java.util
Class Hashtable
java.lang.Object
|
+--java.util.Dictionary
|
+--java.util.HashtableAll Implemented Interfaces:
Cloneable, Map, Serializable
Class Hashtable
java.lang.Object
|
+--java.util.Dictionary
|
+--java.util.HashtableAll Implemented Interfaces:
Cloneable, Map, Serializable
* @(#)Hashtable.java 1.86 01/09/25
*
* Copyright 1994-2000 Sun Microsystems, Inc. All Rights Reserved.
*
* This software is the proprietary information of Sun Microsystems, Inc.
* Use is subject to license terms.
*
*/package java.util;
import java.io.*;/**
* This class implements a hashtable, 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 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.
* When the number of entries in the hashtable exceeds the product of the load
* factor and the current capacity, the capacity is increased by calling the
* <code>rehash</code> method.<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:
* <p><blockquote><pre>
* Hashtable numbers = new Hashtable();
* numbers.put("one", new Integer(1));
* numbers.put("two", new Integer(2));
* numbers.put("three", new Integer(3));
* </pre></blockquote>
* <p>
* To retrieve a number, use the following code:
* <p><blockquote><pre>
* Integer n = (Integer)numbers.get("two");
* if (n != null) {
* System.out.println("two = " + n);
* }
* </pre></blockquote>
* <p>
* As of the Java 2 platform v1.2, this class has been retrofitted to implement Map,
* so that it becomes a part of Java's collection framework. Unlike
* the new collection implementations, Hashtable is synchronized.<p>
*
* The Iterators returned by the iterator and listIterator methods
* of the Collections returned by all of Hashtable'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 remove or add methods, the Iterator will throw a
* 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 values methods are
* <em>not</em> fail-fast.
*
* @author Arthur van Hoff
* @author Josh Bloch
* @version 1.86, 09/25/01
* @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 extends Dictionary implements Map, 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, which is <tt>0.75</tt>.
*
* @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 capacity and load
* factor, which is <tt>0.75</tt>.
*/
public Hashtable() {
this(11, 0.75f);
} /**
* Constructs a new hashtable with the same mappings as the given
* Map. The hashtable is created with a capacity of twice the number
* of entries in the given Map or 11 (whichever is greater), and a
* default load factor, which is <tt>0.75</tt>.
*
* @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 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 keys() {
return getEnumeration(KEYS);
} /**
* Returns an enumeration of the values in this hashtable.
* Use the Enumeration methods on the returned object to fetch the elements
* sequentially.
*
* @return an enumeration of the values in this hashtable.
* @see java.util.Enumeration
* @see #keys()
* @see #values()
* @see Map
*/
public synchronized Enumeration elements() {
return getEnumeration(VALUES);
} /**
* Tests if some key maps into the specified value in this hashtable.
* This operation is more expensive than the <code>containsKey</code>
* method.<p>
*
* Note that this method is identical in functionality to containsValue,
* (which is part of the Map interface in the collections framework).
*
* @param value a value to search for.
* @return <code>true</code> if and only if some key maps to the
* <code>value</code> argument in this hashtable as
* determined by the <tt>equals</tt> method;
* <code>false</code> otherwise.
* @exception NullPointerException if the value is <code>null</code>.
* @see #containsKey(Object)
* @see #containsValue(Object)
* @see Map
*/
public synchronized boolean contains(Object value) {
if (value == null) {
throw new NullPointerException();
} Entry tab[] = table;
for (int i = tab.length ; i-- > 0 ;) {
for (Entry e = tab[i] ; e != null ; e = e.next) {
if (e.value.equals(value)) {
return true;
}
}
}
return false;
} /**
* Returns true if this Hashtable maps one or more keys to this value.<p>
*
* Note that this method is identical in functionality to contains
* (which predates the Map interface).
*
* @param value value whose presence in this Hashtable is to be tested.
* @return <tt>true</tt> if this map maps one or more keys to the
* specified value.
* @see Map
* @since 1.2
*/
public boolean containsValue(Object value) {
return contains(value);
} /**
* Tests if the specified object is a key in this hashtable.
*
* @param key possible key.
* @return <code>true</code> if and only if the specified object
* is a key in this hashtable, as determined by the
* <tt>equals</tt> method; <code>false</code> otherwise.
* @see #contains(Object)
*/
public synchronized boolean containsKey(Object key) {
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
return true;
}
}
return false;
} /**
* Returns the value to which the specified key is mapped in this hashtable.
*
* @param key a key in the hashtable.
* @return the value to which the key is mapped in this hashtable;
* <code>null</code> if the key is not mapped to any value in
* this hashtable.
