Why Functional Interfaces in Java 8 have one Abstract Method?

Question

As we know in Java 8, the concept of functional interfaces are introduced. A Functional Interface has one abstract method and several default or static methods

Answer 1

Functional Interface lets us call an object as if it were a function, which lets us pass verbs(functions) around our program rather than nouns(objects). Implementations of Functional Interface perform a single well-defined action, as any method should, with a name like run, execute, perform, apply, or some other generic verb.[1]

1. Functional Programming Patterns in Scala and Clojure.