PECS: Sum Into and Out Of Lists

Implement two methods that demonstrate PECS signatures: - static double sum(List<? extends Number> src) — producer side, returns the total as double. - static void fill(List<? super Integer> dst, int a, int b, int c) — consumer side, appends a, b, c in order. Then static List<Number> demo() returns a List<Number> produced by calling fill on it and then sum over it — proving one method reads a List<Number> and the other writes into it, exactly as the wildcards promise.

Difficulty: intermediate