* @see #put(Object, Object)
*/
public synchronized Object get(Object key) {
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
return e.value;
}
}
return null;
} /**
* 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.
*/
protected void rehash() {
int oldCapacity = table.length;
Entry oldMap[] = table; int newCapacity = oldCapacity * 2 + 1;
Entry newMap[] = new Entry[newCapacity]; modCount++;
threshold = (int)(newCapacity * loadFactor);
table = newMap; for (int i = oldCapacity ; i-- > 0 ;) {
for (Entry old = oldMap[i] ; old != null ; ) {
Entry e = old;
old = old.next; int index = (e.hash & 0x7FFFFFFF) % newCapacity;
e.next = newMap[index];
newMap[index] = e;
}
}
} /**
* Maps the specified <code>key</code> to the specified
* <code>value</code> in this hashtable. Neither the key nor the
* value can be <code>null</code>. <p>
*
* The value can be retrieved by calling the <code>get</code> method
* with a key that is equal to the original key.
*
* @param key the hashtable key.
* @param value the value.
* @return the previous value of the specified key in this hashtable,
* or <code>null</code> if it did not have one.
* @exception NullPointerException if the key or value is
* <code>null</code>.
* @see Object#equals(Object)
* @see #get(Object)
*/
public synchronized Object put(Object key, Object value) {
// Make sure the value is not null
if (value == null) {
throw new NullPointerException();
} // Makes sure the key is not already in the hashtable.
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
Object old = e.value;
e.value = value;
return old;
}
} modCount++;
if (count >= threshold) {
// Rehash the table if the threshold is exceeded
rehash(); tab = table;
index = (hash & 0x7FFFFFFF) % tab.length;
} // Creates the new entry.
Entry e = new Entry(hash, key, value, tab[index]);
tab[index] = e;
count++;
return null;
} /**
* Removes the key (and its corresponding value) from this
* hashtable. This method does nothing if the key is not in the hashtable.
*
* @param key the key that needs to be removed.
* @return the value to which the key had been mapped in this hashtable,
* or <code>null</code> if the key did not have a mapping.
*/
public synchronized Object remove(Object key) {
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry e = tab[index], prev = null ; e != null ; prev = e, e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
modCount++;
if (prev != null) {
prev.next = e.next;
} else {
tab[index] = e.next;
}
count--;
Object oldValue = e.value;
e.value = null;
return oldValue;
}
}
return null;
} /**
* Copies all of the mappings from the specified Map to this Hashtable
* These mappings will replace any mappings that this Hashtable had for any
* of the keys currently in the specified Map.
*
* @param t Mappings to be stored in this map.
* @throws NullPointerException if the specified map is null.
* @since 1.2
*/
public synchronized void putAll(Map t) {
Iterator i = t.entrySet().iterator();
while (i.hasNext()) {
Map.Entry e = (Map.Entry) i.next();
put(e.getKey(), e.getValue());
}
} /**
* Clears this hashtable so that it contains no keys.
*/
public synchronized void clear() {
Entry tab[] = table;
modCount++;
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 {
Hashtable t = (Hashtable)super.clone();
t.table = new Entry[table.length];
for (int i = table.length ; i-- > 0 ; ) {
t.table[i] = (table[i] != null)
? (Entry)table[i].clone() : null;
}
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();
}
} /**
* Returns a string representation of this <tt>Hashtable</tt> object
* in the form of a set of entries, enclosed in braces and separated
* by the ASCII characters "<tt>, </tt>" (comma and space). Each
* entry is rendered as the key, an equals sign <tt>=</tt>, and the
* associated element, where the <tt>toString</tt> method is used to
* convert the key and element to strings. <p>Overrides to
* <tt>toString</tt> method of <tt>Object</tt>.
*
* @return a string representation of this hashtable.
*/
public synchronized String toString() {
int max = size() - 1;
StringBuffer buf = new StringBuffer();
Iterator it = entrySet().iterator(); buf.append("{");
for (int i = 0; i <= max; i++) {
Map.Entry e = (Map.Entry) (it.next());
buf.append(e.getKey() + "=" + e.getValue());
if (i < max)
buf.append(", ");
}
buf.append("}");
return buf.toString();
}
private Enumeration getEnumeration(int type) {
if (count == 0) {
return emptyEnumerator;
} else {
return new Enumerator(type, false);
}
} private Iterator getIterator(int type) {
if (count == 0) {
return emptyIterator;
} else {
return new Enumerator(type, true);
}
} // Views /**
* 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 keySet = null;
private transient volatile Set entrySet = null;
private transient volatile Collection values = null; /**
* Returns a Set view of the keys contained in this Hashtable. The Set
* is backed by the Hashtable, so changes to the Hashtable are reflected
* in the Set, and vice-versa. The Set supports element removal
* (which removes the corresponding entry from the Hashtable), but not
* element addition.
