vector与ArrayList、hashmap与hashtable区别

一、vector与ArrayList区别

    首先要说明的是vector和arraylist都是list的实现类，都是代表链表的数据结构。

    java.util.Vector;  类中

1. package java.util;
   publicclassVector<E>
   extendsAbstractList<E>
   implementsList<E>,RandomAccess,Cloneable, java.io.Serializable
   {
   protectedObject[] elementData;
   protectedint elementCount;
   protectedint capacityIncrement;
   privatestaticfinallong serialVersionUID =-2767605614048989439L;
   publicVector(int initialCapacity,int capacityIncrement){
   super();
   if(initialCapacity <0)
   thrownewIllegalArgumentException("Illegal Capacity: "+
   initialCapacity);
   this.elementData =newObject[initialCapacity];
   this.capacityIncrement = capacityIncrement;
   }
   publicVector(int initialCapacity){
   this(initialCapacity,0);
   }
   publicVector(){
   this(10);
   }
   publicVector(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);
   }
   publicsynchronizedvoid 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.
   */
   publicsynchronizedvoid trimToSize(){
   modCount++;
   int oldCapacity = elementData.length;
   if(elementCount < oldCapacity){
   elementData =Arrays.copyOf(elementData, elementCount);
   }
   }
   publicsynchronizedvoid 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)
   */
   privatevoid 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
   */
   publicsynchronizedvoid 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)
   */
   publicsynchronizedint capacity(){
   return elementData.length;
   }
   /**
   * Returns the number of components in this vector.
   *
   * @return the number of components in this vector
   */
   publicsynchronizedint 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.
   */
   publicsynchronizedboolean 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
   */
   publicEnumeration<E> elements(){
   returnnewEnumeration<E>(){
   int count =0;
   publicboolean hasMoreElements(){
   return count < elementCount;
   }
   public E nextElement(){
   synchronized(Vector.this){
   if(count < elementCount){
   return(E)elementData[count++];
   }
   }
   thrownewNoSuchElementException("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
   */
   publicboolean 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
   */
   publicint 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)
   */
   publicsynchronizedint 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
   */
   publicsynchronizedint 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
   */
   publicsynchronizedint lastIndexOf(Object o,int index){
   if(index >= elementCount)
   thrownewIndexOutOfBoundsException(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()})
   */
   publicsynchronized E elementAt(int index){
   if(index >= elementCount){
   thrownewArrayIndexOutOfBoundsException(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
   */
   publicsynchronized E firstElement(){
   if(elementCount ==0){
   thrownewNoSuchElementException();
   }
   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
   */
   publicsynchronized E lastElement(){
   if(elementCount ==0){
   thrownewNoSuchElementException();
   }
   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()})
   */
   publicsynchronizedvoid setElementAt(E obj,int index){
   if(index >= elementCount){
   thrownewArrayIndexOutOfBoundsException(index +" >= "+
   elementCount);
   }
   elementData[index]= obj;
   }
   publicsynchronizedvoid removeElementAt(int index){
   modCount++;
   if(index >= elementCount){
   thrownewArrayIndexOutOfBoundsException(index +" >= "+
   elementCount);
   }
   elseif(index <0){
   thrownewArrayIndexOutOfBoundsException(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()})
   */
   publicsynchronizedvoid insertElementAt(E obj,int index){
   modCount++;
   if(index > elementCount){
   thrownewArrayIndexOutOfBoundsException(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
   */
   publicsynchronizedvoid 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.
   */
   publicsynchronizedboolean removeElement(Object obj){
   modCount++;
   int i = indexOf(obj);
   if(i >=0){
   removeElementAt(i);
   returntrue;
   }
   returnfalse;
   }
   /**
   * 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).
   */
   publicsynchronizedvoid 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
   */
   publicsynchronizedObject 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
   thrownewInternalError();
   }
   }
   /**
   * Returns an array containing all of the elements in this Vector
   * in the correct order.
   *
   * @since 1.2
   */
   publicsynchronizedObject[] toArray(){
   returnArrays.copyOf(elementData, elementCount);
   }
   publicsynchronized<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
   */
   publicsynchronized E get(int index){
   if(index >= elementCount)
   thrownewArrayIndexOutOfBoundsException(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
   */
   publicsynchronized E set(int index, E element){
   if(index >= elementCount)
   thrownewArrayIndexOutOfBoundsException(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
   */
   publicsynchronizedboolean add(E e){
   modCount++;
   ensureCapacityHelper(elementCount +1);
   elementData[elementCount++]= e;
   returntrue;
   }
   publicboolean remove(Object o){
   return removeElement(o);
   }
   publicvoid add(int index, E element){
   insertElementAt(element, index);
   }
   publicsynchronized E remove(int index){
   modCount++;
   if(index >= elementCount)
   thrownewArrayIndexOutOfBoundsException(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;
   }
   publicvoid clear(){
   removeAllElements();
   }
   // Bulk Operations
   publicsynchronizedboolean containsAll(Collection<?> c){
   returnsuper.containsAll(c);
   }
   publicsynchronizedboolean 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;
   }
   publicsynchronizedboolean removeAll(Collection<?> c){
   returnsuper.removeAll(c);
   }
   publicsynchronizedboolean retainAll(Collection<?> c){
   returnsuper.retainAll(c);
   }
   publicsynchronizedboolean addAll(int index,Collection<?extends E> c){
   modCount++;
   if(index <0|| index > elementCount)
   thrownewArrayIndexOutOfBoundsException(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
   */
   publicsynchronizedboolean equals(Object o){
   returnsuper.equals(o);
   }
   /**
   * Returns the hash code value for this Vector.
   */
   publicsynchronizedint hashCode(){
   returnsuper.hashCode();
   }
   /**
   * Returns a string representation of this Vector, containing
   * the String representation of each element.
   */
   publicsynchronizedString toString(){
   returnsuper.toString();
   }
   publicsynchronizedList<E> subList(int fromIndex,int toIndex){
   returnCollections.synchronizedList(super.subList(fromIndex, toIndex),
   this);
   }
   protectedsynchronizedvoid 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;
   }
   privatesynchronizedvoid writeObject(java.io.ObjectOutputStream s)
   throws java.io.IOException
   {
   s.defaultWriteObject();
   }
   }

