add a comment |. up vote 0 down vote. In Java, interface doesn't allow you to declare any instance variables. Using a variable declared in an interface as an instance variable will return a compile time error. You can declare a constant variable, using static final which is different from an instance variable.dd a comment |. up vote 0 down vote. In Java, interface doesn't allow you to declare any instance variables. Using a variable declared in an interface as an instance variable will return a compile time error. You can declare a constant variable, using static final which is different from an instance variable.