聊聊java中的哪些Map：（五）HashTable与HashMap的区别

/**
 * This class implements a hash table, which maps keys to values. Any
 * non-<code>null</code> object can be used as a key or as a value. <p>
 *
 * To successfully store and retrieve objects from a hashtable, the
 * objects used as keys must implement the <code>hashCode</code>
 * method and the <code>equals</code> method. <p>
 *
 * An instance of <code>Hashtable</code> has two parameters that affect its
 * performance: <i>initial capacity</i> and <i>load factor</i>.  The
 * <i>capacity</i> is the number of <i>buckets</i> in the hash table, and the
 * <i>initial capacity</i> is simply the capacity at the time the hash table
 * is created.  Note that the hash table is <i>open</i>: in the case of a "hash
 * collision", a single bucket stores multiple entries, which must be searched
 * sequentially.  The <i>load factor</i> is a measure of how full the hash
 * table is allowed to get before its capacity is automatically increased.
 * The initial capacity and load factor parameters are merely hints to
 * the implementation.  The exact details as to when and whether the rehash
 * method is invoked are implementation-dependent.<p>
 *
 * Generally, the default load factor (.75) offers a good tradeoff between
 * time and space costs.  Higher values decrease the space overhead but
 * increase the time cost to look up an entry (which is reflected in most
 * <tt>Hashtable</tt> operations, including <tt>get</tt> and <tt>put</tt>).<p>
 *
 * The initial capacity controls a tradeoff between wasted space and the
 * need for <code>rehash</code> operations, which are time-consuming.
 * No <code>rehash</code> operations will <i>ever</i> occur if the initial
 * capacity is greater than the maximum number of entries the
 * <tt>Hashtable</tt> will contain divided by its load factor.  However,
 * setting the initial capacity too high can waste space.<p>
 *
 * If many entries are to be made into a <code>Hashtable</code>,
 * creating it with a sufficiently large capacity may allow the
 * entries to be inserted more efficiently than letting it perform
 * automatic rehashing as needed to grow the table. <p>
 *
 * This example creates a hashtable of numbers. It uses the names of
 * the numbers as keys:
 * <pre>   {@code
 *   Hashtable<String, Integer> numbers
 *     = new Hashtable<String, Integer>();
 *   numbers.put("one", 1);
 *   numbers.put("two", 2);
 *   numbers.put("three", 3);}</pre>
 *
 * <p>To retrieve a number, use the following code:
 * <pre>   {@code
 *   Integer n = numbers.get("two");
 *   if (n != null) {
 *     System.out.println("two = " + n);
 *   }}</pre>
 *
 * <p>The iterators returned by the <tt>iterator</tt> method of the collections
 * returned by all of this class's "collection view methods" are
 * <em>fail-fast</em>: if the Hashtable is structurally modified at any time
 * after the iterator is created, in any way except through the iterator's own
 * <tt>remove</tt> method, the iterator will throw a {@link
 * ConcurrentModificationException}.  Thus, in the face of concurrent
 * modification, the iterator fails quickly and cleanly, rather than risking
 * arbitrary, non-deterministic behavior at an undetermined time in the future.
 * The Enumerations returned by Hashtable's keys and elements methods are
 * <em>not</em> fail-fast.
 *
 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.  Fail-fast iterators
 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness: <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i>
 *
 * <p>As of the Java 2 platform v1.2, this class was retrofitted to
 * implement the {@link Map} interface, making it a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 *
 * Java Collections Framework</a>.  Unlike the new collection
 * implementations, {@code Hashtable} is synchronized.  If a
 * thread-safe implementation is not needed, it is recommended to use
 * {@link HashMap} in place of {@code Hashtable}.  If a thread-safe
 * highly-concurrent implementation is desired, then it is recommended
 * to use {@link java.util.concurrent.ConcurrentHashMap} in place of
 * {@code Hashtable}.
 *
 * @author  Arthur van Hoff
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @see     Object#equals(java.lang.Object)
 * @see     Object#hashCode()
 * @see     Hashtable#rehash()
 * @see     Collection
 * @see     Map
 * @see     HashMap
 * @see     TreeMap
 * @since JDK1.0
 */
public class Hashtable<K,V>
    extends Dictionary<K,V>
    implements Map<K,V>, Cloneable, java.io.Serializable {
    
}

 Hashtable<String, Integer> numbers
     = new Hashtable<String, Integer>();
    numbers.put("one", 1);
    numbers.put("two", 2);
    numbers.put("three", 3);

Integer n = numbers.get("two");
    if (n != null) {
      System.out.println("two = " + n);
    }

private static class Entry<K,V> implements Map.Entry<K,V> {
    final int hash;
    final K key;
    V value;
    Entry<K,V> next;

    protected Entry(int hash, K key, V value, Entry<K,V> next) {
        this.hash = hash;
        this.key =  key;
        this.value = value;
        this.next = next;
    }