    说明：vector中使用到了synchronized关键字，也就是说vector是线程安全的！同理可知ArrayList是线程不安全的；

                所以vector中操作成员的方法必须保证同步才行，所以效率上就没有ArrayList的高；

                一般情况下，需要保持同步的情况下才使用vector，多数情况下使用ArrayList。

二、hashmap与hashtable

    同vector与arrayList一样，hashmap与hashtable也是相同的道理

看看HashMap的类

    package java.util;
    import java.io.*;
    publicclassHashMap<K,V>
    extendsAbstractMap<K,V>
    implementsMap<K,V>,Cloneable,Serializable
    {
    /**
    * The default initial capacity - MUST be a power of two.
    */
    staticfinalint 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.
    */
    staticfinalint MAXIMUM_CAPACITY =1<<30;
    /**
    * The load factor used when none specified in constructor.
    */
    staticfinalfloat DEFAULT_LOAD_FACTOR =0.75f;
    /**
    * The table, resized as necessary. Length MUST Always be a power of two.
    */
    transientEntry[] table;
    /**
    * The number of key-value mappings contained in this map.
    */
    transientint size;
    /**
    * The next size value at which to resize (capacity * load factor).
    * @serial
    */
    int threshold;
    /**
    * The load factor for the hash table.
    *
    * @serial
    */
    finalfloat 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).
    */
    transientvolatileint 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
    */
    publicHashMap(int initialCapacity,float loadFactor){
    if(initialCapacity <0)
    thrownewIllegalArgumentException("Illegal initial capacity: "+
    initialCapacity);
    if(initialCapacity > MAXIMUM_CAPACITY)
    initialCapacity = MAXIMUM_CAPACITY;
    if(loadFactor <=0||Float.isNaN(loadFactor))
    thrownewIllegalArgumentException("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 =newEntry[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.
    */
    publicHashMap(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).
    */
    publicHashMap(){
    this.loadFactor = DEFAULT_LOAD_FACTOR;
    threshold =(int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
    table =newEntry[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
    */
    publicHashMap(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.
    */
    staticint 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.
    */
    staticint 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
    */
    publicint 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
    */
    publicboolean 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;
    }
    returnnull;
    }
    /**
    * 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;
    }
    returnnull;
    }
    /**
    * 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.
    */
    publicboolean 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.
    */
    finalEntry<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;
    }
    returnnull;
    }
    /**
    * 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);
    returnnull;
    }
    /**
    * 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);
    returnnull;
    }
    // These methods are used when serializing HashSets
    int capacity(){return table.length;}
    float loadFactor(){return loadFactor;}
    }

HashTable类

    publicclassHashtable<K,V>
    extendsDictionary<K,V>
    implementsMap<K,V>,Cloneable, java.io.Serializable{
    /**
    * The hash table data.
    */
    privatetransientEntry[] table;
    /**
    * The total number of entries in the hash table.
    */
    privatetransientint count;
    /**
    * The table is rehashed when its size exceeds this threshold. (The
    * value of this field is (int)(capacity * loadFactor).)
    *
    * @serial
    */
    privateint threshold;
    /**
    * The load factor for the hashtable.
    *
    * @serial
    */
    privatefloat 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).
    */
    privatetransientint modCount =0;
    /** use serialVersionUID from JDK 1.0.2 for interoperability */
    privatestaticfinallong 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.
    */
    publicHashtable(int initialCapacity,float loadFactor){
    if(initialCapacity <0)
    thrownewIllegalArgumentException("Illegal Capacity: "+
    initialCapacity);
    if(loadFactor <=0||Float.isNaN(loadFactor))
    thrownewIllegalArgumentException("Illegal Load: "+loadFactor);
    if(initialCapacity==0)
    initialCapacity =1;
    this.loadFactor = loadFactor;
    table =newEntry[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.
    */
    publicHashtable(int initialCapacity){
    this(initialCapacity,0.75f);
    }
    /**
    * Constructs a new, empty hashtable with a default initial capacity (11)
    * and load factor (0.75).
    */
    publicHashtable(){
    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
    */
    publicHashtable(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.
    */
    publicsynchronizedint 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.
    */
    publicsynchronizedboolean 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
    */
    publicsynchronizedEnumeration<K> keys(){
    returnthis.<K>getEnumeration(KEYS);
    }

1.     HashTable的方法是同步的，HashMap不能同步，所以在线程多的场合要使用HashTable；
   
2. HashTable不允许空值（key和value都不可以为null），hashmap则可以为null
3. HashTable有一个contains()的方法，功能和containsValue()一样
4. HashTable使用Enumeration，而HashMap使用Iterator
5. HashTabe中的hash数组默认大小为11，增长方式为old*2+1，hashmap的hash为16，是2的指数倍
6. 哈希值的使用不同，HashTable直接使用对象hashcode，而HashMap会重新计算hash值

 

 

 

 

来自为知笔记(Wiz)