*
* @return a set view of the keys contained in this map.
* @since 1.2
*/
public Set keySet() {
if (keySet == null)
keySet = Collections.synchronizedSet(new KeySet(), this);
return keySet;
} private class KeySet extends AbstractSet {
public Iterator 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();
}
} /**
* Returns a Set view of the entries contained in this Hashtable.
* Each element in this collection is a Map.Entry. The Set is
* backed by the Hashtable, so changes to the Hashtable are reflected in
* the Set, and vice-versa. The Set supports element removal
* (which removes the corresponding entry from the Hashtable),
* but not element addition.
*
* @return a set view of the mappings contained in this map.
* @see Map.Entry
* @since 1.2
*/
public Set entrySet() {
if (entrySet==null)
entrySet = Collections.synchronizedSet(new EntrySet(), this);
return entrySet;
} private class EntrySet extends AbstractSet {
public Iterator iterator() {
return getIterator(ENTRIES);
} public boolean contains(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry entry = (Map.Entry)o;
Object key = entry.getKey();
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length; for (Entry e = tab[index]; e != null; e = e.next)
if (e.hash==hash && e.equals(entry))
return true;
return false;
} public boolean remove(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry entry = (Map.Entry)o;
Object key = entry.getKey();
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length; for (Entry e = tab[index], prev = null; e != null;
prev = e, e = e.next) {
if (e.hash==hash && e.equals(entry)) {
modCount++;
if (prev != null)
prev.next = e.next;
else
tab[index] = e.next; count--;
e.value = null;
return true;
}
}
return false;
} public int size() {
return count;
} public void clear() {
Hashtable.this.clear();
}
} /**
* Returns a Collection view of the values contained in this Hashtable.
* The Collection is backed by the Hashtable, so changes to the Hashtable
* are reflected in the Collection, and vice-versa. The Collection
* supports element removal (which removes the corresponding entry from
* the Hashtable), but not element addition.
*
* @return a collection view of the values contained in this map.
* @since 1.2
*/
public Collection values() {
if (values==null)
values = Collections.synchronizedCollection(new ValueCollection(),
this);
return values;
} private class ValueCollection extends AbstractCollection {
public Iterator iterator() {
return getIterator(VALUES);
}
public int size() {
return count;
}
public boolean contains(Object o) {
return containsValue(o);
}
public void clear() {
Hashtable.this.clear();
}
} // Comparison and hashing /**
* Compares the specified Object with this Map for equality,
* as per the definition in the Map interface.
*
* @param o object to be compared for equality with this Hashtable
* @return true if the specified Object is equal to this Map.
* @see Map#equals(Object)
* @since 1.2
*/
public synchronized boolean equals(Object o) {
if (o == this)
return true; if (!(o instanceof Map))
return false;
Map t = (Map) o;
if (t.size() != size())
return false; Iterator i = entrySet().iterator();
while (i.hasNext()) {
Map.Entry e = (Map.Entry) i.next();
Object key = e.getKey();
Object value = e.getValue();
if (value == null) {
if (!(t.get(key)==null && t.containsKey(key)))
return false;
} else {
if (!value.equals(t.get(key)))
return false;
}
}
return true;
} /**
* Returns the hash code value for this Map as per the definition in the
* Map interface.
*
* @see Map#hashCode()
* @since 1.2
*/
public synchronized int hashCode() {
int h = 0;
Iterator i = entrySet().iterator();
while (i.hasNext())
h += i.next().hashCode();
return h;
} /**
* Save the state of the Hashtable to a stream (i.e., serialize it).
*
* @serialData The <i>capacity</i> of the Hashtable (the length of the
* bucket array) is emitted (int), followed by the
* <i>size</i> of the Hashtable (the number of key-value
* mappings), followed by the key (Object) and value (Object)
* for each key-value mapping represented by the Hashtable
* The key-value mappings are emitted in no particular order.
*/
private synchronized void writeObject(java.io.ObjectOutputStream s)
throws IOException
{
// Write out the length, threshold, loadfactor
s.defaultWriteObject(); // Write out length, count of elements and then the key/value objects
s.writeInt(table.length);
s.writeInt(count);
for (int index = table.length-1; index >= 0; index--) {
Entry entry = table[index]; while (entry != null) {
s.writeObject(entry.key);
s.writeObject(entry.value);
entry = entry.next;
}
}
} /**
* Reconstitute the Hashtable from a stream (i.e., deserialize it).
