Prime example: javadoc of FunctionalInterface, to be seen here
@Unihedron annotation java.lang.FunctionalInterface: An informative annotation type used to indicate that an interface type declaration is intended to be a functional interface as defined by the Java Language Specification. Conceptually, a functional interface has exactly one abstract method. Since [default...
methods](docs.oracle.com/javase/8/docs/…) have an implementation, they are not abstract. If an interface declares an abstract method overriding one of the public methods of java.lang.Object, that also does not count toward the interface's abstract method count since any implementation of the interface will have an implementation from java.lang.Object or elsewhere.
Prime example: javadoc of FunctionalInterface, to be seen here