HashTable源码与总结

川长思鸟来 2022-12-11 03:15 187阅读 0赞

> 作为最原始的线程安全的map，HashTable的实现原理有必要进行分析总结

# 包含的方法API #

synchronized void clear()
 synchronized Object clone()
 boolean contains(Object value)
 synchronized boolean containsKey(Object key)
 synchronized boolean containsValue(Object value)
 synchronized Enumeration<V> elements()
 synchronized Set<Entry<K, V>> entrySet()
 synchronized boolean equals(Object object)
 synchronized V get(Object key)
 synchronized int hashCode()
 synchronized boolean isEmpty()
 synchronized Set<K> keySet()
 synchronized Enumeration<K> keys()
 synchronized V put(K key, V value)
 synchronized void putAll(Map<? extends K, ? extends V> map)
 synchronized V remove(Object key)
 synchronized int size()
 synchronized String toString()
 synchronized Collection<V> values()

# 直接上源码 #

/* * Copyright (c) 1994, 2017, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */
 
 package java.util;
 
 import java.io.*;
 import java.util.concurrent.ThreadLocalRandom;
 import java.util.function.BiConsumer;
 import java.util.function.Function;
 import java.util.function.BiFunction;
 import sun.misc.SharedSecrets;
 
 /** * 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. * * 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. * * * @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 { 
 
 /** * 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)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
 }
 
 /** * Constructs a new, empty hashtable with the specified initial capacity * and default load factor (0.75). * * @param initialCapacity the initial capacity of the hashtable. * @exception IllegalArgumentException if the initial capacity is less * than zero. */
 public Hashtable(int initialCapacity) { 
 this(initialCapacity, 0.75f);
 }
 
 /** * Constructs a new, empty hashtable with a default initial capacity (11) * and load factor (0.75). */
 public Hashtable() { 
 this(11, 0.75f);
 }
 
 /** * Constructs a new hashtable with the same mappings as the given * Map. The hashtable is created with an initial capacity sufficient to * hold the mappings in the given Map and a default load factor (0.75). * * @param t the map whose mappings are to be placed in this map. * @throws NullPointerException if the specified map is null. * @since 1.2 */
 public Hashtable(Map<? extends K, ? extends V> t) { 
 this(Math.max(2*t.size(), 11), 0.75f);
 putAll(t);
 }
 
 /** * 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);
 }
 
 /** * 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<V> elements() { 
 return this.<V>getEnumeration(VALUES);
 }
 
 /** * Tests if some key maps into the specified value in this hashtable. * This operation is more expensive than the {@link #containsKey * containsKey} method. * * Note that this method is identical in functionality to * {@link #containsValue containsValue}, (which is part of the * {@link 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> */
 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. * * Note that this method is identical in functionality to {@link * #contains contains} (which predates the {@link 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 * @throws NullPointerException if the value is <code>null</code> * @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. * @throws NullPointerException if the key is <code>null</code> * @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, * or {@code null} if this map contains no mapping for the key. * * More formally, if this map contains a mapping from a key * {@code k} to a value {@code v} such that {@code (key.equals(k))}, * then this method returns {@code v}; otherwise it returns * {@code null}. (There can be at most one such mapping.) * * @param key the key whose associated value is to be returned * @return the value to which the specified key is mapped, or * {@code null} if this map contains no mapping for the key * @throws NullPointerException if the specified key is null * @see #put(Object, Object) */
 @SuppressWarnings("unchecked")
 public synchronized V 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 (V)e.value;
 }
 }
 return null;
 }
 
 /** * 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;
 
 /** * 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() { 
 int oldCapacity = table.length;
 Entry<?,?>[] oldMap = table;
 
