openjdk/jdk/src/share/classes/java/util/stream/Collector.java

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package java.util.stream;

import java.util.Collections;
import java.util.EnumSet;
import java.util.Set;
import java.util.function.BiConsumer;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.function.Supplier;

/**
 * A <a href="package-summary.html#Reduction">reduction operation</a> that
 * folds input elements into a mutable result container, optionally transforming
 * the accumulated result into a final representation after all input elements
 * have been processed.
 *
 * <p>Examples of mutable reduction operations include:
 * accumulating elements into a {@code Collection}; concatenating
 * strings using a {@code StringBuilder}; computing summary information about
 * elements such as sum, min, max, or average; computing "pivot table" summaries
 * such as "maximum valued transaction by seller", etc.  Reduction operations
 * can be performed either sequentially or in parallel.
 *
 * <p>The following are examples of using the predefined {@code Collector}
 * implementations in {@link Collectors} with the {@code Stream} API to perform
 * mutable reduction tasks:
 * <pre>{@code
 *     // Accumulate names into a List
 *     List<String> list = people.stream().map(Person::getName).collect(Collectors.toList());
 *
 *     // Accumulate names into a TreeSet
 *     Set<String> list = people.stream().map(Person::getName).collect(Collectors.toCollection(TreeSet::new));
 *
 *     // Convert elements to strings and concatenate them, separated by commas
 *     String joined = things.stream()
 *                           .map(Object::toString)
 *                           .collect(Collectors.joining(", "));
 *
 *     // Find highest-paid employee
 *     Employee highestPaid = employees.stream()
 *                                     .collect(Collectors.maxBy(Comparators.comparing(Employee::getSalary)))
 *                                     .get();
 *
 *     // Group employees by department
 *     Map<Department, List<Employee>> byDept
 *         = employees.stream()
 *                    .collect(Collectors.groupingBy(Employee::getDepartment));
 *
 *     // Find highest-paid employee by department
 *     Map<Department, Optional<Employee>> highestPaidByDept
 *         = employees.stream()
 *                    .collect(Collectors.groupingBy(Employee::getDepartment,
 *                                                   Collectors.maxBy(Comparators.comparing(Employee::getSalary))));
 *
 *     // Partition students into passing and failing
 *     Map<Boolean, List<Student>> passingFailing =
 *         students.stream()
 *                 .collect(Collectors.partitioningBy(s -> s.getGrade() >= PASS_THRESHOLD));
 *
 * }</pre>
 *
 * <p>A {@code Collector} is specified by four functions that work together to
 * accumulate entries into a mutable result container, and optionally perform
 * a final transform on the result.  They are: creation of a new result container,
 * incorporating a new data element into a result container, combining two
 * result containers into one, and performing a final transform on the container.
 * The combiner function is used during parallel operations, where
 * subsets of the input are accumulated into separate result
 * containers, and then the subresults merged into a combined result.  The
 * combiner function may merge one set of subresults into the other and return
 * that, or it may return a new object to describe the combined results.
 *
 * <p>Collectors also have a set of characteristics, such as
 * {@link Characteristics#CONCURRENT}.  These characteristics provide
 * hints that can be used by a reduction implementation to provide better
 * performance.
 *
 * <p>Libraries that implement reduction based on {@code Collector}, such as
 * {@link Stream#collect(Collector)}, must adhere to the following constraints:
 * <ul>
 *     <li>The first argument passed to the accumulator function, both
 *     arguments passed to the combiner function, and the argument passed to the
 *     finisher function must be the result of a previous invocation of the
 *     result supplier, accumulator, or combiner functions.</li>
 *     <li>The implementation should not do anything with the result of any of
 *     the result supplier, accumulator, or combiner functions other than to
 *     pass them again to the accumulator, combiner, or finisher functions,
 *     or return them to the caller of the reduction operation.</li>
 *     <li>If a result is passed to the combiner or finisher
 *     function, and the same object is not returned from that function, it is
 *     never used again.</li>
 *     <li>Once a result is passed to the combiner or finisher function, it
 *     is never passed to the accumulator function again.</li>
 *     <li>For non-concurrent collectors, any result returned from the result
 *     supplier, accumulator, or combiner functions must be serially
 *     thread-confined.  This enables collection to occur in parallel without
 *     the {@code Collector} needing to implement any additional synchronization.
 *     The reduction implementation must manage that the input is properly
 *     partitioned, that partitions are processed in isolation, and combining
 *     happens only after accumulation is complete.</li>
 *     <li>For concurrent collectors, an implementation is free to (but not
 *     required to) implement reduction concurrently.  A concurrent reduction
 *     is one where the accumulator function is called concurrently from
 *     multiple threads, using the same concurrently-modifiable result container,
 *     rather than keeping the result isolated during accumulation.
 *     A concurrent reduction should only be applied if the collector has the
 *     {@link Characteristics#UNORDERED} characteristics or if the
 *     originating data is unordered.</li>
 * </ul>
 *
 * @apiNote
 * Performing a reduction operation with a {@code Collector} should produce a
 * result equivalent to:
 * <pre>{@code
 *     R container = collector.supplier().get();
 *     for (T t : data)
 *         collector.accumulator().accept(container, t);
 *     return collector.finisher().apply(container);
 * }</pre>
 *
 * <p>However, the library is free to partition the input, perform the reduction
 * on the partitions, and then use the combiner function to combine the partial
 * results to achieve a parallel reduction.  Depending on the specific reduction
 * operation, this may perform better or worse, depending on the relative cost
 * of the accumulator and combiner functions.
 *
 * <p>An example of an operation that can be easily modeled by {@code Collector}
 * is accumulating elements into a {@code TreeSet}. In this case, the {@code
 * resultSupplier()} function is {@code () -> new Treeset<T>()}, the
 * {@code accumulator} function is
 * {@code (set, element) -> set.add(element) }, and the combiner
 * function is {@code (left, right) -> { left.addAll(right); return left; }}.
 * (This behavior is implemented by
 * {@code Collectors.toCollection(TreeSet::new)}).
 *
 * TODO Associativity and commutativity
 *
 * @see Stream#collect(Collector)
 * @see Collectors
 *
 * @param <T> the type of input elements to the reduction operation
 * @param <A> the mutable accumulation type of the reduction operation (often
 *           hidden as an implementation detail)
 * @param <R> the result type of the reduction operation
 * @since 1.8
 */
public interface Collector<T, A, R> {
    /**
     * A function that creates and returns a new mutable result container.
     *
     * @return a function which returns a new, mutable result container
     */
    Supplier<A> supplier();

