vector与ArrayList、hashmap与hashtable区别

package java.util;
public class Vector<E>
    extends AbstractList<E>
    implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
   
    protected Object[] elementData;
    protected int elementCount;
    protected int capacityIncrement;
    private static final long serialVersionUID = -2767605614048989439L;
    public Vector(int initialCapacity, int capacityIncrement) {
	super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
	this.elementData = new Object[initialCapacity];
	this.capacityIncrement = capacityIncrement;
    }
 
    public Vector(int initialCapacity) {
	this(initialCapacity, 0);
    }
    public Vector() {
	this(10);
    }
 
    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);
    }
 
    public synchronized void copyInto(Object[] anArray) {
	System.arraycopy(elementData, 0, anArray, 0, elementCount);
    }
    /**
     * Trims the capacity of this vector to be the vector‘s current
     * size. If the capacity of this vector is larger than its current
     * size, then the capacity is changed to equal the size by replacing
     * its internal data array, kept in the field {@code elementData},
     * with a smaller one. An application can use this operation to
     * minimize the storage of a vector.
     */
    public synchronized void trimToSize() {
	modCount++;
	int oldCapacity = elementData.length;
	if (elementCount < oldCapacity) {
            elementData = Arrays.copyOf(elementData, elementCount);
	}
    }
  
    public synchronized void ensureCapacity(int minCapacity) {
	modCount++;
	ensureCapacityHelper(minCapacity);
    }
    /**
     * This implements the unsynchronized semantics of ensureCapacity.
     * Synchronized methods in this class can internally call this
     * method for ensuring capacity without incurring the cost of an
     * extra synchronization.
     *
     * @see #ensureCapacity(int)
     */
    private void ensureCapacityHelper(int minCapacity) {
	int oldCapacity = elementData.length;
	if (minCapacity > oldCapacity) {
	    Object[] oldData = elementData;
	    int newCapacity = (capacityIncrement > 0) ?
		(oldCapacity + capacityIncrement) : (oldCapacity * 2);
    	    if (newCapacity < minCapacity) {
		newCapacity = minCapacity;
	    }
            elementData = Arrays.copyOf(elementData, newCapacity);
	}
    }
    /**
     * Sets the size of this vector. If the new size is greater than the
     * current size, new {@code null} items are added to the end of
     * the vector. If the new size is less than the current size, all
     * components at index {@code newSize} and greater are discarded.
     *
     * @param  newSize   the new size of this vector
     * @throws ArrayIndexOutOfBoundsException if the new size is negative
     */
    public synchronized void setSize(int newSize) {
	modCount++;
	if (newSize > elementCount) {
	    ensureCapacityHelper(newSize);
	} else {
	    for (int i = newSize ; i < elementCount ; i++) {
		elementData[i] = null;
	    }
	}
	elementCount = newSize;
    }
    /**
     * Returns the current capacity of this vector.
     *
     * @return  the current capacity (the length of its internal
     *          data array, kept in the field {@code elementData}
     *          of this vector)
     */
    public synchronized int capacity() {
	return elementData.length;
    }
    /**
     * Returns the number of components in this vector.
     *
     * @return  the number of components in this vector
     */
    public synchronized int size() {
	return elementCount;
    }
    /**
     * Tests if this vector has no components.
     *
     * @return  {@code true} if and only if this vector has
     *          no components, that is, its size is zero;
     *          {@code false} otherwise.
     */
    public synchronized boolean isEmpty() {
	return elementCount == 0;
    }
    /**
     * Returns an enumeration of the components of this vector. The
     * returned {@code Enumeration} object will generate all items in
     * this vector. The first item generated is the item at index {@code 0},
     * then the item at index {@code 1}, and so on.
     *
     * @return  an enumeration of the components of this vector
     * @see     Iterator
     */
    public Enumeration<E> elements() {
	return new Enumeration<E>() {
	    int count = 0;
	    public boolean hasMoreElements() {
		return count < elementCount;
	    }
	    public E nextElement() {
		synchronized (Vector.this) {
		    if (count < elementCount) {
			return (E)elementData[count++];
		    }
		}
		throw new NoSuchElementException("Vector Enumeration");
	    }
	};
    }
    /**
     * Returns {@code true} if this vector contains the specified element.
     * More formally, returns {@code true} if and only if this vector
     * contains at least one element {@code e} such that
     * <tt>(o==null ? e==null : o.equals(e))</tt>.
