ConcurrentSkipListSet.java

/*
 * 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.
 */

/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.io.Serializable;
import java.lang.reflect.Field;
import java.util.AbstractSet;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.Map;
import java.util.NavigableSet;
import java.util.Set;
import java.util.SortedSet;
import java.util.Spliterator;

/**
 * A scalable concurrent {@link NavigableSet} implementation based on
 * a {@link ConcurrentSkipListMap}.  The elements of the set are kept
 * sorted according to their {@linkplain Comparable natural ordering},
 * or by a {@link Comparator} provided at set creation time, depending
 * on which constructor is used.
 *
 * <p>This implementation provides expected average <i>log(n)</i> time
 * cost for the {@code contains}, {@code add}, and {@code remove}
 * operations and their variants.  Insertion, removal, and access
 * operations safely execute concurrently by multiple threads.
 *
 * <p>Iterators and spliterators are
 * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
 *
 * <p>Ascending ordered views and their iterators are faster than
 * descending ones.
 *
 * <p>Beware that, unlike in most collections, the {@code size}
 * method is <em>not</em> a constant-time operation. Because of the
 * asynchronous nature of these sets, determining the current number
 * of elements requires a traversal of the elements, and so may report
 * inaccurate results if this collection is modified during traversal.
 *
 * <p>Bulk operations that add, remove, or examine multiple elements,
 * such as {@link #addAll}, {@link #removeIf} or {@link #forEach},
 * are <em>not</em> guaranteed to be performed atomically.
 * For example, a {@code forEach} traversal concurrent with an {@code
 * addAll} operation might observe only some of the added elements.
 *
 * <p>This class and its iterators implement all of the
 * <em>optional</em> methods of the {@link Set} and {@link Iterator}
 * interfaces. Like most other concurrent collection implementations,
 * this class does not permit the use of {@code null} elements,
 * because {@code null} arguments and return values cannot be reliably
 * distinguished from the absence of elements.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @author Doug Lea
 * @param <E> the type of elements maintained by this set
 * @since 1.6
 */
/*
 * 跳表-集合，利用跳表来存储集合元素
 *
 * ConcurrentSkipListSet线程安全，且支持导航，内部直接使用ConcurrentNavigableMap来完成所有功能
 */
public class ConcurrentSkipListSet<E> extends AbstractSet<E> implements NavigableSet<E>, Cloneable, Serializable {
    private static final long serialVersionUID = -2479143111061671589L;
    
    /**
     * The underlying map. Uses Boolean.TRUE as value for each
     * element.  This field is declared final for the sake of thread
     * safety, which entails some ugliness in clone().
     */
    private final ConcurrentNavigableMap<E, Object> m;
    
    
    
    /*▼ 构造器 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Constructs a new, empty set that orders its elements according to
     * their {@linkplain Comparable natural ordering}.
     */
    public ConcurrentSkipListSet() {
        m = new ConcurrentSkipListMap<E, Object>();
    }
    
    /**
     * Constructs a new, empty set that orders its elements according to
     * the specified comparator.
     *
     * @param comparator the comparator that will be used to order this set.
     *                   If {@code null}, the {@linkplain Comparable natural
     *                   ordering} of the elements will be used.
     */
    public ConcurrentSkipListSet(Comparator<? super E> comparator) {
        m = new ConcurrentSkipListMap<E, Object>(comparator);
    }
    
    /**
     * Constructs a new set containing the elements in the specified
     * collection, that orders its elements according to their
     * {@linkplain Comparable natural ordering}.
     *
     * @param c The elements that will comprise the new set
     *
     * @throws ClassCastException   if the elements in {@code c} are
     *                              not {@link Comparable}, or are not mutually comparable
     * @throws NullPointerException if the specified collection or any
     *                              of its elements are null
     */
    public ConcurrentSkipListSet(Collection<? extends E> c) {
        m = new ConcurrentSkipListMap<E, Object>();
        addAll(c);
    }
    
    /**
     * Constructs a new set containing the same elements and using the
     * same ordering as the specified sorted set.
     *
     * @param s sorted set whose elements will comprise the new set
     *
     * @throws NullPointerException if the specified sorted set or any
     *                              of its elements are null
     */
    public ConcurrentSkipListSet(SortedSet<E> s) {
        m = new ConcurrentSkipListMap<E, Object>(s.comparator());
        addAll(s);
    }
    
