转载地址:http://blog.csdn.net/wangxiaotongfan/article/details/51345426
前一章,我们学习了HashMap。这一章,我们对Hashtable进行学习。
我们先对Hashtable有个整体认识,然后再学习它的源码,最后再通过实例来学会使用Hashtable。
第1部分 Hashtable介绍
第2部分 Hashtable数据结构
第3部分 Hashtable源码解析(基于JDK1.6.0_45)
第4部分 Hashtable遍历方式
第5部分 Hashtable示例
和HashMap一样,Hashtable 也是一个散列表,它存储的内容是键值对(key-value)映射。
Hashtable 继承于Dictionary,实现了Map、Cloneable、java.io.Serializable接口。
Hashtable 的函数都是同步的,这意味着它是线程安全的。它的key、value都不可以为null。此外,Hashtable中的映射不是有序的。
Hashtable 的实例有两个参数影响其性能:初始容量 和 加载因子。容量 是哈希表中桶 的数量,初始容量 就是哈希表创建时的容量。注意,哈希表的状态为 open:在发生“哈希冲突”的情况下,单个桶会存储多个条目,这些条目必须按顺序搜索。加载因子 是对哈希表在其容量自动增加之前可以达到多满的一个尺度。初始容量和加载因子这两个参数只是对该实现的提示。关于何时以及是否调用 rehash 方法的具体细节则依赖于该实现。
通常,默认加载因子是 0.75, 这是在时间和空间成本上寻求一种折衷。加载因子过高虽然减少了空间开销,但同时也增加了查找某个条目的时间(在大多数 Hashtable 操作中,包括 get 和 put 操作,都反映了这一点)。
public Hashtable()
// 指定“容量大小”的构造函数
public Hashtable(int initialCapacity)
// 指定“容量大小”和“加载因子”的构造函数
public Hashtable(int initialCapacity, float loadFactor)
// 包含“子Map”的构造函数
public Hashtable(Map<? extends K, ? extends V> t)
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()
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Hashtable的继承关系
java.lang.Object
↳ java.util.Dictionary<K, V>
↳ java.util.Hashtable<K, V>
public class Hashtable<K,V> extends Dictionary<K,V>
implements Map<K,V>, Cloneable, java.io.Serializable { }
Hashtable与Map关系如下图:
从图中可以看出:
(01) Hashtable继承于Dictionary类,实现了Map接口。Map是”key-value键值对”接口,Dictionary是声明了操作”键值对”函数接口的抽象类。
(02) Hashtable是通过”拉链法”实现的哈希表。它包括几个重要的成员变量:table, count, threshold, loadFactor, modCount。
table是一个Entry[]数组类型,而Entry实际上就是一个单向链表。哈希表的”key-value键值对”都是存储在Entry数组中的。
count是Hashtable的大小,它是Hashtable保存的键值对的数量。
threshold是Hashtable的阈值,用于判断是否需要调整Hashtable的容量。threshold的值=”容量*加载因子”。
loadFactor就是加载因子。
modCount是用来实现fail-fast机制的
第3部分 Hashtable源码解析(基于JDK1.6.0_45) |
为了更了解Hashtable的原理,下面对Hashtable源码代码作出分析。
在阅读源码时,建议参考后面的说明来建立对Hashtable的整体认识,这样更容易理解Hashtable。
package java.util;
import java.io.*;
public class Hashtable<K,V>
extends Dictionary<K,V>
implements Map<K,V>, Cloneable, java.io.Serializable {
private transient Entry[] table;
private transient int count;
private int threshold;
private float loadFactor;
private transient int modCount = 0;
private static final long serialVersionUID = 1421746759512286392L;
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);
}
public Hashtable(int initialCapacity) {
this(initialCapacity, 0.75f);
}
public Hashtable() {
this(11, 0.75f);
}
public Hashtable(Map<? extends K, ? extends V> t) {
this(Math.max(2*t.size(), 11), 0.75f);
putAll(t);
}
public synchronized int size() {
return count;
}
public synchronized boolean isEmpty() {
return count == 0;
}
public synchronized Enumeration<K> keys() {
return this.<K>getEnumeration(KEYS);
}
public synchronized Enumeration<V> elements() {
return this.<V>getEnumeration(VALUES);
}
public synchronized boolean contains(Object value) {
if (value == null) {
throw new NullPointerException();
}
Entry tab[] = table;
for (int i = tab.length ; i-- > 0 ;) {
for (Entry<K,V> e = tab[i] ; e != null ; e = e.next) {
if (e.value.equals(value)) {
return true;
}
}
}
return false;
}
public boolean containsValue(Object value) {
return contains(value);
}
public synchronized boolean containsKey(Object key) {
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
return true;
}
}
return false;
}
public synchronized V get(Object key) {
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
return e.value;
}
}
return null;
}
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<K,V> old = oldMap[i] ; old != null ; ) {
Entry<K,V> e = old;
old = old.next;
int index = (e.hash & 0x7FFFFFFF) % newCapacity;
e.next = newMap[index];
newMap[index] = e;
}
}
}
public synchronized V put(K key, V value) {
if (value == null) {
throw new NullPointerException();
}
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
V old = e.value;
e.value = value;
