If we have some method XYZ() in class SubA and another XYZ() in class SubB that can be invoked over a collection of the parent class Parent, e.g.: ArrayList stuff = ... //some code here -- //details omitted for( Parent item : stuff) item.XYZ(); //all items will be of type SubA or type SubB Circle ALL that apply The dynamic type check on the loop in the line above item.XYZ() will fail at compile time if there is no superclass XYZ() method. The Parent class will never be instantiated, so the Parent class must be an abstract class. If the Parent class can be instantiated, it needs an XYZO method of its own. For this code to compile, there must be a concrete method XYZ() in the Parent superclass to override. For polymorphism to work, a superclass XYZ() method can't be an abstract method Which one of the following would be valid code for the declaration of a Consumer? Consumer (p, q) lambdal = p.setValue(a) ; Consumer lambdal = WeirdClass-> setValue(5); 0 Consumer lambdal = w ->w.setValue (5) ; Consumer lambdal = w -> w.getValue(); Consumer lambdal = a -> foo (a)