    /**
     * For use by submaps
     */
    ConcurrentSkipListSet(ConcurrentNavigableMap<E, Object> m) {
        this.m = m;
    }
    
    /*▲ 构造器 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 存值 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Adds the specified element to this set if it is not already present.
     * More formally, adds the specified element {@code e} to this set if
     * the set contains no element {@code e2} such that {@code e.equals(e2)}.
     * If this set already contains the element, the call leaves the set
     * unchanged and returns {@code false}.
     *
     * @param e element to be added to this set
     *
     * @return {@code true} if this set did not already contain the
     * specified element
     *
     * @throws ClassCastException   if {@code e} cannot be compared
     *                              with the elements currently in this set
     * @throws NullPointerException if the specified element is null
     */
    // 向Set中添加元素
    public boolean add(E e) {
        return m.putIfAbsent(e, Boolean.TRUE) == null;
    }
    
    /*▲ 存值 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 取值 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * @throws java.util.NoSuchElementException {@inheritDoc}
     */
    // 返回遍历当前集合时的首个元素
    public E first() {
        return m.firstKey();
    }
    
    /**
     * @throws java.util.NoSuchElementException {@inheritDoc}
     */
    // 返回遍历当前集合时的最后一个元素
    public E last() {
        return m.lastKey();
    }
    
    /**
     * @throws ClassCastException   {@inheritDoc}
     * @throws NullPointerException if the specified element is null
     */
    // 〖前驱〗获取遍历当前Set时形参e的前驱
    public E lower(E e) {
        return m.lowerKey(e);
    }
    
    /**
     * @throws ClassCastException   {@inheritDoc}
     * @throws NullPointerException if the specified element is null
     */
    // 〖后继〗获取遍历当前Set时形参e的后继
    public E higher(E e) {
        return m.higherKey(e);
    }
    
    /**
     * @throws ClassCastException   {@inheritDoc}
     * @throws NullPointerException if the specified element is null
     */
    // 【前驱】获取遍历当前Set时形参e的前驱（包括e本身）
    public E floor(E e) {
        return m.floorKey(e);
    }
    
    /**
     * @throws ClassCastException   {@inheritDoc}
     * @throws NullPointerException if the specified element is null
     */
    // 【后继】获取遍历当前Set时形参e的后继（包括e本身）
    public E ceiling(E e) {
        return m.ceilingKey(e);
    }
    
    /*▲ 取值 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 移除 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Removes the specified element from this set if it is present.
     * More formally, removes an element {@code e} such that
     * {@code o.equals(e)}, if this set contains such an element.
     * Returns {@code true} if this set contained the element (or
     * equivalently, if this set changed as a result of the call).
     * (This set will not contain the element once the call returns.)
     *
     * @param o object to be removed from this set, if present
     *
     * @return {@code true} if this set contained the specified element
     *
     * @throws ClassCastException   if {@code o} cannot be compared
     *                              with the elements currently in this set
     * @throws NullPointerException if the specified element is null
     */
    // 移除指定的元素，返回值指示是否移除成功
    public boolean remove(Object o) {
        return m.remove(o, Boolean.TRUE);
    }
    
    /**
     * Removes from this set all of its elements that are contained in
     * the specified collection.  If the specified collection is also
     * a set, this operation effectively modifies this set so that its
     * value is the <i>asymmetric set difference</i> of the two sets.
     *
     * @param c collection containing elements to be removed from this set
     *
     * @return {@code true} if this set changed as a result of the call
     *
     * @throws ClassCastException   if the class of an element of this set
     *                              is incompatible with the specified collection
     *                              (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified collection or any
     *                              of its elements are null
     */
    // (匹配则移除)移除当前集合中所有与给定容器中的元素匹配的元素
    public boolean removeAll(Collection<?> c) {
        // Override AbstractSet version to avoid unnecessary call to size()
        boolean modified = false;
        for(Object e : c)
            if(remove(e))
                modified = true;
        return modified;
    }
    
    /**
     * Removes all of the elements from this set.
     */
    // 清空当前集合中所有元素
    public void clear() {
        m.clear();
    }
    
    
    // 移除遍历当前Set时的首个元素（及其关联的内容）
    public E pollFirst() {
        Map.Entry<E, Object> e = m.pollFirstEntry();
        return (e == null) ? null : e.getKey();
    }
    