    @SuppressWarnings("unchecked")
    protected Object clone() {
        return new Entry<>(hash, key, value,
                              (next==null ? null : (Entry<K,V>) next.clone()));
    }

    // Map.Entry Ops

    public K getKey() {
        return key;
    }

    public V getValue() {
        return value;
    }

    public V setValue(V value) {
        if (value == null)
            throw new NullPointerException();

        V oldValue = this.value;
        this.value = value;
        return oldValue;
    }

    public boolean equals(Object o) {
        if (!(o instanceof Map.Entry))
            return false;
        Map.Entry<?,?> e = (Map.Entry<?,?>)o;

        return (key==null ? e.getKey()==null : key.equals(e.getKey())) &&
           (value==null ? e.getValue()==null : value.equals(e.getValue()));
    }

    public int hashCode() {
        return hash ^ Objects.hashCode(value);
    }

    public String toString() {
        return key.toString()+"="+value.toString();
    }
}

Objects.hashCode(key) ^ Objects.hashCode(value);

return hash ^ Objects.hashCode(value);

hash = key.hashCode();

 (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);

/**
 * 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 maximum size of array to allocate.
 * Some VMs reserve some header words in an array.
 * Attempts to allocate larger arrays may result in
 * OutOfMemoryError: Requested array size exceeds VM limit
 */
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

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);
}

public Hashtable(int initialCapacity) {
    this(initialCapacity, 0.75f);
}

public Hashtable() {
    this(11, 0.75f);
}

public Hashtable(Map<? extends K, ? extends V> t) {
    this(Math.max(2*t.size(), 11), 0.75f);
    putAll(t);
}

public synchronized void putAll(Map<? extends K, ? extends V> t) {
    for (Map.Entry<? extends K, ? extends V> e : t.entrySet())
        put(e.getKey(), e.getValue());
}

/**
 * Increases the capacity of and internally reorganizes this
 * hashtable, in order to accommodate and access its entries more
 * efficiently.  This method is called automatically when the
 * number of keys in the hashtable exceeds this hashtable's capacity
 * and load factor.
 */
@SuppressWarnings("unchecked")
protected void rehash() {
    //原始的table容量及table
    int oldCapacity = table.length;
    Entry<?,?>[] oldMap = table;

    // overflow-conscious code
    //注意其扩容的方式，左移1位+1。
    int newCapacity = (oldCapacity << 1) + 1;
    //如果其容量大于最大值
    if (newCapacity - MAX_ARRAY_SIZE > 0) {
        //如果旧的容量就为最大值，那么说明没必要扩容，直接return
        if (oldCapacity == MAX_ARRAY_SIZE)
            // Keep running with MAX_ARRAY_SIZE buckets
            return;
        //反之扩容到最大值
        newCapacity = MAX_ARRAY_SIZE;
    }
    //创建新的数组
    Entry<?,?>[] newMap = new Entry<?,?>[newCapacity];
    //增加modCount
    modCount++;
    //重新计算门槛阈值
    threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
    table = newMap;
    //遍历数组
    for (int i = oldCapacity ; i-- > 0 ;) {
        //遍历数组上的链表
        for (Entry<K,V> old = (Entry<K,V>)oldMap[i] ; old != null ; ) {
            Entry<K,V> e = old;
            old = old.next;
            //注意这个计算槽位的方法
            int index = (e.hash & 0x7FFFFFFF) % newCapacity;
            e.next = (Entry<K,V>)newMap[index];
            newMap[index] = e;
        }
    }
}

newCapacity = (oldCapacity << 1) + 1;

int index = (e.hash & 0x7FFFFFFF) % newCapacity;

private void addEntry(int hash, K key, V value, int index) {
    //修改modecount
    modCount++;
    //tab指向table
    Entry<?,?> tab[] = table;
    //判断是否需要扩容
    if (count >= threshold) {
        // Rehash the table if the threshold is exceeded
        //扩容
        rehash();

        tab = table;
        hash = key.hashCode();
        //重新计算索引
        index = (hash & 0x7FFFFFFF) % tab.length;
    }

    // Creates the new entry.
    @SuppressWarnings("unchecked")
    //在索引处创建元素 将这个元素添加到链表的末尾
    Entry<K,V> e = (Entry<K,V>) tab[index];
    tab[index] = new Entry<>(hash, key, value, e);
    count++;
}

public synchronized V put(K key, V value) {
    // Make sure the value is not null
    //不支持空的value
    if (value == null) {
        throw new NullPointerException();
    }