     *
     * @param o element whose presence in this vector is to be tested
     * @return {@code true} if this vector contains the specified element
     */
    public boolean contains(Object o) {
	return indexOf(o, 0) >= 0;
    }
    /**
     * Returns the index of the first occurrence of the specified element
     * in this vector, or -1 if this vector does not contain the element.
     * More formally, returns the lowest index {@code i} such that
     * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @return the index of the first occurrence of the specified element in
     *         this vector, or -1 if this vector does not contain the element
     */
    public int indexOf(Object o) {
	return indexOf(o, 0);
    }
    /**
     * Returns the index of the first occurrence of the specified element in
     * this vector, searching forwards from {@code index}, or returns -1 if
     * the element is not found.
     * More formally, returns the lowest index {@code i} such that
     * <tt>(i >= index && (o==null ? get(i)==null : o.equals(get(i))))</tt>,
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @param index index to start searching from
     * @return the index of the first occurrence of the element in
     *         this vector at position {@code index} or later in the vector;
     *         {@code -1} if the element is not found.
     * @throws IndexOutOfBoundsException if the specified index is negative
     * @see     Object#equals(Object)
     */
    public synchronized int indexOf(Object o, int index) {
	if (o == null) {
	    for (int i = index ; i < elementCount ; i++)
		if (elementData[i]==null)
		    return i;
	} else {
	    for (int i = index ; i < elementCount ; i++)
		if (o.equals(elementData[i]))
		    return i;
	}
	return -1;
    }
    /**
     * Returns the index of the last occurrence of the specified element
     * in this vector, or -1 if this vector does not contain the element.
     * More formally, returns the highest index {@code i} such that
     * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @return the index of the last occurrence of the specified element in
     *         this vector, or -1 if this vector does not contain the element
     */
    public synchronized int lastIndexOf(Object o) {
	return lastIndexOf(o, elementCount-1);
    }
    /**
     * Returns the index of the last occurrence of the specified element in
     * this vector, searching backwards from {@code index}, or returns -1 if
     * the element is not found.
     * More formally, returns the highest index {@code i} such that
     * <tt>(i <= index && (o==null ? get(i)==null : o.equals(get(i))))</tt>,
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @param index index to start searching backwards from
     * @return the index of the last occurrence of the element at position
     *         less than or equal to {@code index} in this vector;
     *         -1 if the element is not found.
     * @throws IndexOutOfBoundsException if the specified index is greater
     *         than or equal to the current size of this vector
     */
    public synchronized int lastIndexOf(Object o, int index) {
        if (index >= elementCount)
            throw new IndexOutOfBoundsException(index + " >= "+ elementCount);
	if (o == null) {
	    for (int i = index; i >= 0; i--)
		if (elementData[i]==null)
		    return i;
	} else {
	    for (int i = index; i >= 0; i--)
		if (o.equals(elementData[i]))
		    return i;
	}
	return -1;
    }
    /**
     * Returns the component at the specified index.
     *
     * <p>This method is identical in functionality to the {@link #get(int)}
     * method (which is part of the {@link List} interface).
     *
     * @param      index   an index into this vector
     * @return     the component at the specified index
     * @throws ArrayIndexOutOfBoundsException if the index is out of range
     *	       ({@code index < 0 || index >= size()})
     */
    public synchronized E elementAt(int index) {
	if (index >= elementCount) {
	    throw new ArrayIndexOutOfBoundsException(index + " >= " + elementCount);
	}
        return (E)elementData[index];
    }
    /**
     * Returns the first component (the item at index {@code 0}) of
     * this vector.
     *
     * @return     the first component of this vector
     * @throws NoSuchElementException if this vector has no components
     */
    public synchronized E firstElement() {
	if (elementCount == 0) {
	    throw new NoSuchElementException();
	}
	return (E)elementData[0];
    }
    /**
     * Returns the last component of the vector.
     *
     * @return  the last component of the vector, i.e., the component at index
     *          <code>size() - 1</code>.
     * @throws NoSuchElementException if this vector is empty
     */
    public synchronized E lastElement() {
	if (elementCount == 0) {
	    throw new NoSuchElementException();
	}
	return (E)elementData[elementCount - 1];
    }
    /**
     * Sets the component at the specified {@code index} of this
     * vector to be the specified object. The previous component at that
     * position is discarded.
     *
     * <p>The index must be a value greater than or equal to {@code 0}
     * and less than the current size of the vector.
