Java: Variables in interfaces

Concept of variables in interfaces is allowed in Java, but the concept is different. Variables defined in interfaces are always static and final.

Check this for more details.

Java Instance variables and Class variables

aka static/non-static variables.

In object-oriented programming with classes, an instance variable is a variable defined in a class (i.e. a member variable), for which each object of the class has a separate copy. They live in memory for the life of the class.

An instance variable is the opposite of class variable, and it is a special type of instance member.

A class variable is a variable defined in a class (i.e. a member variable) of which a single copy exists, regardless of how many instances of the class exist. We use keyword static for declaring class variables.

Instance variable:
■ Can be marked final
■ Can be marked transient
■ Cannot be marked abstract
■ Cannot be marked synchronized
■ Cannot be marked strictfp
■ Cannot be marked native
■ Cannot be marked static, because then they'd become class variables.

Java default values

There is a simple rule - when you define a member variable, it gets a default value. When you define local value, it do not get any default value - it has to be inicialized before the first use, or an compile-time exception will be thrown.

Default values for member variables:

Data Type	Default Value (for fields)
byte	0
short	0
int	0
long	0L
float	0.0f
double	0.0d
char	'\u0000'
Any object	null
boolean	false