 // overflow-conscious code
 int newCapacity = (oldCapacity << 1) + 1;
 if (newCapacity - MAX_ARRAY_SIZE > 0) { 
 if (oldCapacity == MAX_ARRAY_SIZE)
 // Keep running with MAX_ARRAY_SIZE buckets
 return;
 newCapacity = MAX_ARRAY_SIZE;
 }
 Entry<?,?>[] newMap = new Entry<?,?>[newCapacity];
 
 modCount++;
 threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
 table = newMap;
 
 for (int i = oldCapacity ; i-- > 0 ;) { 
 for (Entry<K,V> old = (Entry<K,V>)oldMap[i] ; old != null ; ) { 
 Entry<K,V> e = old;
 old = old.next;
 
 int index = (e.hash & 0x7FFFFFFF) % newCapacity;
 e.next = (Entry<K,V>)newMap[index];
 newMap[index] = e;
 }
 }
 }
 
 private void addEntry(int hash, K key, V value, int index) { 
 modCount++;
 
 Entry<?,?> tab[] = table;
 if (count >= threshold) { 
 // Rehash the table if the threshold is exceeded
 rehash();
 
 tab = table;
 hash = key.hashCode();
 index = (hash & 0x7FFFFFFF) % tab.length;
 }
 
 // Creates the new entry.
 @SuppressWarnings("unchecked")
 Entry<K,V> e = (Entry<K,V>) tab[index];
 tab[index] = new Entry<>(hash, key, value, e);
 count++;
 }
 
 /** * 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>. * * 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 V put(K key, V 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;
 @SuppressWarnings("unchecked")
 Entry<K,V> entry = (Entry<K,V>)tab[index];
 for(; entry != null ; entry = entry.next) { 
 if ((entry.hash == hash) && entry.key.equals(key)) { 
 V old = entry.value;
 entry.value = value;
 return old;
 }
 }
 
 addEntry(hash, key, value, index);
 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 * @throws NullPointerException if the key is <code>null</code> */
 public synchronized V remove(Object key) { 
 Entry<?,?> tab[] = table;
 int hash = key.hashCode();
 int index = (hash & 0x7FFFFFFF) % tab.length;
 @SuppressWarnings("unchecked")
 Entry<K,V> e = (Entry<K,V>)tab[index];
 for(Entry<K,V> 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--;
 V 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<? extends K, ? extends V> t) { 
 for (Map.Entry<? extends K, ? extends V> e : t.entrySet())
 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(e);
 }
 }
 
 /** * 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. * * @return a string representation of this hashtable */
 public synchronized String toString() { 
 int max = size() - 1;
 if (max == -1)
 return "{}";
 
 StringBuilder sb = new StringBuilder();
 Iterator<Map.Entry<K,V>> it = entrySet().iterator();
 
 sb.append('{');
 for (int i = 0; ; i++) { 
 Map.Entry<K,V> e = it.next();
 K key = e.getKey();
 V value = e.getValue();
 sb.append(key == this ? "(this Map)" : key.toString());
 sb.append('=');
 sb.append(value == this ? "(this Map)" : value.toString());
 
 if (i == max)
 return sb.append('}').toString();
 sb.append(", ");
 }
 }
 
 
 private <T> Enumeration<T> getEnumeration(int type) { 
 if (count == 0) { 
 return Collections.emptyEnumeration();
 } else { 
 return new Enumerator<>(type, false);
 }
 }
 
 private <T> Iterator<T> getIterator(int type) { 
 if (count == 0) { 
 return Collections.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<K> keySet;
 private transient volatile Set<Map.Entry<K,V>> entrySet;
 private transient volatile Collection<V> values;
 
 /** * Returns a {@link Set} view of the keys contained in this map. * The set is backed by the map, so changes to the map are * reflected in the set, and vice-versa. If the map is modified * while an iteration over the set is in progress (except through * the iterator's own <tt>remove</tt> operation), the results of * the iteration are undefined. The set supports element removal, * which removes the corresponding mapping from the map, via the * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>, * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> * operations. It does not support the <tt>add</tt> or <tt>addAll</tt> * operations. * * @since 1.2 */
 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();
 }
 }
 