return old;
}
}
modCount++;
if (count >= threshold) {
rehash();
tab = table;
index = (hash & 0x7FFFFFFF) % tab.length;
}
Entry<K,V> e = tab[index];
tab[index] = new Entry<K,V>(hash, key, value, e);
count++;
return null;
}
public synchronized V remove(Object key) {
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> 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--;
V oldValue = e.value;
e.value = null;
return oldValue;
}
}
return null;
}
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());
}
public synchronized void clear() {
Entry tab[] = table;
modCount++;
for (int index = tab.length; --index >= 0; )
tab[index] = null;
count = 0;
}
public synchronized Object clone() {
try {
Hashtable<K,V> t = (Hashtable<K,V>) super.clone();
t.table = new Entry[table.length];
for (int i = table.length ; i-- > 0 ; ) {
t.table[i] = (table[i] != null)
? (Entry<K,V>) table[i].clone() : null;
}
t.keySet = null;
t.entrySet = null;
t.values = null;
t.modCount = 0;
return t;
} catch (CloneNotSupportedException e) {
throw new InternalError();
}
}
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 (Enumeration<T>)emptyEnumerator;
} else {
return new Enumerator<T>(type, false);
}
}
private <T> Iterator<T> getIterator(int type) {
if (count == 0) {
return (Iterator<T>) emptyIterator;
} else {
return new Enumerator<T>(type, true);
}
}
private transient volatile Set<K> keySet = null;
private transient volatile Set<Map.Entry<K,V>> entrySet = null;
private transient volatile Collection<V> values = null;
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();
}
}
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<K,V> entry = (Map.Entry<K,V>) o;
K key = entry.getKey();
Entry[] tab = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> 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();
}
}
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();
}
}
public synchronized boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof Map))
return false;
Map<K,V> t = (Map<K,V>) 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;
}
public synchronized int hashCode() {
int h = 0;
if (count == 0 || loadFactor < 0)
return h;
loadFactor = -loadFactor;
Entry[] tab = table;
for (int i = 0; i < tab.length; i++)
for (Entry e = tab[i]; e != null; e = e.next)
h += e.key.hashCode() ^ e.value.hashCode();
loadFactor = -loadFactor;
return h;
}
private synchronized void writeObject(java.io.ObjectOutputStream s)
throws IOException
{
s.defaultWriteObject();
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;
}
}
}
private void readObject(java.io.ObjectInputStream s)
throws IOException, ClassNotFoundException
{
s.defaultReadObject();
int origlength = s.readInt();
int elements = s.readInt();
int length = (int)(elements * loadFactor) + (elements / 20) + 3;
if (length > elements && (length & 1) == 0)
length--;
if (origlength > 0 && length > origlength)
length = origlength;
Entry[] table = new Entry[length];
count = 0;
for (; elements > 0; elements--) {
K key = (K)s.readObject();
V value = (V)s.readObject();
reconstitutionPut(table, key, value);
}
this.table = table;
}
private void reconstitutionPut(Entry[] tab, K key, V value)
throws StreamCorruptedException
{
if (value == null) {
throw new java.io.StreamCorruptedException();
}
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
throw new java.io.StreamCorruptedException();
}
}
Entry<K,V> e = tab[index];
tab[index] = new Entry<K,V>(hash, key, value, e);
count++;
}
private static class Entry<K,V> implements Map.Entry<K,V> {
int hash;
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;
}
protected Object clone() {
return new Entry<K,V>(hash, key, value,
(next==null ? null : (Entry<K,V>) next.clone()));
}
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 ^ (value==null ? 0 : value.hashCode());
}
public String toString() {
