Implement countBy on Collectors2.

Signed-off-by: Donald Raab <Donald.Raab@bnymellon.com>
eclipse · Sep 27, 2017 · d593a1b · d593a1b
1 parent 305b981
commit d593a1b
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diff --git a/eclipse-collections/src/main/java/org/eclipse/collections/impl/collector/Collectors2.java b/eclipse-collections/src/main/java/org/eclipse/collections/impl/collector/Collectors2.java
@@ -721,6 +721,21 @@ private Collectors2()
                 EMPTY_CHARACTERISTICS);
     }
 
+    /**
+     * Returns the counts of all of the values returned by applying the specified function to each
+     * item of the Stream.
+     *
+     * @since 9.1
+     */
+    public static <T, K> Collector<T, ?, MutableBag<K>> countBy(Function<? super T, ? extends K> function)
+    {
+        return Collector.of(
+                Bags.mutable::empty,
+                (bag, each) -> bag.with(function.valueOf(each)),
+                MutableBag::withAll,
+                EMPTY_CHARACTERISTICS);
+    }
+
     /**
      * <p>Returns the elements as an MutableMultimap grouping each element using the specified groupBy Function.</p>
      * <p>Examples:</p>

diff --git a/unit-tests/src/test/java/org/eclipse/collections/impl/collector/Collectors2Test.java b/unit-tests/src/test/java/org/eclipse/collections/impl/collector/Collectors2Test.java
@@ -16,6 +16,7 @@
 import java.util.Map;
 import java.util.stream.Collectors;
 
+import org.eclipse.collections.api.bag.MutableBag;
 import org.eclipse.collections.api.collection.primitive.MutableIntCollection;
 import org.eclipse.collections.api.list.MutableList;
 import org.eclipse.collections.api.map.primitive.MutableObjectLongMap;
@@ -1083,4 +1084,24 @@ public void toImmutableBagMultimap2Parallel()
                 LARGE_INTERVAL.reduceInPlace(Collectors2.toImmutableBagMultimap(Object::toString, Object::toString)),
                 this.bigData.parallelStream().collect(Collectors2.toImmutableBagMultimap(Object::toString, Object::toString)));
     }
+
+    @Test
+    public void countBy()
+    {
+        Interval integers = Interval.oneTo(100);
+        MutableBag<Integer> counts = integers.stream().collect(Collectors2.countBy(i -> i % 2));
+        Assert.assertEquals(integers.countBy(i -> i % 2), counts);
+        Assert.assertEquals(50, counts.occurrencesOf(0));
+        Assert.assertEquals(50, counts.occurrencesOf(1));
+    }
+
+    @Test
+    public void countByParallel()
+    {
+        Interval integers = Interval.oneTo(100000);
+        MutableBag<Integer> counts = integers.parallelStream().collect(Collectors2.countBy(i -> i % 2));
+        Assert.assertEquals(integers.countBy(i -> i % 2), counts);
+        Assert.assertEquals(50000, counts.occurrencesOf(0));
+        Assert.assertEquals(50000, counts.occurrencesOf(1));
+    }
 }