     *
     * <p>This method is identical in functionality to the
     * {@link #set(int, Object) set(int, E)}
     * method (which is part of the {@link List} interface). Note that the
     * {@code set} method reverses the order of the parameters, to more closely
     * match array usage.  Note also that the {@code set} method returns the
     * old value that was stored at the specified position.
     *
     * @param      obj     what the component is to be set to
     * @param      index   the specified index
     * @throws ArrayIndexOutOfBoundsException if the index is out of range
     *	       ({@code index < 0 || index >= size()})
     */
    public synchronized void setElementAt(E obj, int index) {
	if (index >= elementCount) {
	    throw new ArrayIndexOutOfBoundsException(index + " >= " +
						     elementCount);
	}
	elementData[index] = obj;
    }
    public synchronized void removeElementAt(int index) {
	modCount++;
	if (index >= elementCount) {
	    throw new ArrayIndexOutOfBoundsException(index + " >= " +
						     elementCount);
	}
	else if (index < 0) {
	    throw new ArrayIndexOutOfBoundsException(index);
	}
	int j = elementCount - index - 1;
	if (j > 0) {
	    System.arraycopy(elementData, index + 1, elementData, index, j);
	}
	elementCount--;
	elementData[elementCount] = null; /* to let gc do its work */
    }
    /**
     * Inserts the specified object as a component in this vector at the
     * specified {@code index}. Each component in this vector with
     * an index greater or equal to the specified {@code index} is
     * shifted upward to have an index one greater than the value it had
     * previously.
     *
     * <p>The index must be a value greater than or equal to {@code 0}
     * and less than or equal to the current size of the vector. (If the
     * index is equal to the current size of the vector, the new element
     * is appended to the Vector.)
     *
     * <p>This method is identical in functionality to the
     * {@link #add(int, Object) add(int, E)}
     * method (which is part of the {@link List} interface).  Note that the
     * {@code add} method reverses the order of the parameters, to more closely
     * match array usage.
     *
     * @param      obj     the component to insert
     * @param      index   where to insert the new component
     * @throws ArrayIndexOutOfBoundsException if the index is out of range
     *	       ({@code index < 0 || index > size()})
     */
    public synchronized void insertElementAt(E obj, int index) {
	modCount++;
	if (index > elementCount) {
	    throw new ArrayIndexOutOfBoundsException(index
						     + " > " + elementCount);
	}
	ensureCapacityHelper(elementCount + 1);
	System.arraycopy(elementData, index, elementData, index + 1, elementCount - index);
	elementData[index] = obj;
	elementCount++;
    }
    /**
     * Adds the specified component to the end of this vector,
     * increasing its size by one. The capacity of this vector is
     * increased if its size becomes greater than its capacity.
     *
     * <p>This method is identical in functionality to the
     * {@link #add(Object) add(E)}
     * method (which is part of the {@link List} interface).
     *
     * @param   obj   the component to be added
     */
    public synchronized void addElement(E obj) {
	modCount++;
	ensureCapacityHelper(elementCount + 1);
	elementData[elementCount++] = obj;
    }
    /**
     * Removes the first (lowest-indexed) occurrence of the argument
     * from this vector. If the object is found in this vector, each
     * component in the vector with an index greater or equal to the
     * object‘s index is shifted downward to have an index one smaller
     * than the value it had previously.
     *
     * <p>This method is identical in functionality to the
     * {@link #remove(Object)} method (which is part of the
     * {@link List} interface).
     *
     * @param   obj   the component to be removed
     * @return  {@code true} if the argument was a component of this
     *          vector; {@code false} otherwise.
     */
    public synchronized boolean removeElement(Object obj) {
	modCount++;
	int i = indexOf(obj);
	if (i >= 0) {
	    removeElementAt(i);
	    return true;
	}
	return false;
    }
    /**
     * Removes all components from this vector and sets its size to zero.
     *
     * <p>This method is identical in functionality to the {@link #clear}
     * method (which is part of the {@link List} interface).
     */
    public synchronized void removeAllElements() {
        modCount++;
	// Let gc do its work
	for (int i = 0; i < elementCount; i++)
	    elementData[i] = null;
	elementCount = 0;
    }
    /**
     * Returns a clone of this vector. The copy will contain a
     * reference to a clone of the internal data array, not a reference
     * to the original internal data array of this {@code Vector} object.
