/**
 * Default initial capacity. 默认容量为10
 */
private static final int DEFAULT_CAPACITY = 10;

/**
 * Shared empty array instance used for empty instances.
 */
private static final Object[] EMPTY_ELEMENTDATA = {};

/**
 * Shared empty array instance used for default sized empty instances. We
 * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
 * first element is added.
 */
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

/**
 * The array buffer into which the elements of the ArrayList are stored.
 * The capacity of the ArrayList is the length of this array buffer. Any
 * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
 * will be expanded to DEFAULT_CAPACITY when the first element is added.
 */
transient Object[] elementData; // non-private to simplify nested class access

/**
 * The size of the ArrayList (the number of elements it contains).
 *
 * @serial
 */
private int size;

/**
 * Constructs an empty list with the specified initial capacity.
 *
 * @param  initialCapacity  the initial capacity of the list
 * @throws IllegalArgumentException if the specified initial capacity
 *         is negative
 */
public ArrayList(int initialCapacity) {
    if (initialCapacity > 0) {
        this.elementData = new Object[initialCapacity];
    } else if (initialCapacity == 0) {
        this.elementData = EMPTY_ELEMENTDATA;
    } else {
        throw new IllegalArgumentException("Illegal Capacity: "+
                                           initialCapacity);
    }
}

/**
 * Constructs an empty list with an initial capacity of ten.
 */
public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

/**
 * Constructs a list containing the elements of the specified
 * collection, in the order they are returned by the collection's
 * iterator.
 *
 * @param c the collection whose elements are to be placed into this list
 * @throws NullPointerException if the specified collection is null
 */
public ArrayList(Collection<? extends E> c) {
    elementData = c.toArray();
    if ((size = elementData.length) != 0) {
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, size, Object[].class);
    } else {
        // replace with empty array.
        this.elementData = EMPTY_ELEMENTDATA;
    }
}

/**
 * The array buffer into which the components of the vector are
 * stored. The capacity of the vector is the length of this array buffer,
 * and is at least large enough to contain all the vector's elements.
 *
 * <p>Any array elements following the last element in the Vector are null.
 *
 * @serial
 */
protected Object[] elementData;

/**
 * The number of valid components in this {@code Vector} object.
 * Components {@code elementData[0]} through
 * {@code elementData[elementCount-1]} are the actual items.
 *
 * @serial
 */
protected int elementCount;

/**
 * The amount by which the capacity of the vector is automatically
 * incremented when its size becomes greater than its capacity.  If
 * the capacity increment is less than or equal to zero, the capacity
 * of the vector is doubled each time it needs to grow.
 *
 * @serial
 */
protected int capacityIncrement;

/**
 * Constructs an empty vector with the specified initial capacity and
 * capacity increment.
 *
 * @param   initialCapacity     the initial capacity of the vector
 * @param   capacityIncrement   the amount by which the capacity is
 *                              increased when the vector overflows
 * @throws IllegalArgumentException if the specified initial capacity
 *         is negative
 */
public Vector(int initialCapacity, int capacityIncrement) {
    super();
    if (initialCapacity < 0)
        throw new IllegalArgumentException("Illegal Capacity: "+
                                           initialCapacity);
    this.elementData = new Object[initialCapacity];
    this.capacityIncrement = capacityIncrement;
}

/**
 * Constructs an empty vector with the specified initial capacity and
 * with its capacity increment equal to zero.
 *
 * @param   initialCapacity   the initial capacity of the vector
 * @throws IllegalArgumentException if the specified initial capacity
 *         is negative
 */
public Vector(int initialCapacity) {
    this(initialCapacity, 0);
}

/**
 * Constructs an empty vector so that its internal data array
 * has size {@code 10} and its standard capacity increment is
 * zero.
 */
public Vector() {
    this(10);
}

/**
 * Constructs a vector containing the elements of the specified
 * collection, in the order they are returned by the collection's
 * iterator.
 *
 * @param c the collection whose elements are to be placed into this
 *       vector
 * @throws NullPointerException if the specified collection is null
 * @since   1.2
 */
public Vector(Collection<? extends E> c) {
    elementData = c.toArray();
    elementCount = elementData.length;
    // c.toArray might (incorrectly) not return Object[] (see 6260652)
    if (elementData.getClass() != Object[].class)
        elementData = Arrays.copyOf(elementData, elementCount, Object[].class);
}

private static class Node<E> {
    E item;
    Node<E> next;
    Node<E> prev;

