Should 'Comparable' be a 'Functional interface'?

Question

The definition of a functional interface is \"A functional interface is an interface that has just one abstract method (aside from the methods of Object ), and thus represen

Answer 1

The issue comes from a subtle difference between a "method" and a "function".

The output value of a function depends ONLY on the arguments that are input to that function.

However the output of a method depends on the arguments that are input to the function but it may also depend on the object's state (instance variables).

That is, any function is a method but not all methods are functions.

For example, the method compare in the interface Comparator depends only on its arguments. However, the method compareTo in the interface Comparable depends on the state of the object to compare to, so it needs to be implemented in a class.

So even Comparable has one abstarct method, semantically it shouldn't be considered as a functional interface.