types supported by the
programming language.
Use lowercase for primitive data types in code
Type
Size (bits)
Value Range
byte
8
-128 to 127
short
16
-32,768 to 32767
int
32
-2,147,483,648 to 2,147,483,647
long
64
-9.2 x 1015 to 9.2 x 1015
float
32
-3.4 x 1038 to 3.4 x 1038
double
64
-1.8 x 10308 to 1.8 x 10308
char
16
0 to 65,535
boolean
1
True or False
Primitive Data Type
Selection Practice

How to choose the right primitive variable type.
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
Byte, short, int and long store whole numbers.
Float and double store fractions. Double is more accurate, so if
accuracy is important, use double.
If you pick something that is too big, you’re wasting memory space.
Wasting memory space slows things down.
If you pick something that is too small, then you’re going to crash.
If you need a character, use char.
If you need a true/false value, use boolean.
What is a good data type for:
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
Someone’s age?
A customer’s identification number for a video rental store in
Nuku’alofa?
A very large number with decimals?
The price of an item?
Using Primitive Variables

2 Steps to using variables
1. Declare
the variable
2. Initialize the variable
Declaring Primitive Variables

Declare the variable – Tell the computer to reserve a space in memory
for the variable.

You need to tell the computer 2 things:
1.
2.
Name of the variable
Type of the variable (What kind of variable you have)
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Primitive types
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byte
short
int
long
float
double
char
boolean
Name
Type
Declaring Primitive Variables
Guidelines

Use a name that is easy to remember.
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Do not use x, y, z
Variable names must start with a letter, underscore or
dollar sign.
Begin variables with a lowercase character. Then, use a
capital letter for each word.
Examples


firstName
customerID
Initializing Primitive Variables

Initialize the variable – Assign an initial value to
a variable.
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Char values must be enclosed in single quotes.
String values must be enclosed in double quotes.
Boolean value should be True or False
Initial Value
Declaring and Initializing Primitive
Variables in 1 line

You can declare and initialize a variable in 1 line.
Demonstration
Declaring and Initializing
Variables
Converting Data Types
Smaller to Larger

It is possible to automatically move a value from
a smaller data type to larger data type

Example
Maximum
Short Value
Maximum Long Value
Is 9.2 x 1015
Converting Data Types
Larger to Smaller

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If a value from a larger data type is moved into a smaller
data type, the value may be too big for the new data type
This will cause an error

Using this:

Results in:

Do not do this!
Maximum
Short Value
is 32767
Converting Data Types


Force conversion of data types by casting it.
To cast, use this format:
(<Desired data type>)<Variable name>
int is bigger
than a byte
Forcing a cast from a larger data
type to a smaller data type is
not recommended!
Without this, we
would get an error

But the final value
Is -24, which is
strange!
If the Cast function isn’t able to convert the value,
you may not get the answer you expect.
Demonstration
Converting Data Types
Arithmetic Operators
Operator
Meaning
Example
+
Addition
11 + 22
-
Subtraction
22 – 11
*
Multiplication
5*6
/
Division
21 / 3
%
Modulus
12 percent
2
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
Declare and Initialize x, y
and z
Get values from x and y
 Adds x and y together
 Assigns the sum of x an y to z
String Concatenation

Addition
 You
can add strings.
Adding strings is called
concatenation.
 This
converts all non
strings into strings.
Make sure you add your
numbers before
converting them to
strings.
Demonstration
String Concatenation
Division

Tricky because result
may not be an integer. In
this case the number gets
cut off. NOT rounded!

Even having a result of
float doesn’t work.
Because Java has rules
for dealing with data
types. The result of a
division between two
integers is always an
integer. So the float just
added a .0 to it.

To get around this, we
need to divide two floats!
Demonstration
Division
Multiplication and Modulus
 Multiplication
Use
asterisk (*) instead of x
 Modulus
 The
remainder of a division
 Can be used to determine whether a number
is divisible by another number
 Can also determine if a number is even or
odd
Order of Operations

When you have a lot of operations, they are performed
in a certain order.
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Parentheses ()
Multiplication or Division operations from left to right
Addition or Subtraction operations from left to right.
Examples:
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3 + 6 + 9 / 3 = 3 + 6 + 3 = 12
(3 + 6 + 9) / 3 = 18 / 3 = 6