An instance variable is a variable that is related to a single instance of a class. Each time an instance of a class is created, the system creates one copy of the instance variables related to that class.
An instance variable is declared inside a class, but not within a method. A variable that is defined inside a class and a method is known as a local variable.
The opposite of an instance variable is a static variable, also referred to as a class variable. A static variable exists across instances of a class.
By default, all variables are created as instance variables.
Static Variables
A static variable, also referred to as a class variable, is a variable that exists across instances of a class.
The opposite of a static variable is an instance variable, which is a variable related to a single instance of a class. Each time an instance of a class is created, the system creates one copy of the instance variables related to that class.
By default, all variables are created as instance variables. To make a class variable, one must explicitly declare the variable as static.
Final Variables
A final variable is a variable that has been initialized to a fixed value that cannot be changed after initialization.
In older programming languages, a final variable would be called a constant.
Variable Naming Constraints
Java variable names must start with one of the following characters:
- Letter
- Underscore
- Dollar sign
After the first character, Java variable names can contain numbers.
Java Variable Naming Reserved Words
Java variable names cannot be one of the following Java reserved words:
- assert
- boolean
- break
- byte
- case
- catch
- char
- class
- const
- continue
- default
- do
- double
- else
- extends
- false
- final
- finally
- float
- for
- goto
- if
- implements
- import
- instanceof
- int
- interface
- long
- native
- new
- null
- package
- private
- protected
- public
- return
- short
- static
- strictfp
- super
- switch
- synchronized
- this
- throw
- throws
- transient
- true
- try
- void
- volatile
- while
