After reading this section, check out Method References in Java 8 (Toby Weston, February 2014) for more insight into method references in bound and unbound non-static method contexts.

The non-static member class’s instance methods can call the enclosing class’s instance methods and access its instance fields. To demonstrate this access, Listing 3 declares an EnclosingClass ...

As stated in the Javadoc and User Guide documentation for @MethodSource, the referenced method must be static unless you are using @TestInstance(Lifecycle.PER_CLASS). That is also discussed in detail ...