### Question:
Quadruple extends Triple<R, S, T> implements Fourth<U>,
                   MappableFourth<U, Quadruple<R, S, T, ?>> { @Override public <A> Quadruple<R, S, T, A> mapFourth(UnaryFunction<U, A> function) { return quadruple(getFirst(), getSecond(), getThird(), function.call(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 shouldBeMappableUsingMapperOnFourthPosition() throws Exception { Quadruple<Integer, String, Boolean, Double> quadruple = tuple(5, "Five", true, 3.6); Mapper<Double, String> mapper = new Mapper<Double, String>() { @Override public String map(Double input) { return input.toString(); } }; Quadruple<Integer, String, Boolean, String> expected = tuple(5, "Five", true, "3.6"); Quadruple<Integer, String, Boolean, String> actual = quadruple.mapFourth(mapper); assertThat(actual, is(expected)); }
@Test(expected = NullPointerException.class) public void shouldThrowNullPointerExceptionIfNullMapperPassedToMapFourth() throws Exception { Quadruple<Integer, String, Boolean, Double> quadruple = tuple(5, "Five", true, 3.6); Mapper<Double, String> mapper = null; quadruple.mapFourth(mapper); }
@Test public void shouldBeMappableUsingUnaryFunctionOnFourthPosition() throws Exception { Quadruple<Integer, String, Boolean, Double> quadruple = tuple(5, "Five", true, 3.6); UnaryFunction<Double, String> function = new UnaryFunction<Double, String>() { @Override public String call(Double input) { return input.toString(); } }; Quadruple<Integer, String, Boolean, String> expected = tuple(5, "Five", true, "3.6"); Quadruple<Integer, String, Boolean, String> actual = quadruple.mapFourth(function); assertThat(actual, is(expected)); }
@Test(expected = NullPointerException.class) public void shouldThrowNullPointerExceptionIfNullUnaryFunctionPassedToMapFourth() throws Exception { Quadruple<Integer, String, Boolean, Double> quadruple = tuple(5, "Five", true, 3.6); UnaryFunction<Double, String> function = null; quadruple.mapFourth(function); }