I'm self learning Java, and I'm stuck on a chapter about Interfaces. I simply cannot understand how they work in Java.

I believe I understand perfectly what Interface means and how they apply in everyday situations and technology. 

But when it comes to Java, code-wise and logic-wise, I'm stuck. I don't get it. How does the concept work? 

Say I have 3 objects, and 1 interface object. 2 Objects are ObjectCalculatingA, ObjectCalculatingB, ObjectMathFunctions, and ObjectInterface.

Supposedly in ObjectInterface there must be some sort of reference to the ObjectMathFunctions, so that ObjectCalculatingA and B can just access the math functions in ObjectMathFunctions without writting them all again in A and B.

Am I right?