    // 移除遍历当前Set时的最后一个元素（及其关联的内容）
    public E pollLast() {
        Map.Entry<E, Object> e = m.pollLastEntry();
        return (e == null) ? null : e.getKey();
    }
    
    /*▲ 移除 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 包含查询 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns {@code true} if this set contains the specified element.
     * More formally, returns {@code true} if and only if this set
     * contains an element {@code e} such that {@code o.equals(e)}.
     *
     * @param o object to be checked for containment in this set
     *
     * @return {@code true} if this set contains the specified element
     *
     * @throws ClassCastException   if the specified element cannot be
     *                              compared with the elements currently in this set
     * @throws NullPointerException if the specified element is null
     */
    // 判断当前集合中是否包含元素o
    public boolean contains(Object o) {
        return m.containsKey(o);
    }
    
    /*▲ 包含查询 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 视图 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * @throws ClassCastException       {@inheritDoc}
     * @throws NullPointerException     if {@code fromElement} or
     *                                  {@code toElement} is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    // 获取【理论区间】为[fromElement, toElement)的Set
    public NavigableSet<E> subSet(E fromElement, E toElement) {
        return subSet(fromElement, true, toElement, false);
    }
    
    /**
     * @throws ClassCastException       {@inheritDoc}
     * @throws NullPointerException     if {@code fromElement} or
     *                                  {@code toElement} is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    // 获取【理论区间】为〖fromElement, toElement〗的Set，区间下限/上限是否为闭区间由fromInclusive和toInclusive参数决定
    public NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {
        return new ConcurrentSkipListSet<E>(m.subMap(fromElement, fromInclusive, toElement, toInclusive));
    }
    
    /**
     * @throws ClassCastException       {@inheritDoc}
     * @throws NullPointerException     if {@code toElement} is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    // 获取【理论区间】上限为toElement(不包含)的Set
    public NavigableSet<E> headSet(E toElement) {
        return headSet(toElement, false);
    }
    
    /**
     * @throws ClassCastException       {@inheritDoc}
     * @throws NullPointerException     if {@code toElement} is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    // 获取【理论区间】上限为toElement的Set，区间上限是否为闭区间由inclusive参数决定
    public NavigableSet<E> headSet(E toElement, boolean inclusive) {
        return new ConcurrentSkipListSet<E>(m.headMap(toElement, inclusive));
    }
    
    /**
     * @throws ClassCastException       {@inheritDoc}
     * @throws NullPointerException     if {@code fromElement} is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    // 获取【理论区间】下限为fromElement(包含)的Set
    public NavigableSet<E> tailSet(E fromElement) {
        return tailSet(fromElement, true);
    }
    
    /**
     * @throws ClassCastException       {@inheritDoc}
     * @throws NullPointerException     if {@code fromElement} is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    // 获取【理论区间】下限为fromElement的Set，区间下限是否为闭区间由inclusive参数决定
    public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
        return new ConcurrentSkipListSet<E>(m.tailMap(fromElement, inclusive));
    }
    
    
    /**
     * Returns a reverse order view of the elements contained in this set.
     * The descending set is backed by this set, so changes to the set are
     * reflected in the descending set, and vice-versa.
     *
     * <p>The returned set has an ordering equivalent to
     * {@link Collections#reverseOrder(Comparator) Collections.reverseOrder}{@code (comparator())}.
     * The expression {@code s.descendingSet().descendingSet()} returns a
     * view of {@code s} essentially equivalent to {@code s}.
     *
     * @return a reverse order view of this set
     */
    // 获取【逆序】Set
    public NavigableSet<E> descendingSet() {
        return new ConcurrentSkipListSet<E>(m.descendingMap());
    }
    
    /*▲ 视图 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 迭代 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns an iterator over the elements in this set in ascending order.
     *
     * @return an iterator over the elements in this set in ascending order
     */
    // 返回当前集合的迭代器
    public Iterator<E> iterator() {
        return m.navigableKeySet().iterator();
    }
    
    /**
     * Returns an iterator over the elements in this set in descending order.
     *
     * @return an iterator over the elements in this set in descending order
     */
    // 返回【逆序】Set的Iterator
    public Iterator<E> descendingIterator() {
        return m.descendingKeySet().iterator();
    }
    