    // Makes sure the key is not already in the hashtable.
    Entry<?,?> tab[] = table;
    int hash = key.hashCode();
    //计算索引
    int index = (hash & 0x7FFFFFFF) % tab.length;
    @SuppressWarnings("unchecked")
    Entry<K,V> entry = (Entry<K,V>)tab[index];
    //遍历 判断其key是否已经存在于链表中，如果存在直接返回
    for(; entry != null ; entry = entry.next) {
        if ((entry.hash == hash) && entry.key.equals(key)) {
            V old = entry.value;
            entry.value = value;
            return old;
        }
    }
    //添加元素
    addEntry(hash, key, value, index);
    添加成功返回null否则返回之前的old元素
    return null;
}

public synchronized V remove(Object key) {
    Entry<?,?> tab[] = table;
    int hash = key.hashCode();
    //计算index
    int index = (hash & 0x7FFFFFFF) % tab.length;
    @SuppressWarnings("unchecked")
    Entry<K,V> e = (Entry<K,V>)tab[index];
    //遍历
    for(Entry<K,V> prev = null ; e != null ; prev = e, e = e.next) {
    //用hash和key是否都相等来判断是否存在于HashTable中
        if ((e.hash == hash) && e.key.equals(key)) {
            modCount++;
            if (prev != null) {
            //改变链表
                prev.next = e.next;
            } else {
            //如果在链表末尾 tab中，则改变tab指针
                tab[index] = e.next;
            }
            count--;
            V oldValue = e.value;
            e.value = null;
            //返回被删除的value
            return oldValue;
        }
    }
    //如果不存在则返回null
    return null;
}

public synchronized V get(Object key) {
    Entry<?,?> tab[] = table;
    int hash = key.hashCode();
    //计算index
    int index = (hash & 0x7FFFFFFF) % tab.length;
    //遍历
    for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) {
        //如果hash相同且key相等说明即为所查找的元素
        if ((e.hash == hash) && e.key.equals(key)) {
            return (V)e.value;
        }
    }
    return null;
}

public synchronized void clear() {
    Entry<?,?> tab[] = table;
    modCount++;
    //遍历table，将其都置为null
    for (int index = tab.length; --index >= 0; )
        tab[index] = null;
    count = 0;
}

/**
 * Creates a shallow copy of this hashtable. All the structure of the
 * hashtable itself is copied, but the keys and values are not cloned.
 * This is a relatively expensive operation.
 *
 * @return  a clone of the hashtable
 */
public synchronized Object clone() {
    try {
        //clone对象
        Hashtable<?,?> t = (Hashtable<?,?>)super.clone();
        //创建数组
        t.table = new Entry<?,?>[table.length];
        //如果某个索引不为空则clone
        for (int i = table.length ; i-- > 0 ; ) {
            t.table[i] = (table[i] != null)
                ? (Entry<?,?>) table[i].clone() : null;
        }
        //其他全部置空
        t.keySet = null;
        t.entrySet = null;
        t.values = null;
        t.modCount = 0;
        return t;
    } catch (CloneNotSupportedException e) {
        // this shouldn't happen, since we are Cloneable
        throw new InternalError(e);
    }
}

 @Override
public synchronized boolean replace(K key, V oldValue, V newValue) {
    Objects.requireNonNull(oldValue);
    Objects.requireNonNull(newValue);
    Entry<?,?> tab[] = table;
    int hash = key.hashCode();
    //计算index
    int index = (hash & 0x7FFFFFFF) % tab.length;
    @SuppressWarnings("unchecked")
    Entry<K,V> e = (Entry<K,V>)tab[index];
    //for循环遍历
    for (; e != null; e = e.next) {
        //判断hash和key是否相等
        if ((e.hash == hash) && e.key.equals(key)) {
           //判断value是否相等
            if (e.value.equals(oldValue)) {
               //如果都相等则替换
                e.value = newValue;
                return true;
            } else {
                return false;
            }
        }
    }
    return false;
}

public synchronized V replace(K key, V value) {
    Objects.requireNonNull(value);
    Entry<?,?> tab[] = table;
    int hash = key.hashCode();
    //计算index
    int index = (hash & 0x7FFFFFFF) % tab.length;
    @SuppressWarnings("unchecked")
    Entry<K,V> e = (Entry<K,V>)tab[index];
    //循环遍历
    for (; e != null; e = e.next) {
    //如果hash和key相等则替换
        if ((e.hash == hash) && e.key.equals(key)) {
            V oldValue = e.value;
            e.value = value;
            return oldValue;
        }
    }
    return null;
}

/**
 * Each of these fields are initialized to contain an instance of the
 * appropriate view the first time this view is requested.  The views are
 * stateless, so there's no reason to create more than one of each.
 */
private transient volatile Set<K> keySet;
private transient volatile Set<Map.Entry<K,V>> entrySet;
private transient volatile Collection<V> values;

public Set<K> keySet() {
    if (keySet == null)
        keySet = Collections.synchronizedSet(new KeySet(), this);
    return keySet;
}

private class KeySet extends AbstractSet<K> {
    public Iterator<K> iterator() {
        return getIterator(KEYS);
    }
    public int size() {
        return count;
    }
    public boolean contains(Object o) {
        return containsKey(o);
    }
    public boolean remove(Object o) {
        return Hashtable.this.remove(o) != null;
    }
    public void clear() {
        Hashtable.this.clear();
    }
}