 /** * Returns a {@link Set} view of the mappings contained in this map. * The set is backed by the map, so changes to the map are * reflected in the set, and vice-versa. If the map is modified * while an iteration over the set is in progress (except through * the iterator's own <tt>remove</tt> operation, or through the * <tt>setValue</tt> operation on a map entry returned by the * iterator) the results of the iteration are undefined. The set * supports element removal, which removes the corresponding * mapping from the map, via the <tt>Iterator.remove</tt>, * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and * <tt>clear</tt> operations. It does not support the * <tt>add</tt> or <tt>addAll</tt> operations. * * @since 1.2 */
 public Set<Map.Entry<K,V>> entrySet() { 
 if (entrySet==null)
 entrySet = Collections.synchronizedSet(new EntrySet(), this);
 return entrySet;
 }
 
 private class EntrySet extends AbstractSet<Map.Entry<K,V>> { 
 public Iterator<Map.Entry<K,V>> iterator() { 
 return getIterator(ENTRIES);
 }
 
 public boolean add(Map.Entry<K,V> o) { 
 return super.add(o);
 }
 
 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;
 
 @SuppressWarnings("unchecked")
 Entry<K,V> e = (Entry<K,V>)tab[index];
 for(Entry<K,V> 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 {@link Collection} view of the values contained in this map. * The collection is backed by the map, so changes to the map are * reflected in the collection, and vice-versa. If the map is * modified while an iteration over the collection is in progress * (except through the iterator's own <tt>remove</tt> operation), * the results of the iteration are undefined. The collection * supports element removal, which removes the corresponding * mapping from the map, via the <tt>Iterator.remove</tt>, * <tt>Collection.remove</tt>, <tt>removeAll</tt>, * <tt>retainAll</tt> and <tt>clear</tt> operations. It does not * support the <tt>add</tt> or <tt>addAll</tt> operations. * * @since 1.2 */
 public Collection<V> values() { 
 if (values==null)
 values = Collections.synchronizedCollection(new ValueCollection(),
 this);
 return values;
 }
 
 private class ValueCollection extends AbstractCollection<V> { 
 public Iterator<V> 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;
 
 try { 
 Iterator<Map.Entry<K,V>> i = entrySet().iterator();
 while (i.hasNext()) { 
 Map.Entry<K,V> e = i.next();
 K key = e.getKey();
 V 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;
 }
 }
 } catch (ClassCastException unused) { 
 return false;
 } catch (NullPointerException unused) { 
 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() { 
 /* * This code detects the recursion caused by computing the hash code * of a self-referential hash table and prevents the stack overflow * that would otherwise result. This allows certain 1.1-era * applets with self-referential hash tables to work. This code * abuses the loadFactor field to do double-duty as a hashCode * in progress flag, so as not to worsen the space performance. * A negative load factor indicates that hash code computation is * in progress. */
 int h = 0;
 if (count == 0 || loadFactor < 0)
 return h; // Returns zero
 
 loadFactor = -loadFactor; // Mark hashCode computation in progress
 Entry<?,?>[] tab = table;
 for (Entry<?,?> entry : tab) { 
 while (entry != null) { 
 h += entry.hashCode();
 entry = entry.next;
 }
 }
 
 loadFactor = -loadFactor; // Mark hashCode computation complete
 
 return h;
 }
 
 @Override
 public synchronized V getOrDefault(Object key, V defaultValue) { 
 V result = get(key);
 return (null == result) ? defaultValue : result;
 }
 
 @SuppressWarnings("unchecked")
 @Override
 public synchronized void forEach(BiConsumer<? super K, ? super V> action) { 
 Objects.requireNonNull(action); // explicit check required in case
 // table is empty.
 final int expectedModCount = modCount;
 
 Entry<?, ?>[] tab = table;
 for (Entry<?, ?> entry : tab) { 
 while (entry != null) { 
 action.accept((K)entry.key, (V)entry.value);
 entry = entry.next;
 
 if (expectedModCount != modCount) { 
 throw new ConcurrentModificationException();
 }
 }
 }
 }
 
