Method Reference. Cannot make a static reference to the non-static method

As peter-walser pointed out, since MyCass::mymethod is an instance method it requires an instance to be converted to a Function instance.

The static in front of your interface declaration just makes it a static interface, it does not turn each method into a static one.

A possible solution would be to declare the method inside the class as static:

class MyCass{
   static boolean mymethod(String input){
       return true;
   }
}

To understand better how it works, you can consider the code equivalente to the method reference MyCass::mymethod that is (assuming the above modified declaration of MyClass):

new FunctionalInterface{
  boolean function(String file){
    return MyClass.mymethod(file);
  }
}

Your original code would attempt to sort-of translate into:

new FunctionalInterface{
  boolean function(String file){
    return _missing_object_.mymethod(); # mymethod is not static
  }
}

Another possibility is using a BiFunction instead of your FunctionalInterface. In that case the first argument of apply would be the object and the second would be the argument to mymethod.

Method references to non-static methods require an instance to operate on.

In the case of the listFiles method, the argument is a FileFilter with accept(File file). As you operate on an instance (the argument), you can refer to its instance methods:

listFiles(File::isHidden)

which is shorthand for

listFiles(f -> f.isHidden())

Now why can't you use test(MyCass::mymethod)? Because you simply don't have an instance of MyCass to operate on.

You can however create an instance, and then pass a method reference to your instance method:

MyCass myCass = new MyCass(); // the instance
test(myCass::mymethod); // pass a non-static method reference

or

test(new MyCass()::mymethod);

Edit: MyCass would need to be declared static (static class MyCass) in order to be accessible from the main method.