     *
     * @return  a clone of this vector
     */
    public synchronized Object clone() {
	try {
	    Vector<E> v = (Vector<E>) super.clone();
	    v.elementData = Arrays.copyOf(elementData, elementCount);
	    v.modCount = 0;
	    return v;
	} catch (CloneNotSupportedException e) {
	    // this shouldn‘t happen, since we are Cloneable
	    throw new InternalError();
	}
    }
    /**
     * Returns an array containing all of the elements in this Vector
     * in the correct order.
     *
     * @since 1.2
     */
    public synchronized Object[] toArray() {
        return Arrays.copyOf(elementData, elementCount);
    }
    public synchronized <T> T[] toArray(T[] a) {
        if (a.length < elementCount)
            return (T[]) Arrays.copyOf(elementData, elementCount, a.getClass());
	System.arraycopy(elementData, 0, a, 0, elementCount);
        if (a.length > elementCount)
            a[elementCount] = null;
        return a;
    }
    // Positional Access Operations
    /**
     * Returns the element at the specified position in this Vector.
     *
     * @param index index of the element to return
     * @return object at the specified index
     * @throws ArrayIndexOutOfBoundsException if the index is out of range
     *            ({@code index < 0 || index >= size()})
     * @since 1.2
     */
    public synchronized E get(int index) {
	if (index >= elementCount)
	    throw new ArrayIndexOutOfBoundsException(index);
	return (E)elementData[index];
    }
    /**
     * Replaces the element at the specified position in this Vector with the
     * specified element.
     *
     * @param index index of the element to replace
     * @param element element to be stored at the specified position
     * @return the element previously at the specified position
     * @throws ArrayIndexOutOfBoundsException if the index is out of range
     *	       ({@code index < 0 || index >= size()})
     * @since 1.2
     */
    public synchronized E set(int index, E element) {
	if (index >= elementCount)
	    throw new ArrayIndexOutOfBoundsException(index);
	Object oldValue = elementData[index];
	elementData[index] = element;
	return (E)oldValue;
    }
    /**
     * Appends the specified element to the end of this Vector.
     *
     * @param e element to be appended to this Vector
     * @return {@code true} (as specified by {@link Collection#add})
     * @since 1.2
     */
    public synchronized boolean add(E e) {
	modCount++;
	ensureCapacityHelper(elementCount + 1);
	elementData[elementCount++] = e;
        return true;
    }
    public boolean remove(Object o) {
        return removeElement(o);
    }
    public void add(int index, E element) {
        insertElementAt(element, index);
    }
    public synchronized E remove(int index) {
	modCount++;
	if (index >= elementCount)
	    throw new ArrayIndexOutOfBoundsException(index);
	Object oldValue = elementData[index];
	int numMoved = elementCount - index - 1;
	if (numMoved > 0)
	    System.arraycopy(elementData, index+1, elementData, index,
			     numMoved);
	elementData[--elementCount] = null; // Let gc do its work
	return (E)oldValue;
    }
    public void clear() {
        removeAllElements();
    }
    // Bulk Operations
 
    public synchronized boolean containsAll(Collection<?> c) {
        return super.containsAll(c);
    }
    public synchronized boolean addAll(Collection<? extends E> c) {
	modCount++;
        Object[] a = c.toArray();
        int numNew = a.length;
	ensureCapacityHelper(elementCount + numNew);
        System.arraycopy(a, 0, elementData, elementCount, numNew);
        elementCount += numNew;
	return numNew != 0;
    }
    public synchronized boolean removeAll(Collection<?> c) {
        return super.removeAll(c);
    }
    public synchronized boolean retainAll(Collection<?> c)  {
        return super.retainAll(c);
    }
    public synchronized boolean addAll(int index, Collection<? extends E> c) {
	modCount++;
	if (index < 0 || index > elementCount)
	    throw new ArrayIndexOutOfBoundsException(index);
        Object[] a = c.toArray();
	int numNew = a.length;
	ensureCapacityHelper(elementCount + numNew);
	int numMoved = elementCount - index;
	if (numMoved > 0)
	    System.arraycopy(elementData, index, elementData, index + numNew,
			     numMoved);
        System.arraycopy(a, 0, elementData, index, numNew);
	elementCount += numNew;
	return numNew != 0;
    }
    /**
     * Compares the specified Object with this Vector for equality.  Returns
     * true if and only if the specified Object is also a List, both Lists
     * have the same size, and all corresponding pairs of elements in the two
     * Lists are <em>equal</em>.  (Two elements {@code e1} and
     * {@code e2} are <em>equal</em> if {@code (e1==null ? e2==null :
     * e1.equals(e2))}.)  In other words, two Lists are defined to be
     * equal if they contain the same elements in the same order.
