Past exam of the year 1 of the computer science course of the University of Oxford / 2023 / Imperative programming / 6 / b

Permitted; returns "AB". The receiver's static type is A, which offers show(A). The argument also has static type A, so that signature is selected. All three variables refer to the same object of runtime type C; the most-derived implementation of show(A) is inherited from B. Its super.show(o) calls A.show(A), producing "A", and it appends "B". C overrides show(B), not this selected signature.

Permitted; returns "AB". Through a receiver of static type A, only show(A) is visible. The argument's static type C is a subtype of A, so it can be passed to that method. Dynamic dispatch selects B.show(A) on the receiver object, exactly as in part (i). The argument's more specific type cannot expose overloads absent from the receiver's static type.

Permitted; returns "AB". A receiver of static type B exposes show(A) and show(B), but the argument's static type A is not a subtype of B. Only show(A) is applicable. Its effective implementation on the C receiver is B.show(A), which concatenates the superclass result "A" and "B".

Permitted; returns "BC". Through static type B, both show(A) and show(B) accept an argument of type C; method overloading selects the more specific show(B). The receiver's runtime type is C, so dynamic dispatch invokes C.show(B). Its super.show(o) calls B.show(B), returning "B", and the override appends "C". The show(C) overload is not visible through receiver type B.

Permitted; returns "BC". The receiver's static type is C, but the argument's static type is only B. Thus show(C) is inapplicable; among show(A) and show(B), the latter is more specific. Its implementation is C.show(B), whose superclass call returns "B" before "C" is appended. The argument happens to reference a C object, but method overloading uses its static type.

Permitted; returns "C". Both receiver and argument have static type C, so the most-specific applicable overload is show(C). That method is defined in C and returns its own one-letter result without calling super. The six results, in source order, are therefore AB, AB, AB, BC, BC, C.