Why abstraction in java is called as a mathematical entity?

Abstract data classes are analogous to mathematical constructions. In Euclidean geometry, for example, the axioms would be like the methods in an abstract Java class. Since the terms in the axioms, like point and line, are undefined, they are in need of “instantiation.” The use of Cartesian geometry to define points and lines could be considered an instantiation. Of course, this breaks down a bit, since everything in mathematics is ultimately based on undefined terms.

I don’t know if Java has anything like the generic classes in .Net programming. These would be even closer to mathematical theories, since the generic classes allow you to work with types that really are undefined and that have to be instantiated.

^ what he said