    Node(Node<E> prev, E element, Node<E> next) {
        this.item = element;
        this.next = next;
        this.prev = prev;
    }
}

transient int size = 0;  //存储的元素个数

/**
 * Pointer to first node.
 * Invariant: (first == null && last == null) ||
 *            (first.prev == null && first.item != null)
 */
transient Node<E> first;  //第一个节点

/**
 * Pointer to last node.
 * Invariant: (first == null && last == null) ||
 *            (last.next == null && last.item != null)
 */
transient Node<E> last;  //最后一个节点

/**
 * Constructs an empty list.
 */
public LinkedList() {
}

/**
 * Constructs a list containing the elements of the specified
 * collection, in the order they are returned by the collection's
 * iterator.
 *
 * @param  c the collection whose elements are to be placed into this list
 * @throws NullPointerException if the specified collection is null
 */
public LinkedList(Collection<? extends E> c) {
    this();
    addAll(c);
}

/** The lock protecting all mutators */
final transient ReentrantLock lock = new ReentrantLock();

/** The array, accessed only via getArray/setArray. */
private transient volatile Object[] array;

/**
 * Creates an empty list.
 */
public CopyOnWriteArrayList() {
    setArray(new Object[0]);
}

/**
 * Creates a list containing the elements of the specified
 * collection, in the order they are returned by the collection's
 * iterator.
 *
 * @param c the collection of initially held elements
 * @throws NullPointerException if the specified collection is null
 */
public CopyOnWriteArrayList(Collection<? extends E> c) {
    Object[] elements;
    if (c.getClass() == CopyOnWriteArrayList.class)
        elements = ((CopyOnWriteArrayList<?>)c).getArray();
    else {
        elements = c.toArray();
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elements.getClass() != Object[].class)
            elements = Arrays.copyOf(elements, elements.length, Object[].class);
    }
    setArray(elements);
}

/**
 * Creates a list holding a copy of the given array.
 *
 * @param toCopyIn the array (a copy of this array is used as the
 *        internal array)
 * @throws NullPointerException if the specified array is null
 */
public CopyOnWriteArrayList(E[] toCopyIn) {
    setArray(Arrays.copyOf(toCopyIn, toCopyIn.length, Object[].class));
}

/**
 * Constructs an empty <tt>HashMap</tt> with the specified initial
 * capacity and load factor.
 *
 * @param  initialCapacity the initial capacity
 * @param  loadFactor      the load factor
 * @throws IllegalArgumentException if the initial capacity is negative
 *         or the load factor is nonpositive
 */
public HashMap(int initialCapacity, float loadFactor) {
    if (initialCapacity < 0)
        throw new IllegalArgumentException("Illegal initial capacity: " +
                                           initialCapacity);
    if (initialCapacity > MAXIMUM_CAPACITY)
        initialCapacity = MAXIMUM_CAPACITY;
    if (loadFactor <= 0 || Float.isNaN(loadFactor))
        throw new IllegalArgumentException("Illegal load factor: " +
                                           loadFactor);
    this.loadFactor = loadFactor;
    this.threshold = tableSizeFor(initialCapacity);
}

/**
 * Constructs an empty <tt>HashMap</tt> with the specified initial
 * capacity and the default load factor (0.75).
 *
 * @param  initialCapacity the initial capacity.
 * @throws IllegalArgumentException if the initial capacity is negative.
 */
public HashMap(int initialCapacity) {
    this(initialCapacity, DEFAULT_LOAD_FACTOR);
}

/**
 * Constructs an empty <tt>HashMap</tt> with the default initial capacity
 * (16) and the default load factor (0.75).
 */
public HashMap() {
    this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}

/**
 * Constructs a new <tt>HashMap</tt> with the same mappings as the
 * specified <tt>Map</tt>.  The <tt>HashMap</tt> is created with
 * default load factor (0.75) and an initial capacity sufficient to
 * hold the mappings in the specified <tt>Map</tt>.
 *
 * @param   m the map whose mappings are to be placed in this map
 * @throws  NullPointerException if the specified map is null
 */
public HashMap(Map<? extends K, ? extends V> m) {
    this.loadFactor = DEFAULT_LOAD_FACTOR;
    putMapEntries(m, false);
}