     *
     * @param o the Object to be compared for equality with this Vector
     * @return true if the specified Object is equal to this Vector
     */
    public synchronized boolean equals(Object o) {
        return super.equals(o);
    }
    /**
     * Returns the hash code value for this Vector.
     */
    public synchronized int hashCode() {
        return super.hashCode();
    }
    /**
     * Returns a string representation of this Vector, containing
     * the String representation of each element.
     */
    public synchronized String toString() {
        return super.toString();
    }
    public synchronized List<E> subList(int fromIndex, int toIndex) {
        return Collections.synchronizedList(super.subList(fromIndex, toIndex),
                                            this);
    }
    protected synchronized void removeRange(int fromIndex, int toIndex) {
	modCount++;
	int numMoved = elementCount - toIndex;
        System.arraycopy(elementData, toIndex, elementData, fromIndex,
                         numMoved);
	// Let gc do its work
	int newElementCount = elementCount - (toIndex-fromIndex);
	while (elementCount != newElementCount)
	    elementData[--elementCount] = null;
    }
    private synchronized void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException
    {
	s.defaultWriteObject();
    }
}

package java.util;
import java.io.*;
public class HashMap<K,V>
    extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable
{
    /**
     * The default initial capacity - MUST be a power of two.
     */
    static final int DEFAULT_INITIAL_CAPACITY = 16;
    /**
     * The maximum capacity, used if a higher value is implicitly specified
     * by either of the constructors with arguments.
     * MUST be a power of two <= 1<<30.
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;
    /**
     * The load factor used when none specified in constructor.
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;
    /**
     * The table, resized as necessary. Length MUST Always be a power of two.
     */
    transient Entry[] table;
    /**
     * The number of key-value mappings contained in this map.
     */
    transient int size;
    /**
     * The next size value at which to resize (capacity * load factor).
     * @serial
     */
    int threshold;
    /**
     * The load factor for the hash table.
     *
     * @serial
     */
    final float loadFactor;
    /**
     * The number of times this HashMap has been structurally modified
     * Structural modifications are those that change the number of mappings in
     * the HashMap or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the HashMap fail-fast.  (See ConcurrentModificationException).
     */
    transient volatile int modCount;
    /**
     * 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);
        // Find a power of 2 >= initialCapacity
        int capacity = 1;
        while (capacity < initialCapacity)
            capacity <<= 1;
        this.loadFactor = loadFactor;
        threshold = (int)(capacity * loadFactor);
        table = new Entry[capacity];
        init();
    }
    /**
     * 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;
        threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
        table = new Entry[DEFAULT_INITIAL_CAPACITY];
        init();
    }
    /**
     * 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(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
                      DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
        putAllForCreate(m);
    }
    // internal utilities
    /**
     * Initialization hook for subclasses. This method is called
     * in all constructors and pseudo-constructors (clone, readObject)
     * after HashMap has been initialized but before any entries have
     * been inserted.  (In the absence of this method, readObject would
     * require explicit knowledge of subclasses.)
     */
    void init() {
    }
    /**
     * Applies a supplemental hash function to a given hashCode, which
     * defends against poor quality hash functions.  This is critical
     * because HashMap uses power-of-two length hash tables, that
     * otherwise encounter collisions for hashCodes that do not differ
     * in lower bits. Note: Null keys always map to hash 0, thus index 0.
     */
    static int hash(int h) {
        // This function ensures that hashCodes that differ only by
        // constant multiples at each bit position have a bounded
        // number of collisions (approximately 8 at default load factor).
        h ^= (h >>> 20) ^ (h >>> 12);
        return h ^ (h >>> 7) ^ (h >>> 4);
    }
    /**
     * Returns index for hash code h.
     */
    static int indexFor(int h, int length) {
        return h & (length-1);
    }
    /**
     * Returns the number of key-value mappings in this map.
     *
     * @return the number of key-value mappings in this map
     */
    public int size() {
        return size;
    }
    /**
     * Returns <tt>true</tt> if this map contains no key-value mappings.
     *
     * @return <tt>true</tt> if this map contains no key-value mappings
     */
    public boolean isEmpty() {
        return size == 0;
    }
    /**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
     * key.equals(k))}, then this method returns {@code v}; otherwise
     * it returns {@code null}.  (There can be at most one such mapping.)
     *
     * <p>A return value of {@code null} does not <i>necessarily</i>
     * indicate that the map contains no mapping for the key; it‘s also
     * possible that the map explicitly maps the key to {@code null}.
