Improve $.groupBy(function, binaryOperator) method for chain.

Author: javadev

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

@@ -807,23 +807,11 @@ public <K, E> Map<K, List<E>> groupBy(final Function<E, K> func) {
 
     public static <K, E> Map<K, Optional<E>> groupBy(final Iterable<E> iterable, final Function<E, K> func,
         final BinaryOperator<E> binaryOperator) {
-        final Map<K, List<E>> retVal = newLinkedHashMap();
-        for (E e : iterable) {
-            final K key = func.apply(e);
-            List<E> val;
-            if (retVal.containsKey(key)) {
-                val = retVal.get(key);
-            } else {
-                val = newArrayList();
-            }
-            val.add(e);
-            retVal.put(key, val);
-        }
-        final Map<K, Optional<E>> retVal2 = newLinkedHashMap();
-        for (Map.Entry<K, List<E>> entry : retVal.entrySet()) {
-            retVal2.put(entry.getKey(), reduce(entry.getValue(), binaryOperator));
+        final Map<K, Optional<E>> retVal = newLinkedHashMap();
+        for (Map.Entry<K, List<E>> entry : groupBy(iterable, func).entrySet()) {
+            retVal.put(entry.getKey(), reduce(entry.getValue(), binaryOperator));
         }
-        return retVal2;
+        return retVal;
     }
 
     @SuppressWarnings("unchecked")

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

@@ -251,6 +251,11 @@ public <F> Chain<Map<F, List<T>>> groupBy(final Function<T, F> func) {
             return new Chain<Map<F, List<T>>>($.groupBy(value(), 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>>>($.groupBy(value(), func, binaryOperator));
+        }
+
         public Chain<Map<Object, List<T>>> indexBy(final String property) {
             return new Chain<Map<Object, List<T>>>($.indexBy(value(), property));
         }

src/test/java/com/github/underscore/lodash/LodashTest.java

@@ -642,6 +642,17 @@ public void chain() {
         $.chain(new LinkedHashMap<Integer, Integer>().entrySet()).sortBy("");
         $.chain(new Integer[] {0}).groupBy(new Function<Integer, Integer>() {
             public Integer apply(Integer value) { return value; } });
+        $.chain(new Integer[] {0}).groupBy(
+            new Function<Integer, Integer>() {
+            public Integer apply(Integer num) {
+                return num;
+            }
+            },
+            new BinaryOperator<Integer>() {
+            public Integer apply(Integer a, Integer b) {
+                return a;
+            }
+            });
         $.chain(new Integer[] {0}).indexBy("");
         $.chain(new Integer[] {0}).countBy(new Function<Integer, Integer>() {
             public Integer apply(Integer value) { return value; } });