src/java.base/share/classes/java/util/stream/Collectors.java

@@ -1,7 +1,7 @@
 /*
- * Copyright (c) 2012, 2017, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2012, 2018, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
  * under the terms of the GNU General Public License version 2 only, as
  * published by the Free Software Foundation.  Oracle designates this

@@ -1883,10 +1883,106 @@
                 (r, t) -> r.accept(mapper.applyAsDouble(t)),
                 (l, r) -> { l.combine(r); return l; }, CH_ID);
     }
 
     /**
+     * Returns a {@code Collector} that is a composite of two downstream collectors.
+     * Every element passed to the resulting collector is processed by both downstream
+     * collectors, then their results are merged using the specified merge function
+     * into the final result.
+     *
+     * <p>The resulting collector functions do the following:
+     *
+     * <ul>
+     * <li>supplier: creates a result container that contains result containers
+     * obtained by calling each collector's supplier
+     * <li>accumulator: calls each collector's accumulator with its result container
+     * and the input element
+     * <li>combiner: calls each collector's combiner with two result containers
+     * <li>finisher: calls each collector's finisher with its result container,
+     * then calls the supplied merger and returns its result.
+     * </ul>
+     *
+     * <p>The resulting collector is {@link Collector.Characteristics#UNORDERED} if both downstream
+     * collectors are unordered and {@link Collector.Characteristics#CONCURRENT} if both downstream
+     * collectors are concurrent.
+     *
+     * @param <T>         the type of the input elements
+     * @param <R1>        the result type of the first collector
+     * @param <R2>        the result type of the second collector
+     * @param <R>         the final result type
+     * @param downstream1 the first downstream collector
+     * @param downstream2 the second downstream collector
+     * @param merger      the function which merges two results into the single one
+     * @return a {@code Collector} which aggregates the results of two supplied collectors.
+     * @since 12
+     */
+    public static <T, R1, R2, R>
+    Collector<T, ?, R> teeing(Collector<? super T, ?, R1> downstream1,
+                              Collector<? super T, ?, R2> downstream2,
+                              BiFunction<? super R1, ? super R2, R> merger) {
+        return teeing0(downstream1, downstream2, merger);
+    }
+
+    private static <T, A1, A2, R1, R2, R>
+    Collector<T, ?, R> teeing0(Collector<? super T, A1, R1> downstream1,
+                               Collector<? super T, A2, R2> downstream2,
+                               BiFunction<? super R1, ? super R2, R> merger) {
+        Objects.requireNonNull(downstream1, "downstream1");
+        Objects.requireNonNull(downstream2, "downstream2");
+        Objects.requireNonNull(merger, "merger");
+
+        Supplier<A1> c1Supplier = Objects.requireNonNull(downstream1.supplier(), "downstream1 supplier");
+        Supplier<A2> c2Supplier = Objects.requireNonNull(downstream2.supplier(), "downstream2 supplier");
+        BiConsumer<A1, ? super T> c1Accumulator =
+                Objects.requireNonNull(downstream1.accumulator(), "downstream1 accumulator");
+        BiConsumer<A2, ? super T> c2Accumulator =
+                Objects.requireNonNull(downstream2.accumulator(), "downstream2 accumulator");
+        BinaryOperator<A1> c1Combiner = Objects.requireNonNull(downstream1.combiner(), "downstream1 combiner");
+        BinaryOperator<A2> c2Combiner = Objects.requireNonNull(downstream2.combiner(), "downstream2 combiner");
+        Function<A1, R1> c1Finisher = Objects.requireNonNull(downstream1.finisher(), "downstream1 finisher");
+        Function<A2, R2> c2Finisher = Objects.requireNonNull(downstream2.finisher(), "downstream2 finisher");
+
+        Set<Collector.Characteristics> characteristics;
+        Set<Collector.Characteristics> c1Characteristics = downstream1.characteristics();
+        Set<Collector.Characteristics> c2Characteristics = downstream2.characteristics();
+        if (CH_ID.containsAll(c1Characteristics) || CH_ID.containsAll(c2Characteristics)) {
+            characteristics = CH_NOID;
+        } else {
+            EnumSet<Collector.Characteristics> c = EnumSet.noneOf(Collector.Characteristics.class);
+            c.addAll(c1Characteristics);
+            c.retainAll(c2Characteristics);
+            c.remove(Collector.Characteristics.IDENTITY_FINISH);
+            characteristics = Collections.unmodifiableSet(c);
+        }
+
+        class PairBox {
+            A1 left = c1Supplier.get();
+            A2 right = c2Supplier.get();
+
+            void add(T t) {
+                c1Accumulator.accept(left, t);
+                c2Accumulator.accept(right, t);
+            }
+
+            PairBox combine(PairBox other) {
+                left = c1Combiner.apply(left, other.left);
+                right = c2Combiner.apply(right, other.right);
+                return this;
+            }
+
+            R get() {
+                R1 r1 = c1Finisher.apply(left);
+                R2 r2 = c2Finisher.apply(right);
+                return merger.apply(r1, r2);
+            }
+        }
+
+        return new CollectorImpl<>(PairBox::new, PairBox::add, PairBox::combine, PairBox::get, characteristics);
+    }
+
+    /**
      * Implementation class used by partitioningBy.
      */
     private static final class Partition<T>
             extends AbstractMap<Boolean, T>
             implements Map<Boolean, T> {