JDK8中java.util.HashSet中未使用的方法

查看

java.util.HashSet

的实现时：

您可以看到，有些方法

writeObject

和

readObject

根本没有被使用，而且它们是私有的，因此它们既不能通过扩展类来使用

为什么这些方法在Java8、JDK8的实现中

为清晰起见，复制代码：

/*
 * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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 */

package java.util;

import java.io.InvalidObjectException;

/**
 * This class implements the <tt>Set</tt> interface, backed by a hash table
 * (actually a <tt>HashMap</tt> instance).  It makes no guarantees as to the
 * iteration order of the set; in particular, it does not guarantee that the
 * order will remain constant over time.  This class permits the <tt>null</tt>
 * element.
 *
 * <p>This class offers constant time performance for the basic operations
 * (<tt>add</tt>, <tt>remove</tt>, <tt>contains</tt> and <tt>size</tt>),
 * assuming the hash function disperses the elements properly among the
 * buckets.  Iterating over this set requires time proportional to the sum of
 * the <tt>HashSet</tt> instance's size (the number of elements) plus the
 * "capacity" of the backing <tt>HashMap</tt> instance (the number of
 * buckets).  Thus, it's very important not to set the initial capacity too
 * high (or the load factor too low) if iteration performance is important.
 *
 * <p><strong>Note that this implementation is not synchronized.</strong>
 * If multiple threads access a hash set concurrently, and at least one of
 * the threads modifies the set, it <i>must</i> be synchronized externally.
 * This is typically accomplished by synchronizing on some object that
 * naturally encapsulates the set.
 *
 * If no such object exists, the set should be "wrapped" using the
 * {@link Collections#synchronizedSet Collections.synchronizedSet}
 * method.  This is best done at creation time, to prevent accidental
 * unsynchronized access to the set:<pre>
 *   Set s = Collections.synchronizedSet(new HashSet(...));</pre>
 *
 * <p>The iterators returned by this class's <tt>iterator</tt> method are
 * <i>fail-fast</i>: if the set is modified at any time after the iterator is
 * created, in any way except through the iterator's own <tt>remove</tt>
 * method, the Iterator throws a {@link ConcurrentModificationException}.
 * Thus, in the face of concurrent modification, the iterator fails quickly
 * and cleanly, rather than risking arbitrary, non-deterministic behavior at
 * an undetermined time in the future.
 *
 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.  Fail-fast iterators
 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness: <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i>
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @param <E> the type of elements maintained by this set
 *
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @see     Collection
 * @see     Set
 * @see     TreeSet
 * @see     HashMap
 * @since   1.2
 */

public class HashSet<E>
    extends AbstractSet<E>
    implements Set<E>, Cloneable, java.io.Serializable
{
    static final long serialVersionUID = -5024744406713321676L;

    private transient HashMap<E,Object> map;

    // Dummy value to associate with an Object in the backing Map
    private static final Object PRESENT = new Object();

    /**
     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
     * default initial capacity (16) and load factor (0.75).
     */
    public HashSet() {
        map = new HashMap<>();
    }

    /**
     * Constructs a new set containing the elements in the specified
     * collection.  The <tt>HashMap</tt> is created with default load factor
     * (0.75) and an initial capacity sufficient to contain the elements in
     * the specified collection.
     *
     * @param c the collection whose elements are to be placed into this set
     * @throws NullPointerException if the specified collection is null
     */
    public HashSet(Collection<? extends E> c) {
        map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
        addAll(c);
    }

    /**
     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
     * the specified initial capacity and the specified load factor.
     *
     * @param      initialCapacity   the initial capacity of the hash map
     * @param      loadFactor        the load factor of the hash map
     * @throws     IllegalArgumentException if the initial capacity is less
     *             than zero, or if the load factor is nonpositive
     */
    public HashSet(int initialCapacity, float loadFactor) {
        map = new HashMap<>(initialCapacity, loadFactor);
    }

    /**
     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
     * the specified initial capacity and default load factor (0.75).
     *
     * @param      initialCapacity   the initial capacity of the hash table
     * @throws     IllegalArgumentException if the initial capacity is less
     *             than zero
     */
    public HashSet(int initialCapacity) {
        map = new HashMap<>(initialCapacity);
    }

    /**
     * Constructs a new, empty linked hash set.  (This package private
     * constructor is only used by LinkedHashSet.) The backing
     * HashMap instance is a LinkedHashMap with the specified initial
     * capacity and the specified load factor.
     *
     * @param      initialCapacity   the initial capacity of the hash map
     * @param      loadFactor        the load factor of the hash map
     * @param      dummy             ignored (distinguishes this
     *             constructor from other int, float constructor.)
     * @throws     IllegalArgumentException if the initial capacity is less
     *             than zero, or if the load factor is nonpositive
     */
    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }

    /**
     * Returns an iterator over the elements in this set.  The elements
     * are returned in no particular order.
     *
     * @return an Iterator over the elements in this set
     * @see ConcurrentModificationException
     */
    public Iterator<E> iterator() {
        return map.keySet().iterator();
    }

    /**
     * Returns the number of elements in this set (its cardinality).
     *
     * @return the number of elements in this set (its cardinality)
     */
    public int size() {
        return map.size();
    }

    /**
     * Returns <tt>true</tt> if this set contains no elements.
     *
     * @return <tt>true</tt> if this set contains no elements
     */
    public boolean isEmpty() {
        return map.isEmpty();
    }

    /**
     * Returns <tt>true</tt> if this set contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this set
     * contains an element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
     *
     * @param o element whose presence in this set is to be tested
     * @return <tt>true</tt> if this set contains the specified element
     */
    public boolean contains(Object o) {
        return map.containsKey(o);
    }

    /**
     * Adds the specified element to this set if it is not already present.
     * More formally, adds the specified element <tt>e</tt> to this set if
     * this set contains no element <tt>e2</tt> such that
     * <tt>(e==null&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;e.equals(e2))</tt>.
     * If this set already contains the element, the call leaves the set
     * unchanged and returns <tt>false</tt>.
     *
     * @param e element to be added to this set
     * @return <tt>true</tt> if this set did not already contain the specified
     * element
     */
    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }

    /**
     * Removes the specified element from this set if it is present.
     * More formally, removes an element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>,
     * if this set contains such an element.  Returns <tt>true</tt> 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 <tt>true</tt> if the set contained the specified element
     */
    public boolean remove(Object o) {
        return map.remove(o)==PRESENT;
    }

    /**
     * Removes all of the elements from this set.
     * The set will be empty after this call returns.
     */
    public void clear() {
        map.clear();
    }

    /**
     * Returns a shallow copy of this <tt>HashSet</tt> instance: the elements
     * themselves are not cloned.
     *
     * @return a shallow copy of this set
     */
    @SuppressWarnings("unchecked")
    public Object clone() {
        try {
            HashSet<E> newSet = (HashSet<E>) super.clone();
            newSet.map = (HashMap<E, Object>) map.clone();
            return newSet;
        } catch (CloneNotSupportedException e) {
            throw new InternalError(e);
        }
    }

    /**
     * Save the state of this <tt>HashSet</tt> instance to a stream (that is,
     * serialize it).
     *
     * @serialData The capacity of the backing <tt>HashMap</tt> instance
     *             (int), and its load factor (float) are emitted, followed by
     *             the size of the set (the number of elements it contains)
     *             (int), followed by all of its elements (each an Object) in
     *             no particular order.
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        // Write out any hidden serialization magic
        s.defaultWriteObject();

        // Write out HashMap capacity and load factor
        s.writeInt(map.capacity());
        s.writeFloat(map.loadFactor());

        // Write out size
        s.writeInt(map.size());

        // Write out all elements in the proper order.
        for (E e : map.keySet())
            s.writeObject(e);
    }

    /**
     * Reconstitute the <tt>HashSet</tt> instance from a stream (that is,
     * deserialize it).
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        // Read in any hidden serialization magic
        s.defaultReadObject();

        // Read capacity and verify non-negative.
        int capacity = s.readInt();
        if (capacity < 0) {
            throw new InvalidObjectException("Illegal capacity: " +
                                             capacity);
        }

        // Read load factor and verify positive and non NaN.
        float loadFactor = s.readFloat();
        if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
            throw new InvalidObjectException("Illegal load factor: " +
                                             loadFactor);
        }

        // Read size and verify non-negative.
        int size = s.readInt();
        if (size < 0) {
            throw new InvalidObjectException("Illegal size: " +
                                             size);
        }

        // Set the capacity according to the size and load factor ensuring that
        // the HashMap is at least 25% full but clamping to maximum capacity.
        capacity = (int) Math.min(size * Math.min(1 / loadFactor, 4.0f),
                HashMap.MAXIMUM_CAPACITY);

        // Create backing HashMap
        map = (((HashSet<?>)this) instanceof LinkedHashSet ?
               new LinkedHashMap<E,Object>(capacity, loadFactor) :
               new HashMap<E,Object>(capacity, loadFactor));

        // Read in all elements in the proper order.
        for (int i=0; i<size; i++) {
            @SuppressWarnings("unchecked")
                E e = (E) s.readObject();
            map.put(e, PRESENT);
        }
    }