return key.toString()+"="+value.toString();
}
}
private static final int KEYS = 0;
private static final int VALUES = 1;
private static final int ENTRIES = 2;
private class Enumerator<T> implements Enumeration<T>, Iterator<T> {
Entry[] table = Hashtable.this.table;
int index = table.length;
Entry<K,V> entry = null;
Entry<K,V> lastReturned = null;
int type;
boolean iterator;
protected int expectedModCount = modCount;
Enumerator(int type, boolean iterator) {
this.type = type;
this.iterator = iterator;
}
public boolean hasMoreElements() {
Entry<K,V> e = entry;
int i = index;
Entry[] t = table;
while (e == null && i > 0) {
e = t[--i];
}
entry = e;
index = i;
return e != null;
}
public T nextElement() {
Entry<K,V> et = entry;
int i = index;
Entry[] t = table;
while (et == null && i > 0) {
et = t[--i];
}
entry = et;
index = i;
if (et != null) {
Entry<K,V> e = lastReturned = entry;
entry = e.next;
return type == KEYS ? (T)e.key : (type == VALUES ? (T)e.value : (T)e);
}
throw new NoSuchElementException("Hashtable Enumerator");
}
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;
for (Entry<K,V> 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 Enumeration emptyEnumerator = new EmptyEnumerator();
private static Iterator emptyIterator = new EmptyIterator();
private static class EmptyEnumerator implements Enumeration<Object> {
EmptyEnumerator() {
}
public boolean hasMoreElements() {
return false;
}
public Object nextElement() {
throw new NoSuchElementException("Hashtable Enumerator");
}
}
private static class EmptyIterator implements Iterator<Object> {
EmptyIterator() {
}
public boolean hasNext() {
return false;
}
public Object next() {
throw new NoSuchElementException("Hashtable Iterator");
}
public void remove() {
throw new IllegalStateException("Hashtable Iterator");
}
}
}
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说明: 在详细介绍Hashtable的代码之前,我们需要了解:和Hashmap一样,Hashtable也是一个散列表,它也是通过“拉链法”解决哈希冲突的。
第3.1部分 Hashtable的“拉链法”相关内容 |
private transient Entry[] table;
Hashtable中的key-value都是存储在table数组中的。
private static class Entry<K,V> implements Map.Entry<K,V> {
int hash;
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;
}
protected Object clone() {
return new Entry<K,V>(hash, key, value,
(next==null ? null : (Entry<K,V>) next.clone()));
}
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 ^ (value==null ? 0 : value.hashCode());
}
public String toString() {
return key.toString()+"="+value.toString();
}
}
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从中,我们可以看出 Entry 实际上就是一个单向链表。这也是为什么我们说Hashtable是通过拉链法解决哈希冲突的。
Entry 实现了Map.Entry 接口,即实现getKey(), getValue(), setValue(V value), equals(Object o), hashCode()这些函数。这些都是基本的读取/修改key、value值的函数。
Hashtable共包括4个构造函数
public Hashtable() {
this(11, 0.75f);
}
public Hashtable(int initialCapacity) {
this(initialCapacity, 0.75f);
}
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);
}
public Hashtable(Map<? extends K, ? extends V> t) {
this(Math.max(2*t.size(), 11), 0.75f);
putAll(t);
}
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clear() 的作用是清空Hashtable。它是将Hashtable的table数组的值全部设为null
public synchronized void clear() {
Entry tab[] = table;
modCount++;
for (int index = tab.length; --index >= 0; )
tab[index] = null;
count = 0;
}
3.3.2 contains() 和 containsValue() |
contains() 和 containsValue() 的作用都是判断Hashtable是否包含“值(value)”
public boolean containsValue(Object value) {
return contains(value);
}
public synchronized boolean contains(Object value) {
if (value == null) {
throw new NullPointerException();
}
Entry tab[] = table;
for (int i = tab.length ; i-- > 0 ;) {
for (Entry<K,V> e = tab[i] ; e != null ; e = e.next) {
if (e.value.equals(value)) {
return true;
}
}
}
return false;
}
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containsKey() 的作用是判断Hashtable是否包含key
public synchronized boolean containsKey(Object key) {
