/* * Copyright (c) 2012, 2017, 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 * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ package java.util.stream; import java.util.AbstractMap; import java.util.AbstractSet; import java.util.ArrayList; import java.util.Collection; import java.util.Collections; import java.util.Comparator; import java.util.DoubleSummaryStatistics; import java.util.EnumSet; import java.util.HashMap; import java.util.HashSet; import java.util.IntSummaryStatistics; import java.util.Iterator; import java.util.List; import java.util.LongSummaryStatistics; import java.util.Map; import java.util.Objects; import java.util.Optional; import java.util.Set; import java.util.StringJoiner; import java.util.concurrent.ConcurrentHashMap; import java.util.concurrent.ConcurrentMap; import java.util.function.BiConsumer; import java.util.function.BiFunction; import java.util.function.BinaryOperator; import java.util.function.Consumer; import java.util.function.Function; import java.util.function.Predicate; import java.util.function.Supplier; import java.util.function.ToDoubleFunction; import java.util.function.ToIntFunction; import java.util.function.ToLongFunction; /** * Implementations of {@link Collector} that implement various useful reduction * operations, such as accumulating elements into collections, summarizing * elements according to various criteria, etc. * *
The following are examples of using the predefined collectors to perform * common mutable reduction tasks: * *
{@code * // Accumulate names into a List * List* * @since 1.8 */ public final class Collectors { static final Setlist = people.stream() * .map(Person::getName) * .collect(Collectors.toList()); * * // Accumulate names into a TreeSet * Set set = 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(", ")); * * // Compute sum of salaries of employee * int total = employees.stream() * .collect(Collectors.summingInt(Employee::getSalary)); * * // Group employees by department * Map > byDept = employees.stream() * .collect(Collectors.groupingBy(Employee::getDepartment)); * * // Compute sum of salaries by department * Map totalByDept = employees.stream() * .collect(Collectors.groupingBy(Employee::getDepartment, * Collectors.summingInt(Employee::getSalary))); * * // Partition students into passing and failing * Map > passingFailing = students.stream() * .collect(Collectors.partitioningBy(s -> s.getGrade() >= PASS_THRESHOLD)); * * }