Add $.groupBy(iterator, function, binaryOperator) method for chain.

Author: javadev

src/main/java/com/github/underscore/$.java

@@ -2274,6 +2274,11 @@ public <F> Chain<Map<F, List<T>>> groupBy(final Function<T, F> func) {
             return new Chain<Map<F, List<T>>>($.groupBy(list, func));
         }
 
+        public <F> Chain<Map<F, Optional<T>>> groupBy(final Function<T, F> func,
+            final BinaryOperator<T> binaryOperator) {
+            return new Chain<Map<F, Optional<T>>>($.groupBy(list, func, binaryOperator));
+        }
+
         public Chain<Map<Object, List<T>>> indexBy(final String property) {
             return new Chain<Map<Object, List<T>>>($.indexBy(list, property));
         }

src/test/java/com/github/underscore/CollectionsTest.java

@@ -1519,6 +1519,19 @@ public Double apply(Double a, Double b) {
             }
             });
         assertEquals("{1.0=Optional.of(1.3), 2.0=Optional.of(4.5)}", resultObj.toString());
+        final Map<Double, Optional<Double>> resultChain =
+        $.chain(asList(1.3, 2.1, 2.4)).groupBy(
+            new Function<Double, Double>() {
+            public Double apply(Double num) {
+                return Math.floor(num);
+            }
+            },
+            new BinaryOperator<Double>() {
+            public Double apply(Double a, Double b) {
+                return a + b;
+            }
+            }).item();
+        assertEquals("{1.0=Optional.of(1.3), 2.0=Optional.of(4.5)}", resultChain.toString());
     }
 
 /*