HashTable 简介
- HashTable数据结构也是哈希表(或称散列表)，基本与HashMap类型，不同的是，HashTable的key value 都不可为空，且是线程安全的；
- 为了能够存储和检索HashTable中的对象，作为HashTable中key的对象必须实现了hashCode 和 equals 方法;具体来讲，如果我们使用自定义的类作为key,若重写（Override）了hashCode,也必须重写equals，因为二者具有相同的语义，即对象a b的hashCode相等，其a.equals(b)必然为true(在a b !=null情况下)。简而言之，若重写，二者都要重写，且要保持相同语义，若不重写，二个都不重写
- HashTable有两个参数影响其性能：capacity loadFactot，都是为了时空的平衡（在HashMap的简介里给了详细的）。
HashTable 数据结构
HashTable UML
HashTable 源码
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;
/**
*
* An instance of Hashtable has two parameters that affect its
* performance: initial capacity and load factor. The
* capacity is the number of buckets in the hash table, and the
* initial capacity is simply the capacity at the time the hash table
* is created. Note that the hash table is open: in the case of a "hash
* collision", a single bucket stores multiple entries, which must be searched
* sequentially. The load factor 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.
*
* 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
* Hashtable operations, including get and put).
*
* The initial capacity controls a tradeoff between wasted space and the
* need for rehash operations, which are time-consuming.
* No rehash operations will ever occur if the initial
* capacity is greater than the maximum number of entries the
* Hashtable will contain divided by its load factor. However,
* setting the initial capacity too high can waste space.
*
* If many entries are to be made into a Hashtable,
* 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.
*
* This example creates a hashtable of numbers. It uses the names of
* the numbers as keys:
*
{@code
* Hashtable numbers
* = new Hashtable();
* numbers.put("one", 1);
* numbers.put("two", 2);
* numbers.put("three", 3);}
*
* To retrieve a number, use the following code:
*
{@code
* Integer n = numbers.get("two");
* if (n != null) {
* System.out.println("two = " + n);
* }}
*
* The iterators returned by the iterator method of the collections
* returned by all of this class's "collection view methods" are
* fail-fast: if the Hashtable is structurally modified at any time
* after the iterator is created, in any way except through the iterator's own
* remove 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
* not fail-fast.
*
*
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 ConcurrentModificationException on a best-effort basis.
* Therefore, it would be wrong to write a program that depended on this
* exception for its correctness: the fail-fast behavior of iterators
* should be used only to detect bugs.
*
*
As of the Java 2 platform v1.2, this class was retrofitted to
* implement the {@link Map} interface, making it a member of the
*
*
* Java Collections Framework. 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
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)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 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 true if this hashtable maps no keys to values;
* false 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 this.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 this.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 true if and only if some key maps to the
* value argument in this hashtable as
* determined by the equals method;
* false otherwise.
* @exception NullPointerException if the value is null
*/
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 true if this map maps one or more keys to the
* specified value
* @throws NullPointerException if the value is null
* @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 true if and only if the specified object
* is a key in this hashtable, as determined by the
* equals method; false otherwise.
* @throws NullPointerException if the key is null
* @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 old = (Entry)oldMap[i] ; old != null ; ) {
Entry e = old;
old = old.next;
int index = (e.hash & 0x7FFFFFFF) % newCapacity;
e.next = (Entry)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 e = (Entry) tab[index];
tab[index] = new Entry<>(hash, key, value, e);
count++;
}
/**
* Maps the specified key to the specified
* value in this hashtable. Neither the key nor the
* value can be null.
*
* The value can be retrieved by calling the get 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 null if it did not have one
* @exception NullPointerException if the key or value is
* null
* @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 entry = (Entry)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 null if the key did not have a mapping
* @throws NullPointerException if the key is null
*/
public synchronized V remove(Object key) {
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
@SuppressWarnings("unchecked")
Entry e = (Entry)tab[index];
for(Entry 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 t) {
for (Map.Entry 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 Hashtable object
* in the form of a set of entries, enclosed in braces and separated
* by the ASCII characters ", " (comma and space). Each
* entry is rendered as the key, an equals sign =, and the
* associated element, where the toString 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> it = entrySet().iterator();
sb.append('{');
for (int i = 0; ; i++) {
Map.Entry 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 Enumeration getEnumeration(int type) {
if (count == 0) {
return Collections.emptyEnumeration();
} else {
return new Enumerator<>(type, false);
}
}
private Iterator 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 keySet;
private transient volatile Set> entrySet;
private transient volatile Collection 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 remove operation), the results of
* the iteration are undefined. The set supports element removal,
* which removes the corresponding mapping from the map, via the
* Iterator.remove, Set.remove,
* removeAll, retainAll, and clear
* operations. It does not support the add or addAll
* operations.
*
* @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 {@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 remove operation, or through the
* setValue 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 Iterator.remove,
* Set.remove, removeAll, retainAll and
* clear operations. It does not support the
* add or addAll operations.
*
* @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 add(Map.Entry 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 e = (Entry)tab[index];
for(Entry 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 remove operation),
* the results of the iteration are undefined. The collection
* supports element removal, which removes the corresponding
* mapping from the map, via the Iterator.remove,
* Collection.remove, removeAll,
* retainAll and clear operations. It does not
* support the add or addAll operations.
*
* @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;
try {
Iterator> i = entrySet().iterator();
while (i.hasNext()) {
Map.Entry 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 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 function) {
Objects.requireNonNull(function); // explicit check required in case
// table is empty.
final int expectedModCount = modCount;
Entry[] tab = (Entry[])table;
for (Entry 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 entry = (Entry)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 e = (Entry)tab[index];
for (Entry 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 e = (Entry)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 e = (Entry)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 mappingFunction) {
Objects.requireNonNull(mappingFunction);
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
@SuppressWarnings("unchecked")
Entry e = (Entry)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 remappingFunction) {
Objects.requireNonNull(remappingFunction);
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
@SuppressWarnings("unchecked")
Entry e = (Entry)tab[index];
for (Entry 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 remappingFunction) {
Objects.requireNonNull(remappingFunction);
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
@SuppressWarnings("unchecked")
Entry e = (Entry)tab[index];
for (Entry 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 remappingFunction) {
Objects.requireNonNull(remappingFunction);
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
@SuppressWarnings("unchecked")
Entry e = (Entry)tab[index];
for (Entry 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 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 e = (Entry)tab[index];
tab[index] = new Entry<>(hash, key, value, e);
count++;
}
/**
* Hashtable bucket collision list entry
*/
private static class Entry implements Map.Entry {
final int hash;
final K key;
V value;
Entry next;
protected Entry(int hash, K key, V value, Entry 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) 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 implements Enumeration, Iterator {
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 e = (Entry)tab[index];
for(Entry 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();
}
}
}
}
HashTable 关键技术分析
HashTable 示例
面试session
- 谈谈你对HashTable的认识？
- 谈谈HashTable和HashMap的异同？