1. Tonga Institute of Higher Education
Object Variables
Tonga Institute of Higher Education

2. What is a Scope?
The scope of a variable defines what parts of a program can see the variable or method

3. Scope: Brackets Brackets can change visibility of variables
A variable is valid from when it is declared to where the braces end.
This variable is not
seen because it was
defined inside of another
pair of brackets

4. Variable Visibility and Brackets
Demonstration
Variable Visibility and Brackets

5. Scope: Global Variables vs. Local Variables
Variables defined outside of a method but inside of a class
Local Variables
Variables defined inside of brackets
They are only visible by code inside the brackets
If a variable name is already being used globally, you can still declare it locally.
When looking for a value, start local. Go global if you can’t find the variable

6. Scope with Duplicate Variable Names
Demonstration
Scope with Duplicate Variable Names

7. How to Define a Variable
public static String firstName
Access Specifier
Special Keyword
Name
Type

8. Access Specifiers Access Specifier Name public static String firstName
Special Keyword
Name
Type
Default – This is usable by:
Code in the same class
Code in classes in the same package
Public – This can be used by everything:
Code in classes in different packages
Private – This can only be used by:
Code inside the same class
Protected – This can be used by:
Note: This does not
include inheritance
visibility. Therefore, Default
and Protected look like
they are the same.

9. Customer and CustomerDriver
Demonstration
Customer and CustomerDriver

10. Special Keywords This is optional final static abstract
public static String firstName
Access Specifier
Special Keyword
Name
Type
This is optional
final
For Constants - Variables with values that will never change
Code Convention: Use all capitals and separate words with underscores
Ex: QUESTION_MESSAGE, ERROR_MESSAGE
static
Covered in next slide
abstract
Will be covered later in Object Inheritance lecture

11. Static Keyword
There is only one copy of a static variable. All instances and the class use the same value
If the variable is defined outside of a method but inside the class, it is a class variable. This variable is accessible in 2 ways:
<instance name>.<class variable>
<class name>.<class variable>
Built-in Java classes will often have static methods if an object is not required… like calculating square root.

12. Class Variables, Instance Variables and Local Variables
Global Variables
Variables defined outside of a method but inside of a class
Instance Variable
Each instance uses it’s own value
Access these with dot notation
<instance name>.<instance variable>
Class Variables
Static keyword is used
All instances and the class use the same value
Every member that a static method uses must also be static
Access these with dot notation from an instance
<instance name>.<class variable>
You can also access variables directly from the class name
<class name>.<class variable>
Local Variables
Variables defined inside of a method
They are only visible by code inside the same method

13. Demonstration
Item and Item Driver

14. Variable Type and Name Access Specifier Name
public static String firstName
Access Specifier
Special Keyword
Name
Type
The type can be a primitive or an object
Example: int, boolean, String, Integer
The name should be:
One word
The first letter is lower case.
The first letter of each internal word is capitalized.
Keep your variable names simple and descriptive.
Example:
String firstName
int phoneNumber

15. Demonstration
Item and Item Driver

16. Code Conventions Generally, make all variables private
Use accessors to access your variables
Accessors – methods used to access a class’ variables
Get method – a method used to get the value of a variable
Set method – a method used to set the value of a variable

17. Accessors: Get Methods
Used to retrieve variable values from a class
No parameter
Return value
A good place to call methods that calculate return values

18. Accessors: Set Methods
Used to set values for variables
Require a parameter
No return value
A good place to call validation methods

19. PlaneTicket and PlanetTicketDriver
Demonstration
PlaneTicket and PlanetTicketDriver