*/
private synchronized void readObject(java.io.ObjectInputStream s)
throws IOException, ClassNotFoundException
{
// Read in the length, threshold, and loadfactor
s.defaultReadObject(); // Read the original length of the array and number of elements
int origlength = s.readInt();
int elements = s.readInt(); // Compute new size with a bit of room 5% to grow but
// No larger than the original size. Make the length
// odd if it's large enough, this helps distribute the entries.
// Guard against the length ending up zero, that's not valid.
int length = (int)(elements * loadFactor) + (elements / 20) + 3;
if (length > elements && (length & 1) == 0)
length--;
if (origlength > 0 && length > origlength)
length = origlength; table = new Entry[length];
count = 0; // Read the number of elements and then all the key/value objects
for (; elements > 0; elements--) {
Object key = s.readObject();
Object value = s.readObject();
put(key, value);
}
}
/**
* Hashtable collision list.
*/
private static class Entry implements Map.Entry {
int hash;
Object key;
Object value;
Entry next; protected Entry(int hash, Object key, Object value, Entry next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
} protected Object clone() {
return new Entry(hash, key, value,
(next==null ? null : (Entry)next.clone()));
} // Map.Entry Ops public Object getKey() {
return key;
} public Object getValue() {
return value;
} public Object setValue(Object value) {
if (value == null)
throw new NullPointerException(); Object 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 ^ (value==null ? 0 : value.hashCode());
} public String toString() {
return key.toString()+"="+value.toString();
}
} // Types of Enumerations/Iterations
private static final int KEYS = 0;
private static final int VALUES = 1;
private static final int ENTRIES = 2; /**
* A hashtable enumerator class. This class implements both the
* Enumeration and Iterator interfaces, but individual instances
* can be created with the Iterator methods disabled. This is necessary
* to avoid unintentionally increasing the capabilities granted a user
* by passing an Enumeration.
*/
private class Enumerator implements Enumeration, Iterator {
Entry[] table = Hashtable.this.table;
int index = table.length;
Entry entry = null;
Entry lastReturned = null;
int type; /**
* Indicates whether this Enumerator is serving as an Iterator
* or an Enumeration. (true -> Iterator).
*/
boolean iterator; /**
* The modCount value that the iterator believes that the backing
* List should have. If this expectation is violated, the iterator
* has detected concurrent modification.
*/
protected int expectedModCount = modCount; Enumerator(int type, boolean iterator) {
this.type = type;
this.iterator = iterator;
} public boolean hasMoreElements() {
Entry e = entry;
int i = index;
Entry t[] = table;
/* Use locals for faster loop iteration */
while (e == null && i > 0) {
e = t[--i];
}
entry = e;
index = i;
return e != null;
} public Object nextElement() {
Entry et = entry;
int i = index;
Entry t[] = table;
/* Use locals for faster loop iteration */
while (et == null && i > 0) {
et = t[--i];
}
entry = et;
index = i;
if (et != null) {
Entry e = lastReturned = entry;
entry = e.next;
return type == KEYS ? e.key : (type == VALUES ? e.value : e);
}
throw new NoSuchElementException("Hashtable Enumerator");
} // Iterator methods
public boolean hasNext() {
return hasMoreElements();
} public Object next() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
return nextElement();
} public void remove() {
if (!iterator)
throw new UnsupportedOperationException();
if (lastReturned == null)
throw new IllegalStateException("Hashtable Enumerator");
if (modCount != expectedModCount)
throw new ConcurrentModificationException(); synchronized(Hashtable.this) {
Entry[] tab = Hashtable.this.table;
int index = (lastReturned.hash & 0x7FFFFFFF) % tab.length; for (Entry e = tab[index], prev = null; e != null;
prev = e, e = e.next) {
if (e == lastReturned) {
modCount++;
expectedModCount++;
if (prev == null)
tab[index] = e.next;
else
prev.next = e.next;
count--;
lastReturned = null;
return;
}
}
throw new ConcurrentModificationException();
}
}
}
private static EmptyEnumerator emptyEnumerator = new EmptyEnumerator();
private static EmptyIterator emptyIterator = new EmptyIterator(); /**
* A hashtable enumerator class for empty hash tables, specializes
* the general Enumerator
*/
private static class EmptyEnumerator implements Enumeration { EmptyEnumerator() {
} public boolean hasMoreElements() {
return false;
} public Object nextElement() {
throw new NoSuchElementException("Hashtable Enumerator");
}
}
/**
* A hashtable iterator class for empty hash tables
*/
private static class EmptyIterator implements Iterator { EmptyIterator() {
} public boolean hasNext() {
return false;
} public Object next() {
throw new NoSuchElementException("Hashtable Iterator");
} public void remove() {
throw new IllegalStateException("Hashtable Iterator");
} }}