     * The {@link #containsKey containsKey} operation may be used to
     * distinguish these two cases.
     *
     * @see #put(Object, Object)
     */
    public V get(Object key) {
        if (key == null)
            return getForNullKey();
        int hash = hash(key.hashCode());
        for (Entry<K,V> e = table[indexFor(hash, table.length)];
             e != null;
             e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k)))
                return e.value;
        }
        return null;
    }
    /**
     * Offloaded version of get() to look up null keys.  Null keys map
     * to index 0.  This null case is split out into separate methods
     * for the sake of performance in the two most commonly used
     * operations (get and put), but incorporated with conditionals in
     * others.
     */
    private V getForNullKey() {
        for (Entry<K,V> e = table[0]; e != null; e = e.next) {
            if (e.key == null)
                return e.value;
        }
        return null;
    }
    /**
     * Returns <tt>true</tt> if this map contains a mapping for the
     * specified key.
     *
     * @param   key   The key whose presence in this map is to be tested
     * @return <tt>true</tt> if this map contains a mapping for the specified
     * key.
     */
    public boolean containsKey(Object key) {
        return getEntry(key) != null;
    }
    /**
     * Returns the entry associated with the specified key in the
     * HashMap.  Returns null if the HashMap contains no mapping
     * for the key.
     */
    final Entry<K,V> getEntry(Object key) {
        int hash = (key == null) ? 0 : hash(key.hashCode());
        for (Entry<K,V> e = table[indexFor(hash, table.length)];
             e != null;
             e = e.next) {
            Object k;
            if (e.hash == hash &&
                ((k = e.key) == key || (key != null && key.equals(k))))
                return e;
        }
        return null;
    }
    /**
     * Associates the specified value with the specified key in this map.
     * If the map previously contained a mapping for the key, the old
     * value is replaced.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
     *         (A <tt>null</tt> return can also indicate that the map
     *         previously associated <tt>null</tt> with <tt>key</tt>.)
     */
    public V put(K key, V value) {
        if (key == null)
            return putForNullKey(value);
        int hash = hash(key.hashCode());
        int i = indexFor(hash, table.length);
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }
        modCount++;
        addEntry(hash, key, value, i);
        return null;
    }
    /**
     * Offloaded version of put for null keys
     */
    private V putForNullKey(V value) {
        for (Entry<K,V> e = table[0]; e != null; e = e.next) {
            if (e.key == null) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }
        modCount++;
        addEntry(0, null, value, 0);
        return null;
    }
    // These methods are used when serializing HashSets
    int   capacity()     { return table.length; }
    float loadFactor()   { return loadFactor;   }
}

public class Hashtable<K,V>
    extends Dictionary<K,V>
    implements Map<K,V>, Cloneable, java.io.Serializable {
    /**
     * The hash table data.
     */
    private transient Entry[] table;
    /**
     * The total number of entries in the hash table.
     */
    private transient int count;
    /**
     * The table is rehashed when its size exceeds this threshold.  (The
     * value of this field is (int)(capacity * loadFactor).)
     *
     * @serial
     */
    private int threshold;
    /**
     * The load factor for the hashtable.
     *
     * @serial
     */
    private float loadFactor;
    /**
     * The number of times this Hashtable has been structurally modified
     * Structural modifications are those that change the number of entries in
     * the Hashtable or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the Hashtable fail-fast.  (See ConcurrentModificationException).
     */
    private transient int modCount = 0;
    /** use serialVersionUID from JDK 1.0.2 for interoperability */
    private static final long serialVersionUID = 1421746759512286392L;
    /**
     * 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)(initialCapacity * loadFactor);
    }
    /**
     * 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);
    }
    /**
     * Returns the number of keys in this hashtable.
     *
     * @return  the number of keys in this hashtable.
     */
    public synchronized int size() {
	return count;
    }
    /**
     * Tests if this hashtable maps no keys to values.
     *
     * @return  <code>true</code> if this hashtable maps no keys to values;
     *          <code>false</code> otherwise.
     */
    public synchronized boolean isEmpty() {
	return count == 0;
    }
    /**
     * Returns an enumeration of the keys in this hashtable.
     *
     * @return  an enumeration of the keys in this hashtable.
     * @see     Enumeration
     * @see     #elements()
     * @see	#keySet()
     * @see	Map
     */
    public synchronized Enumeration<K> keys() {
	return this.<K>getEnumeration(KEYS);
    }