 @SuppressWarnings("unchecked")
 @Override
 public synchronized void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) { 
 Objects.requireNonNull(function); // explicit check required in case
 // table is empty.
 final int expectedModCount = modCount;
 
 Entry<K, V>[] tab = (Entry<K, V>[])table;
 for (Entry<K, V> entry : tab) { 
 while (entry != null) { 
 entry.value = Objects.requireNonNull(
 function.apply(entry.key, entry.value));
 entry = entry.next;
 
 if (expectedModCount != modCount) { 
 throw new ConcurrentModificationException();
 }
 }
 }
 }
 
 @Override
 public synchronized V putIfAbsent(K key, V value) { 
 Objects.requireNonNull(value);
 
 // Makes sure the key is not already in the hashtable.
 Entry<?,?> tab[] = table;
 int hash = key.hashCode();
 int index = (hash & 0x7FFFFFFF) % tab.length;
 @SuppressWarnings("unchecked")
 Entry<K,V> entry = (Entry<K,V>)tab[index];
 for (; entry != null; entry = entry.next) { 
 if ((entry.hash == hash) && entry.key.equals(key)) { 
 V old = entry.value;
 if (old == null) { 
 entry.value = value;
 }
 return old;
 }
 }
 
 addEntry(hash, key, value, index);
 return null;
 }
 
 @Override
 public synchronized boolean remove(Object key, Object value) { 
 Objects.requireNonNull(value);
 
 Entry<?,?> tab[] = table;
 int hash = key.hashCode();
 int index = (hash & 0x7FFFFFFF) % tab.length;
 @SuppressWarnings("unchecked")
 Entry<K,V> e = (Entry<K,V>)tab[index];
 for (Entry<K,V> prev = null; e != null; prev = e, e = e.next) { 
 if ((e.hash == hash) && e.key.equals(key) && e.value.equals(value)) { 
 modCount++;
 if (prev != null) { 
 prev.next = e.next;
 } else { 
 tab[index] = e.next;
 }
 count--;
 e.value = null;
 return true;
 }
 }
 return false;
 }
 
 @Override
 public synchronized boolean replace(K key, V oldValue, V newValue) { 
 Objects.requireNonNull(oldValue);
 Objects.requireNonNull(newValue);
 Entry<?,?> tab[] = table;
 int hash = key.hashCode();
 int index = (hash & 0x7FFFFFFF) % tab.length;
 @SuppressWarnings("unchecked")
 Entry<K,V> e = (Entry<K,V>)tab[index];
 for (; e != null; e = e.next) { 
 if ((e.hash == hash) && e.key.equals(key)) { 
 if (e.value.equals(oldValue)) { 
 e.value = newValue;
 return true;
 } else { 
 return false;
 }
 }
 }
 return false;
 }
 
 @Override
 public synchronized V replace(K key, V value) { 
 Objects.requireNonNull(value);
 Entry<?,?> tab[] = table;
 int hash = key.hashCode();
 int index = (hash & 0x7FFFFFFF) % tab.length;
 @SuppressWarnings("unchecked")
 Entry<K,V> e = (Entry<K,V>)tab[index];
 for (; e != null; e = e.next) { 
 if ((e.hash == hash) && e.key.equals(key)) { 
 V oldValue = e.value;
 e.value = value;
 return oldValue;
 }
 }
 return null;
 }
 
 @Override
 public synchronized V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) { 
 Objects.requireNonNull(mappingFunction);
 
 Entry<?,?> tab[] = table;
 int hash = key.hashCode();
 int index = (hash & 0x7FFFFFFF) % tab.length;
 @SuppressWarnings("unchecked")
 Entry<K,V> e = (Entry<K,V>)tab[index];
 for (; e != null; e = e.next) { 
 if (e.hash == hash && e.key.equals(key)) { 
 // Hashtable not accept null value
 return e.value;
 }
 }
 