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
return true;
}
}
return false;
}
elements() 的作用是返回“所有value”的枚举对象
public synchronized Enumeration<V> elements() {
return this.<V>getEnumeration(VALUES);
}
private <T> Enumeration<T> getEnumeration(int type) {
if (count == 0) {
return (Enumeration<T>)emptyEnumerator;
} else {
return new Enumerator<T>(type, false);
}
}
从中,我们可以看出:
(01) 若Hashtable的实际大小为0,则返回“空枚举类”对象emptyEnumerator;
(02) 否则,返回正常的Enumerator的对象。(Enumerator实现了迭代器和枚举两个接口)
我们先看看emptyEnumerator对象是如何实现的
private static Enumeration emptyEnumerator = new EmptyEnumerator();
// 空枚举类
// 当Hashtable的实际大小为0;此时,又要通过Enumeration遍历Hashtable时,返回的是“空枚举类”的对象。
private static class EmptyEnumerator implements Enumeration<Object> {
EmptyEnumerator() {
}
public boolean hasMoreElements() {
return false;
}
public Object nextElement() {
throw new NoSuchElementException("Hashtable Enumerator");
}
}
我们在来看看Enumeration类
Enumerator的作用是提供了“通过elements()遍历Hashtable的接口” 和 “通过entrySet()遍历Hashtable的接口”。因为,它同时实现了 “Enumerator接口”和“Iterator接口”。
private class Enumerator<T> implements Enumeration<T>, Iterator<T> {
Entry[] table = Hashtable.this.table;
int index = table.length;
Entry<K,V> entry = null;
Entry<K,V> lastReturned = null;
int type;
boolean iterator;
protected int expectedModCount = modCount;
Enumerator(int type, boolean iterator) {
this.type = type;
this.iterator = iterator;
}
public boolean hasMoreElements() {
Entry<K,V> e = entry;
int i = index;
Entry[] t = table;
while (e == null && i > 0) {
e = t[--i];
}
entry = e;
index = i;
return e != null;
}
public T nextElement() {
Entry<K,V> et = entry;
int i = index;
Entry[] t = table;
while (et == null && i > 0) {
et = t[--i];
}
entry = et;
index = i;
if (et != null) {
Entry<K,V> e = lastReturned = entry;
entry = e.next;
return type == KEYS ? (T)e.key : (type == VALUES ? (T)e.value : (T)e);
}
throw new NoSuchElementException("Hashtable Enumerator");
}
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;
for (Entry<K,V> 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();
}
}
}
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entrySet(), keySet(), keys(), values()的实现方法和elements()差不多,而且源码中已经明确的给出了注释。这里就不再做过多说明了。
get() 的作用就是获取key对应的value,没有的话返回null
public synchronized V get(Object key) {
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
return e.value;
}
}
return null;
}
put() 的作用是对外提供接口,让Hashtable对象可以通过put()将“key-value”添加到Hashtable中。
public synchronized V put(K key, V value) {
if (value == null) {
throw new NullPointerException();
}
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
V old = e.value;
e.value = value;
return old;
}
}
modCount++;
if (count >= threshold) {
rehash();
tab = table;
index = (hash & 0x7FFFFFFF) % tab.length;
}
Entry<K,V> e = tab[index];
tab[index] = new Entry<K,V>(hash, key, value, e);
count++;
return null;
}
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putAll() 的作用是将“Map(t)”的中全部元素逐一添加到Hashtable中
1 public synchronized void putAll(Map<? extends K, ? extends V> t) {
2 for (Map.Entry<? extends K, ? extends V> e : t.entrySet())
3 put(e.getKey(), e.getValue());
4 }
remove() 的作用就是删除Hashtable中键为key的元素
public synchronized V remove(Object key) {
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> 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--;
V oldValue = e.value;
e.value = null;
return oldValue;
}
}
return null;
}
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第3.4部分 Hashtable实现的Cloneable接口 |
Hashtable实现了Cloneable接口,即实现了clone()方法。
clone()方法的作用很简单,就是克隆一个Hashtable对象并返回。
public synchronized Object clone() {
try {
Hashtable<K,V> t = (Hashtable<K,V>) super.clone();
t.table = new Entry[table.length];
for (int i = table.length ; i-- > 0 ; ) {
t.table[i] = (table[i] != null)
? (Entry<K,V>) table[i].clone() : null;
}
t.keySet = null;
t.entrySet = null;
t.values = null;
t.modCount = 0;
return t;
