Tonga Institute of Higher Education Object Variables Tonga Institute of Higher Education

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

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

Variable Visibility and Brackets Demonstration Variable Visibility and Brackets

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

Scope with Duplicate Variable Names Demonstration Scope with Duplicate Variable Names

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

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.

Customer and CustomerDriver Demonstration Customer and CustomerDriver

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

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.

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

Demonstration Item and Item Driver

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

Demonstration Item and Item Driver

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

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

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

PlaneTicket and PlanetTicketDriver Demonstration PlaneTicket and PlanetTicketDriver