modifiers in variables in an interface are public, static, final by default. But how can I do if I want it to be private so that no other classes can call this variable
Declaring a class to implement an interface is a promise that this class provides these methods, or extensions thereof. Increasing/extending access to the methods is allowed, but decreasing access is not, because then we would break our promise.
This is similar to classes and extensions of them, subclasses. A subclass has to provide all methods of the superclass with at least the visibility they were declared with.
The reason for this is the statically typed nature of Java. If a type may differ from its initial declaration in such a way that it breaks (access to) the type, then it makes the whole static-typing obsolete.
Fields (also called member variables) are different in the way that their access cannot be extended. Declaring another member variable of similar name on a subclass is exactly that, another variable. By doing this, you hide the superclass' variable, and you may have to use the keyword super to access it.