/**
 * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance
 * with the specified initial capacity and load factor.
 *
 * @param  initialCapacity the initial capacity
 * @param  loadFactor      the load factor
 * @throws IllegalArgumentException if the initial capacity is negative
 *         or the load factor is nonpositive
 */
public LinkedHashMap(int initialCapacity, float loadFactor) {
    super(initialCapacity, loadFactor);
    accessOrder = false;
}

/**
 * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance
 * with the specified initial capacity and a default load factor (0.75).
 *
 * @param  initialCapacity the initial capacity
 * @throws IllegalArgumentException if the initial capacity is negative
 */
public LinkedHashMap(int initialCapacity) {
    super(initialCapacity);
    accessOrder = false;
}

/**
 * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance
 * with the default initial capacity (16) and load factor (0.75).
 */
public LinkedHashMap() {
    super();
    accessOrder = false;
}

/**
 * Constructs an insertion-ordered <tt>LinkedHashMap</tt> instance with
 * the same mappings as the specified map.  The <tt>LinkedHashMap</tt>
 * instance is created with a default load factor (0.75) and an initial
 * capacity sufficient to hold the mappings in the specified map.
 *
 * @param  m the map whose mappings are to be placed in this map
 * @throws NullPointerException if the specified map is null
 */
public LinkedHashMap(Map<? extends K, ? extends V> m) {
    super();
    accessOrder = false;
    putMapEntries(m, false);
}

/**
 * Constructs an empty <tt>LinkedHashMap</tt> instance with the
 * specified initial capacity, load factor and ordering mode.
 *
 * @param  initialCapacity the initial capacity
 * @param  loadFactor      the load factor
 * @param  accessOrder     the ordering mode - <tt>true</tt> for
 *         access-order, <tt>false</tt> for insertion-order
 * @throws IllegalArgumentException if the initial capacity is negative
 *         or the load factor is nonpositive
 */
public LinkedHashMap(int initialCapacity,
                     float loadFactor,
                     boolean accessOrder) {
    super(initialCapacity, loadFactor);
    this.accessOrder = accessOrder;
}

/**
 * Constructs a new, empty hashtable with the specified initial
 * capacity and the specified load factor.
 *
 * @param      initialCapacity   the initial capacity of the hashtable.
 * @param      loadFactor        the load factor of the hashtable.
 * @exception  IllegalArgumentException  if the initial capacity is less
 *             than zero, or if the load factor is nonpositive.
 */
public Hashtable(int initialCapacity, float loadFactor) {
    if (initialCapacity < 0)
        throw new IllegalArgumentException("Illegal Capacity: "+
                                           initialCapacity);
    if (loadFactor <= 0 || Float.isNaN(loadFactor))
        throw new IllegalArgumentException("Illegal Load: "+loadFactor);

    if (initialCapacity==0)
        initialCapacity = 1;
    this.loadFactor = loadFactor;
    table = new Entry<?,?>[initialCapacity];
    threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
}

/**
 * Constructs a new, empty hashtable with the specified initial capacity
 * and default load factor (0.75).
 *
 * @param     initialCapacity   the initial capacity of the hashtable.
 * @exception IllegalArgumentException if the initial capacity is less
 *              than zero.
 */
public Hashtable(int initialCapacity) {
    this(initialCapacity, 0.75f);
}

/**
 * Constructs a new, empty hashtable with a default initial capacity (11)
 * and load factor (0.75).
 */
public Hashtable() {
    this(11, 0.75f);
}

/**
 * Constructs a new hashtable with the same mappings as the given
 * Map.  The hashtable is created with an initial capacity sufficient to
 * hold the mappings in the given Map and a default load factor (0.75).
 *
 * @param t the map whose mappings are to be placed in this map.
 * @throws NullPointerException if the specified map is null.
 * @since   1.2
 */
public Hashtable(Map<? extends K, ? extends V> t) {
    this(Math.max(2*t.size(), 11), 0.75f);
    putAll(t);
}

/**
 * The comparator used to maintain order in this tree map, or
 * null if it uses the natural ordering of its keys.
 *
 * @serial
 */
private final Comparator<? super K> comparator;

private transient Entry<K,V> root;

/**
 * The number of entries in the tree
 */
private transient int size = 0;

/**
 * The number of structural modifications to the tree.
 */
private transient int modCount = 0;