    /**
     * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
     * and <em>fail-fast</em> {@link Spliterator} over the elements in this
     * set.
     *
     * <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
     * {@link Spliterator#DISTINCT}.  Overriding implementations should document
     * the reporting of additional characteristic values.
     *
     * @return a {@code Spliterator} over the elements in this set
     * @since 1.8
     */
    public Spliterator<E> spliterator() {
        return new HashMap.KeySpliterator<E,Object>(map, 0, -1, 0, 0);
    }
}

/*
*版权所有（c）1997年、2013年，Oracle和/或其附属公司。版权所有。
*ORACLE专有/机密。使用受许可条款的约束。
*
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*/
包java.util；
导入java.io.InvalidObject异常；
/**
*此类实现了Set接口，该接口由哈希表支持
*（实际上是一个HashMap实例）。它不保证
*集合的迭代顺序；特别是，它不能保证
*随着时间的推移，秩序将保持不变。此类允许空值
*元素。
*
*该类为基本操作提供恒定时间性能
*（添加、删除、包含和大小），
*假设散列函数将元素正确地分散在
*水桶。在这个集合上迭代需要与时间之和成比例的时间
*HashSet实例的大小（元素数）加上
*备份HashMap实例的“容量”（数量
*桶）。因此，不设置初始容量非常重要
*如果迭代性能很重要，则为高（或负载系数太低）。
*
*请注意，此实现未同步。
*如果多个线程同时访问哈希集，并且
*线程修改集合，它必须在外部同步。
*这通常是通过在某个
*自然地封装了集合。
*
*如果不存在这样的对象，则应使用
*{@link Collections#synchronizedSet Collections.synchronizedSet}
*方法。最好在创建时执行此操作，以防止意外事件
*对集合的非同步访问：
*Set s=Collections.synchronizedSet（新哈希集（…）；
*
*这个类的迭代器方法返回的迭代器是
*fail fast（快速失败）：如果在调用迭代器后的任何时间修改集合
*以任何方式创建，除了通过迭代器自己的移除
*方法，迭代器抛出{@link ConcurrentModificationException}。
*因此，面对并发修改，迭代器很快就会失败
*干净地，而不是冒着任意的、不确定的行为的风险
*未来不确定的时间。
*
*请注意，无法保证迭代器的快速失败行为
*因为一般来说，不可能在未来做出任何硬保证
*存在未同步的并发修改。快速失效迭代器
*尽最大努力抛出ConcurrentModificationException。
*因此，编写依赖于此的程序是错误的
*正确性的例外：迭代器的快速失败行为
*应仅用于检测错误。
*
*该类是
* .
*
*@param此集合维护的元素类型
*
*@作者Josh Bloch
*@作者Neal Gafter
*@见收藏
*@见集
*@见树集
*@see HashMap
*@自1.2
*/
公共类哈希集
扩展抽象集
实现Set、Cloneable、java.io.Serializable
{
静态最终长serialVersionUID=-502474406713321676L；
私有瞬态HashMap；
//要与背景贴图中的对象关联的虚拟值
私有静态最终对象存在=新对象（）；
/**
*构造一个新的空集；备份HashMap实例
*默认初始容量（16）和负载系数（0.75）。
*/
公共哈希集（）{
map=新的HashMap（）；
}
/**
*构造一个新的集合，其中包含指定集合中的元素
*HashMap是使用默认加载因子创建的
*（0.75）且初始容量足以容纳
*指定的集合。
*
*@param c将其元素放入此集合的集合
*如果指定的集合为空，@将引发NullPointerException
*/
公共HashSet（Collection它们用于序列化

您可以在此处阅读更多信息：

我还可以推荐Joshua Bloch的《有效Java》一书中的序列化线索。
它们被Java对象序列化机制使用。可以在其规范中找到解释：

对象序列化的标准实现实际上忽略了各自方法的可见性。
这是如何工作的？Netbeans 8.0在它们下面加下划线表示未使用，我也看不出它们是某些接口的实现？@skiwi我更新了我的答案。这些是反射式调用的特殊方法。Netbeans应该在分析“未使用”时，可能会将其更改为考虑到这一点简而言之，这些方法描述了序列化/反序列化应该如何完成，也可以在类外使用它们，它们用很少的声明来声明。谁是NETBESE然后抱怨<代码> WraveObjs<代码> ReS.<代码> Read Objult/Cuth>不被使用？是否不认为这些可能被用于序列化？问题？@skiwi Fair point。如果还没有问题，请提交一个问题：是的，似乎NetBeans正在对此进行监督。例如，Eclipse不会生成有关此方法的任何警告。外部化接口支持
writeExternal（）
和
readExternal（）标准序列化支持
writeObject
和
readObject
。哦，你说得对。我会更正我的答案。