    /**
     * Returns a {@link Spliterator} over the elements in this set.
     *
     * <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
     * {@link Spliterator#NONNULL}, {@link Spliterator#DISTINCT},
     * {@link Spliterator#SORTED} and {@link Spliterator#ORDERED}, with an
     * encounter order that is ascending order.  Overriding implementations
     * should document the reporting of additional characteristic values.
     *
     * <p>The {@linkplain Spliterator#getComparator() spliterator's comparator}
     * is {@code null} if the {@linkplain #comparator() set's comparator}
     * is {@code null}.
     * Otherwise, the spliterator's comparator is the same as or imposes the
     * same total ordering as the set's comparator.
     *
     * @return a {@code Spliterator} over the elements in this set
     *
     * @since 1.8
     */
    // 返回描述此集合中元素的Spliterator
    public Spliterator<E> spliterator() {
        return (m instanceof ConcurrentSkipListMap) ? ((ConcurrentSkipListMap<E, ?>) m).keySpliterator() : ((ConcurrentSkipListMap.SubMap<E, ?>) m).new SubMapKeyIterator();
    }
    
    /*▲ 迭代 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 杂项 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns the number of elements in this set.  If this set
     * contains more than {@code Integer.MAX_VALUE} elements, it
     * returns {@code Integer.MAX_VALUE}.
     *
     * <p>Beware that, unlike in most collections, this method is
     * <em>NOT</em> a constant-time operation. Because of the
     * asynchronous nature of these sets, determining the current
     * number of elements requires traversing them all to count them.
     * Additionally, it is possible for the size to change during
     * execution of this method, in which case the returned result
     * will be inaccurate. Thus, this method is typically not very
     * useful in concurrent applications.
     *
     * @return the number of elements in this set
     */
    // 返回当前集合的元素数量
    public int size() {
        return m.size();
    }
    
    /**
     * Returns {@code true} if this set contains no elements.
     *
     * @return {@code true} if this set contains no elements
     */
    // 判断当前集合是否为空
    public boolean isEmpty() {
        return m.isEmpty();
    }
    
    // 返回当前集合使用的外部比较器Comparator
    public Comparator<? super E> comparator() {
        return m.comparator();
    }
    
    /*▲ 杂项 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /** Initializes map field; for use in clone. */
    private void setMap(ConcurrentNavigableMap<E, Object> map) {
        Field mapField = java.security.AccessController.doPrivileged((java.security.PrivilegedAction<Field>) () -> {
            try {
                Field f = ConcurrentSkipListSet.class.getDeclaredField("m");
                f.setAccessible(true);
                return f;
            } catch(ReflectiveOperationException e) {
                throw new Error(e);
            }
        });
        try {
            mapField.set(this, map);
        } catch(IllegalAccessException e) {
            throw new Error(e);
        }
    }
    
    
    
    /**
     * Compares the specified object with this set for equality.  Returns
     * {@code true} if the specified object is also a set, the two sets
     * have the same size, and every member of the specified set is
     * contained in this set (or equivalently, every member of this set is
     * contained in the specified set).  This definition ensures that the
     * equals method works properly across different implementations of the
     * set interface.
     *
     * @param o the object to be compared for equality with this set
     *
     * @return {@code true} if the specified object is equal to this set
     */
    public boolean equals(Object o) {
        // Override AbstractSet version to avoid calling size()
        if(o == this)
            return true;
        if(!(o instanceof Set))
            return false;
        Collection<?> c = (Collection<?>) o;
        try {
            return containsAll(c) && c.containsAll(this);
        } catch(ClassCastException | NullPointerException unused) {
            return false;
        }
    }
    
    /**
     * Returns a shallow copy of this {@code ConcurrentSkipListSet}
     * instance. (The elements themselves are not cloned.)
     *
     * @return a shallow copy of this set
     */
    public ConcurrentSkipListSet<E> clone() {
        try {
            @SuppressWarnings("unchecked")
            ConcurrentSkipListSet<E> clone = (ConcurrentSkipListSet<E>) super.clone();
            clone.setMap(new ConcurrentSkipListMap<E, Object>(m));
            return clone;
        } catch(CloneNotSupportedException e) {
            throw new InternalError();
        }
    }
    
}