/**
 * Constructs a new, empty tree map, using the natural ordering of its
 * keys.  All keys inserted into the map must implement the {@link
 * Comparable} interface.  Furthermore, all such keys must be
 * <em>mutually comparable</em>: {@code k1.compareTo(k2)} must not throw
 * a {@code ClassCastException} for any keys {@code k1} and
 * {@code k2} in the map.  If the user attempts to put a key into the
 * map that violates this constraint (for example, the user attempts to
 * put a string key into a map whose keys are integers), the
 * {@code put(Object key, Object value)} call will throw a
 * {@code ClassCastException}.
 */
public TreeMap() {
    comparator = null;
}

/**
 * Constructs a new, empty tree map, ordered according to the given
 * comparator.  All keys inserted into the map must be <em>mutually
 * comparable</em> by the given comparator: {@code comparator.compare(k1,
 * k2)} must not throw a {@code ClassCastException} for any keys
 * {@code k1} and {@code k2} in the map.  If the user attempts to put
 * a key into the map that violates this constraint, the {@code put(Object
 * key, Object value)} call will throw a
 * {@code ClassCastException}.
 *
 * @param comparator the comparator that will be used to order this map.
 *        If {@code null}, the {@linkplain Comparable natural
 *        ordering} of the keys will be used.
 */
public TreeMap(Comparator<? super K> comparator) {
    this.comparator = comparator;
}

/**
 * Constructs a new tree map containing the same mappings as the given
 * map, ordered according to the <em>natural ordering</em> of its keys.
 * All keys inserted into the new map must implement the {@link
 * Comparable} interface.  Furthermore, all such keys must be
 * <em>mutually comparable</em>: {@code k1.compareTo(k2)} must not throw
 * a {@code ClassCastException} for any keys {@code k1} and
 * {@code k2} in the map.  This method runs in n*log(n) time.
 *
 * @param  m the map whose mappings are to be placed in this map
 * @throws ClassCastException if the keys in m are not {@link Comparable},
 *         or are not mutually comparable
 * @throws NullPointerException if the specified map is null
 */
public TreeMap(Map<? extends K, ? extends V> m) {
    comparator = null;
    putAll(m);
}

/**
 * Constructs a new tree map containing the same mappings and
 * using the same ordering as the specified sorted map.  This
 * method runs in linear time.
 *
 * @param  m the sorted map whose mappings are to be placed in this map,
 *         and whose comparator is to be used to sort this map
 * @throws NullPointerException if the specified map is null
 */
public TreeMap(SortedMap<K, ? extends V> m) {
    comparator = m.comparator();
    try {
        buildFromSorted(m.size(), m.entrySet().iterator(), null, null);
    } catch (java.io.IOException cannotHappen) {
    } catch (ClassNotFoundException cannotHappen) {
    }
}

/* ---------------- Constants -------------- */

/**
 * The largest possible table capacity.  This value must be
 * exactly 1<<30 to stay within Java array allocation and indexing
 * bounds for power of two table sizes, and is further required
 * because the top two bits of 32bit hash fields are used for
 * control purposes.
 */
private static final int MAXIMUM_CAPACITY = 1 << 30;

/**
 * The default initial table capacity.  Must be a power of 2
 * (i.e., at least 1) and at most MAXIMUM_CAPACITY.
 */
private static final int DEFAULT_CAPACITY = 16;

/**
 * The largest possible (non-power of two) array size.
 * Needed by toArray and related methods.
 */
static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

/**
 * The default concurrency level for this table. Unused but
 * defined for compatibility with previous versions of this class.
 */
private static final int DEFAULT_CONCURRENCY_LEVEL = 16;

/**
 * The load factor for this table. Overrides of this value in
 * constructors affect only the initial table capacity.  The
 * actual floating point value isn't normally used -- it is
 * simpler to use expressions such as {@code n - (n >>> 2)} for
 * the associated resizing threshold.
 */
private static final float LOAD_FACTOR = 0.75f;

/**
 * The bin count threshold for using a tree rather than list for a
 * bin.  Bins are converted to trees when adding an element to a
 * bin with at least this many nodes. The value must be greater
 * than 2, and should be at least 8 to mesh with assumptions in
 * tree removal about conversion back to plain bins upon
 * shrinkage.
 */
static final int TREEIFY_THRESHOLD = 8;

/**
 * The bin count threshold for untreeifying a (split) bin during a
 * resize operation. Should be less than TREEIFY_THRESHOLD, and at
 * most 6 to mesh with shrinkage detection under removal.
 */
static final int UNTREEIFY_THRESHOLD = 6;

