Handle null mapping function in maxBy and minBy (#39)

src/main/java/com/ginsberg/gatherers4j/Gatherers4j.java

@@ -117,6 +117,7 @@ public static <INPUT> LastGatherer<INPUT> last(final int count) {
     public static <INPUT, MAPPED extends Comparable<MAPPED>> MinMaxGatherer<INPUT, MAPPED> maxBy(
             final Function<INPUT, MAPPED> function
     ) {
+        mustNotBeNull(function, "Mapping function must not be null");
         return new MinMaxGatherer<>(true, function);
     }
 
@@ -131,6 +132,7 @@ public static <INPUT, MAPPED extends Comparable<MAPPED>> MinMaxGatherer<INPUT, M
     public static <INPUT, MAPPED extends Comparable<MAPPED>> MinMaxGatherer<INPUT, MAPPED> minBy(
             final Function<INPUT, MAPPED> function
     ) {
+        mustNotBeNull(function, "Mapping function must not be null");
         return new MinMaxGatherer<>(false, function);
     }
 

src/main/java/com/ginsberg/gatherers4j/MinMaxGatherer.java

@@ -64,7 +64,6 @@ public BiConsumer<State<INPUT, MAPPED>, Downstream<? super INPUT>> finisher() {
             if (state.bestSoFar != null) {
                 downstream.push(state.bestSoFar);
             }
-            // TODO: What happens if we have no best?
         };
     }
 

src/test/java/com/ginsberg/gatherers4j/MinMaxGathererTest.java

@@ -23,6 +23,7 @@
 import java.util.stream.Stream;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
 class MinMaxGathererTest {
 
@@ -71,6 +72,13 @@ void firstValueSelectedWhenMultipleExist() {
             assertThat(output).containsExactly(new TestObject("A", 1));
         }
 
+        @Test
+        void mappingFunctionMustNotBeNull() {
+            assertThatThrownBy(() ->
+                    Stream.empty().gather(Gatherers4j.maxBy(null))
+            ).isInstanceOf(IllegalArgumentException.class);
+        }
+
         @Test
         void maxValue() {
             // Arrange
@@ -132,6 +140,13 @@ void firstValueSelectedWhenMultipleExist() {
             assertThat(output).containsExactly(new TestObject("A", 1));
         }
 
+        @Test
+        void mappingFunctionMustNotBeNull() {
+            assertThatThrownBy(() ->
+                    Stream.empty().gather(Gatherers4j.minBy(null))
+            ).isInstanceOf(IllegalArgumentException.class);
+        }
+
         @Test
         void minValue() {
             // Arrange