Class variables also known as static variables are declared with the static keyword in a class, but outside a method, constructor or a block. There would only be one copy of each class variable per class, regardless of how many objects are created from it.
* Static variables are rarely used other than being declared as constants. Constants are variables that are declared as public/private, final, and static. Constant variables never change from their initial value.
* Static variables are stored in the static memory. It is rare to use static variables other than declared final and used as either public or private constants.
* Static variables are created when the program starts and destroyed when the program stops.
* Visibility is similar to instance variables. However, most static variables are declared public since they must be available for users of the class.
* Default values are same as instance variables. For numbers, the default value is 0; for Booleans, it is false; and for object references, it is null. Values can be assigned during the declaration or within the constructor. Additionally, values can be assigned in special static initializer blocks.
* Static variables can be accessed by calling with the class name ClassName.VariableName.
* A static variable cannot be serialized. When deserializing a value can be available for static variables. The value comes from the class declaration rather than from the stream.
```
public class Main {
public static Integer I;
public static void main(String args[]) {
System.out.println(1/Main.I); //java.lang.NullPointerException
}
}
```