/**
 * The smallest table capacity for which bins may be treeified.
 * (Otherwise the table is resized if too many nodes in a bin.)
 * The value should be at least 4 * TREEIFY_THRESHOLD to avoid
 * conflicts between resizing and treeification thresholds.
 */
static final int MIN_TREEIFY_CAPACITY = 64;

/**
 * Minimum number of rebinnings per transfer step. Ranges are
 * subdivided to allow multiple resizer threads.  This value
 * serves as a lower bound to avoid resizers encountering
 * excessive memory contention.  The value should be at least
 * DEFAULT_CAPACITY.
 */
private static final int MIN_TRANSFER_STRIDE = 16;

/**
 * The number of bits used for generation stamp in sizeCtl.
 * Must be at least 6 for 32bit arrays.
 */
private static int RESIZE_STAMP_BITS = 16;

/**
 * The maximum number of threads that can help resize.
 * Must fit in 32 - RESIZE_STAMP_BITS bits.
 */
private static final int MAX_RESIZERS = (1 << (32 - RESIZE_STAMP_BITS)) - 1;

/**
 * The bit shift for recording size stamp in sizeCtl.
 */
private static final int RESIZE_STAMP_SHIFT = 32 - RESIZE_STAMP_BITS;

/*
 * Encodings for Node hash fields. See above for explanation.
 */
static final int MOVED     = -1; // hash for forwarding nodes
static final int TREEBIN   = -2; // hash for roots of trees
static final int RESERVED  = -3; // hash for transient reservations
static final int HASH_BITS = 0x7fffffff; // usable bits of normal node hash

/** Number of CPUS, to place bounds on some sizings */
static final int NCPU = Runtime.getRuntime().availableProcessors();

/** For serialization compatibility. */
private static final ObjectStreamField[] serialPersistentFields = {
    new ObjectStreamField("segments", Segment[].class),
    new ObjectStreamField("segmentMask", Integer.TYPE),
    new ObjectStreamField("segmentShift", Integer.TYPE)
};

    /* ---------------- Fields -------------- */

    /**
     * The array of bins. Lazily initialized upon first insertion.
     * Size is always a power of two. Accessed directly by iterators.
     */
    transient volatile Node<K,V>[] table;

    /**
     * The next table to use; non-null only while resizing.
     */
    private transient volatile Node<K,V>[] nextTable;

    /**
     * Base counter value, used mainly when there is no contention,
     * but also as a fallback during table initialization
     * races. Updated via CAS.
     */
    private transient volatile long baseCount;

    /**
     * Table initialization and resizing control.  When negative, the
     * table is being initialized or resized: -1 for initialization,
     * else -(1 + the number of active resizing threads).  Otherwise,
     * when table is null, holds the initial table size to use upon
     * creation, or 0 for default. After initialization, holds the
     * next element count value upon which to resize the table.
     */
    private transient volatile int sizeCtl;

    /**
     * The next table index (plus one) to split while resizing.
     */
    private transient volatile int transferIndex;

    /**
     * Spinlock (locked via CAS) used when resizing and/or creating CounterCells.
     */
    private transient volatile int cellsBusy;

    /**
     * Table of counter cells. When non-null, size is a power of 2.
     */
    private transient volatile CounterCell[] counterCells;

    // views
    private transient KeySetView<K,V> keySet;
    private transient ValuesView<K,V> values;
    private transient EntrySetView<K,V> entrySet;

/**
 * Creates a new, empty map with the default initial table size (16).
 */
public ConcurrentHashMap() {
}

/**
 * Creates a new, empty map with an initial table size
 * accommodating the specified number of elements without the need
 * to dynamically resize.
 *
 * @param initialCapacity The implementation performs internal
 * sizing to accommodate this many elements.
 * @throws IllegalArgumentException if the initial capacity of
 * elements is negative
 */
public ConcurrentHashMap(int initialCapacity) {
    if (initialCapacity < 0)
        throw new IllegalArgumentException();
    int cap = ((initialCapacity >= (MAXIMUM_CAPACITY >>> 1)) ?
               MAXIMUM_CAPACITY :
               tableSizeFor(initialCapacity + (initialCapacity >>> 1) + 1));
    this.sizeCtl = cap;
}

