why can't a class make an interface method private?

All methods of an interface must be public. And if the method must remain public in a class that implemets it, then that is bad, because we might not want it to be public!
So what is the reason behind Java not allowing interface methods to become private inside a class?

Thank you.

Because the purpose of interfaces is to publicly declare a behaviour that a class has, therefore it doesn't make sense to be able to restrict the access to the method.

If you do need to do this then it's most likely that you have a design issue to resolve. Maybe splitting the interface into two (or more) parts and possibly having 1 extend the other would help. That way your class only needs to implement the interface(s) that defines the behaviour it has.

A couple times I found myself wanting to force a subclass to implement some method that really ought to be private. Is that what you're looking to do? Java won't let us, and it turns out it doesn't make much sense. You have to give implementers the freedom to implement any way they please.

Thanks everyone.

Oh, I probably read you wrong. I was thinking you meant to put a private method in an interface, but maybe you meant for the implementing class to make a public interface method private in the implementation.

The interface is often called a contract, which just means it makes promises to clients that must be kept. If I write a method that depends on an interface, I rely on the promise that all the interface methods will be available:

size() has to be there for all implementations of List. If you could write a new List implementation that made size private, and if you passed it into my method, what should happen? There's no good answer, so the compiler just doesn't let it happen. (Ok, there are some answers we only have to look at loosely typed languages to see, but Java isn't one of em.)