    /**
     * A function that folds a new value into a mutable result container.
     *
     * @return a function which folds a new value into a mutable result container
     */
    BiConsumer<A, T> accumulator();

    /**
     * A function that accepts two partial results and merges them.  The
     * combiner function may fold state from one argument into the other and
     * return that, or may return a new result object.
     *
     * @return a function which combines two partial results into a cumulative
     * result
     */
    BinaryOperator<A> combiner();

    /**
     * Perform the final transformation from the intermediate accumulation type
     * {@code A} to the final result representation {@code R}.
     *
     * <p>If the characteristic {@code IDENTITY_TRANSFORM} is
     * set, this function may be presumed to be an identity transform with an
     * unchecked cast from {@code A} to {@code R}.
     *
     * @return a function which transforms the intermediate result to the final
     * result
     */
    Function<A, R> finisher();

    /**
     * Returns a {@code Set} of {@code Collector.Characteristics} indicating
     * the characteristics of this Collector.  This set should be immutable.
     *
     * @return an immutable set of collector characteristics
     */
    Set<Characteristics> characteristics();

    /**
     * Returns a new {@code Collector} described by the given {@code supplier},
     * {@code accumulator}, and {@code combiner} functions.  The resulting
     * {@code Collector} has the {@code Collector.Characteristics.IDENTITY_FINISH}
     * characteristic.
     *
     * @param supplier The supplier function for the new collector
     * @param accumulator The accumulator function for the new collector
     * @param combiner The combiner function for the new collector
     * @param characteristics The collector characteristics for the new
     *                        collector
     * @param <T> The type of input elements for the new collector
     * @param <R> The type of intermediate accumulation result, and final result,
     *           for the new collector
     * @return the new {@code Collector}
     */
    public static<T, R> Collector<T, R, R> of(Supplier<R> supplier,
                                              BiConsumer<R, T> accumulator,
                                              BinaryOperator<R> combiner,
                                              Characteristics... characteristics) {
        Set<Characteristics> cs = (characteristics.length == 0)
                                  ? Collectors.CH_ID
                                  : Collections.unmodifiableSet(EnumSet.of(Collector.Characteristics.IDENTITY_FINISH,
                                                                           characteristics));
        return new Collectors.CollectorImpl<>(supplier, accumulator, combiner, cs);
    }

    /**
     * Returns a new {@code Collector} described by the given {@code supplier},
     * {@code accumulator}, {@code combiner}, and {@code finisher} functions.
     *
     * @param supplier The supplier function for the new collector
     * @param accumulator The accumulator function for the new collector
     * @param combiner The combiner function for the new collector
     * @param finisher The finisher function for the new collector
     * @param characteristics The collector characteristics for the new
     *                        collector
     * @param <T> The type of input elements for the new collector
     * @param <A> The intermediate accumulation type of the new collector
     * @param <R> The final result type of the new collector
     * @return the new {@code Collector}
     */
    public static<T, A, R> Collector<T, A, R> of(Supplier<A> supplier,
                                                 BiConsumer<A, T> accumulator,
                                                 BinaryOperator<A> combiner,
                                                 Function<A, R> finisher,
                                                 Characteristics... characteristics) {
        Set<Characteristics> cs = Collectors.CH_NOID;
        if (characteristics.length > 0) {
            cs = EnumSet.noneOf(Characteristics.class);
            Collections.addAll(cs, characteristics);
            cs = Collections.unmodifiableSet(cs);
        }
        return new Collectors.CollectorImpl<>(supplier, accumulator, combiner, finisher, cs);
    }

    /**
     * Characteristics indicating properties of a {@code Collector}, which can
     * be used to optimize reduction implementations.
     */
    enum Characteristics {
        /**
         * Indicates that this collector is <em>concurrent</em>, meaning that
         * the result container can support the accumulator function being
         * called concurrently with the same result container from multiple
         * threads.
         *
         * <p>If a {@code CONCURRENT} collector is not also {@code UNORDERED},
         * then it should only be evaluated concurrently if applied to an
         * unordered data source.
         */
        CONCURRENT,

        /**
         * Indicates that the result container has no intrinsic order, such as
         * a {@link Set}.
         */
        UNORDERED,

        /**
         * Indicates that the finisher function is the identity function and
         * can be elided.  If set, it must be the case that an unchecked cast
         * from A to R will succeed.
         */
        IDENTITY_FINISH
    }
}