/**
 * Creates a new map with the same mappings as the given map.
 *
 * @param m the map
 */
public ConcurrentHashMap(Map<? extends K, ? extends V> m) {
    this.sizeCtl = DEFAULT_CAPACITY;
    putAll(m);
}

/**
 * Creates a new, empty map with an initial table size based on
 * the given number of elements ({@code initialCapacity}) and
 * initial table density ({@code loadFactor}).
 *
 * @param initialCapacity the initial capacity. The implementation
 * performs internal sizing to accommodate this many elements,
 * given the specified load factor.
 * @param loadFactor the load factor (table density) for
 * establishing the initial table size
 * @throws IllegalArgumentException if the initial capacity of
 * elements is negative or the load factor is nonpositive
 *
 * @since 1.6
 */
public ConcurrentHashMap(int initialCapacity, float loadFactor) {
    this(initialCapacity, loadFactor, 1);
}

/**
 * Creates a new, empty map with an initial table size based on
 * the given number of elements ({@code initialCapacity}), table
 * density ({@code loadFactor}), and number of concurrently
 * updating threads ({@code concurrencyLevel}).
 *
 * @param initialCapacity the initial capacity. The implementation
 * performs internal sizing to accommodate this many elements,
 * given the specified load factor.
 * @param loadFactor the load factor (table density) for
 * establishing the initial table size
 * @param concurrencyLevel the estimated number of concurrently
 * updating threads. The implementation may use this value as
 * a sizing hint.
 * @throws IllegalArgumentException if the initial capacity is
 * negative or the load factor or concurrencyLevel are
 * nonpositive
 */
public ConcurrentHashMap(int initialCapacity,
                         float loadFactor, int concurrencyLevel) {
    if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)
        throw new IllegalArgumentException();
    if (initialCapacity < concurrencyLevel)   // Use at least as many bins
        initialCapacity = concurrencyLevel;   // as estimated threads
    long size = (long)(1.0 + (long)initialCapacity / loadFactor);
    int cap = (size >= (long)MAXIMUM_CAPACITY) ?
        MAXIMUM_CAPACITY : tableSizeFor((int)size);
    this.sizeCtl = cap;
}

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);
}

/**
 * The backing map.
 */
private transient NavigableMap<E,Object> m;

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

/**
 * Constructs a set backed by the specified navigable map.
 */
TreeSet(NavigableMap<E,Object> m) {
    this.m = m;
}

/**
 * Constructs a new, empty tree set, sorted according to the
 * natural ordering of its elements.  All elements inserted into
 * the set must implement the {@link Comparable} interface.
 * Furthermore, all such elements must be <i>mutually
 * comparable</i>: {@code e1.compareTo(e2)} must not throw a
 * {@code ClassCastException} for any elements {@code e1} and
 * {@code e2} in the set.  If the user attempts to add an element
 * to the set that violates this constraint (for example, the user
 * attempts to add a string element to a set whose elements are
 * integers), the {@code add} call will throw a
 * {@code ClassCastException}.
 */
public TreeSet() {
    this(new TreeMap<E,Object>());
}

/**
 * Constructs a new, empty tree set, sorted according to the specified
 * comparator.  All elements inserted into the set must be <i>mutually
 * comparable</i> by the specified comparator: {@code comparator.compare(e1,
 * e2)} must not throw a {@code ClassCastException} for any elements
 * {@code e1} and {@code e2} in the set.  If the user attempts to add
 * an element to the set that violates this constraint, the
 * {@code add} call will throw a {@code ClassCastException}.
 *
 * @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 TreeSet(Comparator<? super E> comparator) {
    this(new TreeMap<>(comparator));
}

/**
 * Constructs a new tree set containing the elements in the specified
 * collection, sorted according to the <i>natural ordering</i> of its
 * elements.  All elements inserted into the set must implement the
 * {@link Comparable} interface.  Furthermore, all such elements must be
 * <i>mutually comparable</i>: {@code e1.compareTo(e2)} must not throw a
 * {@code ClassCastException} for any elements {@code e1} and
 * {@code e2} in the set.
 *
 * @param c collection whose elements 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 is null
 */
public TreeSet(Collection<? extends E> c) {
    this();
    addAll(c);
}

/**
 * Constructs a new tree 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 is null
 */
public TreeSet(SortedSet<E> s) {
    this(s.comparator());
    addAll(s);
}

void foo(Set s) {
    Set copy = new LinkedHashSet(s);
    ...
}