### Question:
Quadruple extends Triple<R, S, T> implements Fourth<U>,
                   MappableFourth<U, Quadruple<R, S, T, ?>> { @Override public <A> Quadruple<R, A, T, U> mapSecond(UnaryFunction<S, A> function) { return quadruple(getFirst(), function.call(getSecond()), getThird(), getFourth()); } Quadruple(R first, S second, T third, U fourth); static Quadruple<R, S, T, U> quadruple(R first, S second, T third, U fourth); @Override U getFourth(); @Override Quadruple<A, S, T, U> mapFirst(UnaryFunction<R, A> function); @Override Quadruple<R, A, T, U> mapSecond(UnaryFunction<S, A> function); @Override Quadruple<R, S, A, U> mapThird(UnaryFunction<T, A> function); @Override Quadruple<R, S, T, A> mapFourth(UnaryFunction<U, A> function); @Override Iterable<Object> getValues();  }### Answer:
@Test public void shouldBeMappableUsingMapperOnSecondPosition() throws Exception { Quadruple<Integer, String, Boolean, Double> quadruple = tuple(5, "Five", true, 3.6); Mapper<String, Integer> mapper = new Mapper<String, Integer>() { @Override public Integer map(String input) { return input.length(); } }; Quadruple<Integer, Integer, Boolean, Double> expected = tuple(5, 4, true, 3.6); Quadruple<Integer, Integer, Boolean, Double> actual = quadruple.mapSecond(mapper); assertThat(actual, is(expected)); }
@Test(expected = NullPointerException.class) public void shouldThrowNullPointerExceptionIfNullMapperPassedToMapSecond() throws Exception { Quadruple<Integer, String, Boolean, Double> quadruple = tuple(5, "Five", true, 3.6); Mapper<String, Integer> mapper = null; quadruple.mapSecond(mapper); }
@Test public void shouldBeMappableUsingUnaryFunctionOnSecondPosition() throws Exception { Quadruple<Integer, String, Boolean, Double> quadruple = tuple(5, "Five", true, 3.6); UnaryFunction<String, Integer> function = new UnaryFunction<String, Integer>() { @Override public Integer call(String input) { return input.length(); } }; Quadruple<Integer, Integer, Boolean, Double> expected = tuple(5, 4, true, 3.6); Quadruple<Integer, Integer, Boolean, Double> actual = quadruple.mapSecond(function); assertThat(actual, is(expected)); }
@Test(expected = NullPointerException.class) public void shouldThrowNullPointerExceptionIfNullUnaryFunctionPassedToMapSecond() throws Exception { Quadruple<Integer, String, Boolean, Double> quadruple = tuple(5, "Five", true, 3.6); UnaryFunction<String, Integer> function = null; quadruple.mapSecond(function); }