A variable is any data that is stored in memory and has an identifier or name. The data is either an object or a primitive data type such as int and boolean.
Java is a strict typing language which means that all variables must be a data type. That data type can be a primitive data type or a class.
All variables are declared inside class data types. In other words, there are no global variables allowed in Java. The closest thing in Java to a global variable is a static data member defined in a class. Such a variable can be referenced as:
<class type>.<variable identifier>
Every variable must be declared before it can be used. Every variable must be defined before it can be used. The first step is the is to declare the variable. This step tells the compiler how to create and initialize the variable at the appropriate time.
The syntax for declaring a variable is:
[static] <access modifier> <data type> <identifier> [ = <value>]
The static keyword is optional, hence being in square brackets.
A static variable can be modified. The static keyword means that
there is only one instance of the variable shared between all the
objects of that class type.
The possible access modifiers are: public, protected, private, and default access. The access modifer determines what other code can access the variable directly.
See the lesson on Access Modifiers for more details.
Any data type can be used. The data type must be known to the compiler. This restricts the data types to primitive data types, class data types defined in the same package, and class data types imported from other packages.
Any valid identifer can be used so long as it is unique within the class declaration.
A variable can be initialize where it is being declared. This is done by using an equal sign followed by an expression. The expression is evaluated to the value. The expression can be a constant, an object, a new object, or an expression whose evaluated value matches the variable's data type.
Initializing in a variable declaration is optional except in the case of static variables. Static variables must be initialized in the declaration statement. This is because static variables are defined when a program starts running, before any object of the class type is defined.
All non-static variables are initialized when the object is defined, when it is created. These data member variables can be initialized by the value in their declaration statements or by the class construct.