 V newValue = mappingFunction.apply(key);
 if (newValue != null) { 
 addEntry(hash, key, newValue, index);
 }
 
 return newValue;
 }
 
 @Override
 public synchronized V computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction) { 
 Objects.requireNonNull(remappingFunction);
 
 Entry<?,?> tab[] = table;
 int hash = key.hashCode();
 int index = (hash & 0x7FFFFFFF) % tab.length;
 @SuppressWarnings("unchecked")
 Entry<K,V> e = (Entry<K,V>)tab[index];
 for (Entry<K,V> prev = null; e != null; prev = e, e = e.next) { 
 if (e.hash == hash && e.key.equals(key)) { 
 V newValue = remappingFunction.apply(key, e.value);
 if (newValue == null) { 
 modCount++;
 if (prev != null) { 
 prev.next = e.next;
 } else { 
 tab[index] = e.next;
 }
 count--;
 } else { 
 e.value = newValue;
 }
 return newValue;
 }
 }
 return null;
 }
 
 @Override
 public synchronized V compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction) { 
 Objects.requireNonNull(remappingFunction);
 
 Entry<?,?> tab[] = table;
 int hash = key.hashCode();
 int index = (hash & 0x7FFFFFFF) % tab.length;
 @SuppressWarnings("unchecked")
 Entry<K,V> e = (Entry<K,V>)tab[index];
 for (Entry<K,V> prev = null; e != null; prev = e, e = e.next) { 
 if (e.hash == hash && Objects.equals(e.key, key)) { 
 V newValue = remappingFunction.apply(key, e.value);
 if (newValue == null) { 
 modCount++;
 if (prev != null) { 
 prev.next = e.next;
 } else { 
 tab[index] = e.next;
 }
 count--;
 } else { 
 e.value = newValue;
 }
 return newValue;
 }
 }
 
 V newValue = remappingFunction.apply(key, null);
 if (newValue != null) { 
 addEntry(hash, key, newValue, index);
 }
 
 return newValue;
 }
 
 @Override
 public synchronized V merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction) { 
 Objects.requireNonNull(remappingFunction);
 
 Entry<?,?> tab[] = table;
 int hash = key.hashCode();
 int index = (hash & 0x7FFFFFFF) % tab.length;
 @SuppressWarnings("unchecked")
 Entry<K,V> e = (Entry<K,V>)tab[index];
 for (Entry<K,V> prev = null; e != null; prev = e, e = e.next) { 
 if (e.hash == hash && e.key.equals(key)) { 
 V newValue = remappingFunction.apply(e.value, value);
 if (newValue == null) { 
 modCount++;
 if (prev != null) { 
 prev.next = e.next;
 } else { 
 tab[index] = e.next;
 }
 count--;
 } else { 
 e.value = newValue;
 }
 return newValue;
 }
 }
 
 if (value != null) { 
 addEntry(hash, key, value, index);
 }
 
 return value;
 }
 
 /** * Save the state of the Hashtable to a stream (i.e., serialize it). * * @serialData The capacity of the Hashtable (the length of the * bucket array) is emitted (int), followed by the * size 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 void writeObject(java.io.ObjectOutputStream s)
 throws IOException { 
 Entry<Object, Object> entryStack = null;
 
 synchronized (this) { 
 // Write out the threshold and loadFactor
 s.defaultWriteObject();
 
 // Write out the length and count of elements
 s.writeInt(table.length);
 s.writeInt(count);
 
 // Stack copies of the entries in the table
 for (int index = 0; index < table.length; index++) { 
 Entry<?,?> entry = table[index];
 
 while (entry != null) { 
 entryStack =
 new Entry<>(0, entry.key, entry.value, entryStack);
 entry = entry.next;
 }
 }
 }
 
