An interface can't have behavior or state because it is intended to specify only an interaction contract, no implementation details. 'No behavior' is enforced by not allowing method/constructor bodies or static/instance initializing blocks. 'No state' is enforced by only allowing static final fields. Therefore, the class can have a state (static state), but the instance state is not inferred by the interface.
BTW : A constant in Java is defined by a static final field (and by convention the name uses UPPERCASEAND_UNDERSCORES).