 // Write out the key/value objects from the stacked entries
 while (entryStack != null) { 
 s.writeObject(entryStack.key);
 s.writeObject(entryStack.value);
 entryStack = entryStack.next;
 }
 }
 
 /** * Reconstitute the Hashtable from a stream (i.e., deserialize it). */
 private void readObject(java.io.ObjectInputStream s)
 throws IOException, ClassNotFoundException
 { 
 // Read in the threshold and loadFactor
 s.defaultReadObject();
 
 // Validate loadFactor (ignore threshold - it will be re-computed)
 if (loadFactor <= 0 || Float.isNaN(loadFactor))
 throw new StreamCorruptedException("Illegal Load: " + loadFactor);
 
 // Read the original length of the array and number of elements
 int origlength = s.readInt();
 int elements = s.readInt();
 
 // Validate # of elements
 if (elements < 0)
 throw new StreamCorruptedException("Illegal # of Elements: " + elements);
 
 // Clamp original length to be more than elements / loadFactor
 // (this is the invariant enforced with auto-growth)
 origlength = Math.max(origlength, (int)(elements / loadFactor) + 1);
 
 // Compute new length with a bit of room 5% + 3 to grow but
 // no larger than the clamped original length. 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 + elements / 20) / loadFactor) + 3;
 if (length > elements && (length & 1) == 0)
 length--;
 length = Math.min(length, origlength);
 
 if (length < 0) { // overflow
 length = origlength;
 }
 
 // Check Map.Entry[].class since it's the nearest public type to
 // what we're actually creating.
 SharedSecrets.getJavaOISAccess().checkArray(s, Map.Entry[].class, length);
 table = new Entry<?,?>[length];
 threshold = (int)Math.min(length * loadFactor, MAX_ARRAY_SIZE + 1);
 count = 0;
 
 // Read the number of elements and then all the key/value objects
 for (; elements > 0; elements--) { 
 @SuppressWarnings("unchecked")
 K key = (K)s.readObject();
 @SuppressWarnings("unchecked")
 V value = (V)s.readObject();
 // sync is eliminated for performance
 reconstitutionPut(table, key, value);
 }
 }
 
 /** * The put method used by readObject. This is provided because put * is overridable and should not be called in readObject since the * subclass will not yet be initialized. * * This differs from the regular put method in several ways. No * checking for rehashing is necessary since the number of elements * initially in the table is known. The modCount is not incremented and * there's no synchronization because we are creating a new instance. * Also, no return value is needed. */
 private void reconstitutionPut(Entry<?,?>[] tab, K key, V value)
 throws StreamCorruptedException
 { 
 if (value == null) { 
 throw new java.io.StreamCorruptedException();
 }
 // Makes sure the key is not already in the hashtable.
 // This should not happen in deserialized version.
 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)) { 
 throw new java.io.StreamCorruptedException();
 }
 }
 // Creates the new entry.
 @SuppressWarnings("unchecked")
 Entry<K,V> e = (Entry<K,V>)tab[index];
 tab[index] = new Entry<>(hash, key, value, e);
 count++;
 }
 
 /** * Hashtable bucket collision list entry */
 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();
 }
 }
 
 // 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<T> implements Enumeration<T>, Iterator<T> { 
 Entry<?,?>[] table = Hashtable.this.table;
 int index = table.length;
 Entry<?,?> entry;
 Entry<?,?> lastReturned;
 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 * Hashtable 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;
 }
 
 @SuppressWarnings("unchecked")
 public T 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 ? (T)e.key : (type == VALUES ? (T)e.value : (T)e);
 }
 throw new NoSuchElementException("Hashtable Enumerator");
 }
 
 // Iterator methods
 public boolean hasNext() { 
 return hasMoreElements();
 }
 
 public T 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;
 
 @SuppressWarnings("unchecked")
 Entry<K,V> e = (Entry<K,V>)tab[index];
 for(Entry<K,V> 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();
 }
 }
 }
 }