} catch (CloneNotSupportedException e) {
throw new InternalError();
}
}
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第3.5部分 Hashtable实现的Serializable接口 |
Hashtable实现java.io.Serializable,分别实现了串行读取、写入功能。
串行写入函数就是将Hashtable的“总的容量,实际容量,所有的Entry”都写入到输出流中
串行读取函数:根据写入方式读出将Hashtable的“总的容量,实际容量,所有的Entry”依次读出
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;
}
}
}
private 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;
Entry[] table = new Entry[length];
count = 0;
// Read the number of elements and then all the key/value objects
for (; elements > 0; elements--) {
K key = (K)s.readObject();
V value = (V)s.readObject();
// synch could be eliminated for performance
reconstitutionPut(table, key, value);
}
this.table = table;
}
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第一步:根据entrySet()获取Hashtable的“键值对”的Set集合。
第二步:通过Iterator迭代器遍历“第一步”得到的集合。
Integer integ = null;
Iterator iter = table.entrySet().iterator();
while(iter.hasNext()) {
Map.Entry entry = (Map.Entry)iter.next();
key = (String)entry.getKey();
integ = (Integer)entry.getValue();
}
4.2 通过Iterator遍历Hashtable的键 |
第一步:根据keySet()获取Hashtable的“键”的Set集合。
第二步:通过Iterator迭代器遍历“第一步”得到的集合。
String key = null;
Integer integ = null;
Iterator iter = table.keySet().iterator();
while (iter.hasNext()) {
key = (String)iter.next();
integ = (Integer)table.get(key);
}
4.3 通过Iterator遍历Hashtable的值 |
第一步:根据value()获取Hashtable的“值”的集合。
第二步:通过Iterator迭代器遍历“第一步”得到的集合。
Integer value = null;
Collection c = table.values();
Iterator iter= c.iterator();
while (iter.hasNext()) {
value = (Integer)iter.next();
}
4.4 通过Enumeration遍历Hashtable的键 |
第一步:根据keys()获取Hashtable的集合。
第二步:通过Enumeration遍历“第一步”得到的集合。
Enumeration enu = table.keys()
while(enu.hasMoreElements()) {
System.out.println(enu.nextElement())
}
4.5 通过Enumeration遍历Hashtable的值 |
第一步:根据elements()获取Hashtable的集合。
第二步:通过Enumeration遍历“第一步”得到的集合。
Enumeration enu = table.elements()
while(enu.hasMoreElements()) {
System.out.println(enu.nextElement())
}
遍历测试程序如下:
import java.util.*;
public class HashtableIteratorTest {
public static void main(String[] args) {
int val = 0;
String key = null;
Integer value = null;
Random r = new Random();
Hashtable table = new Hashtable();
for (int i=0; i<12; i++) {
val = r.nextInt(100);
key = String.valueOf(val);
value = r.nextInt(5);
table.put(key, value);
System.out.println(" key:"+key+" value:"+value);
}
iteratorHashtableByEntryset(table) ;
iteratorHashtableByKeyset(table) ;
iteratorHashtableJustValues(table);
enumHashtableKey(table);
}
private static void enumHashtableKey(Hashtable table) {
if (table == null)
return ;
System.out.println("\nenumeration Hashtable");
Enumeration enu = table.keys();
while(enu.hasMoreElements()) {
System.out.println(enu.nextElement());
}
}
private static void enumHashtableValue(Hashtable table) {
if (table == null)
return ;
System.out.println("\nenumeration Hashtable");
Enumeration enu = table.elements();
while(enu.hasMoreElements()) {
System.out.println(enu.nextElement());
}
}
private static void iteratorHashtableByEntryset(Hashtable table) {
if (table == null)
return ;
System.out.println("\niterator Hashtable By entryset");
String key = null;
Integer integ = null;
Iterator iter = table.entrySet().iterator();
while(iter.hasNext()) {
Map.Entry entry = (Map.Entry)iter.next();
key = (String)entry.getKey();
integ = (Integer)entry.getValue();
System.out.println(key+" -- "+integ.intValue());
}
}
private static void iteratorHashtableByKeyset(Hashtable table) {
if (table == null)
return ;
System.out.println("\niterator Hashtable By keyset");
String key = null;
Integer integ = null;
Iterator iter = table.keySet().iterator();
while (iter.hasNext()) {
key = (String)iter.next();
integ = (Integer)table.get(key);
System.out.println(key+" -- "+integ.intValue());
}
}
private static void iteratorHashtableJustValues(Hashtable table) {
if (table == null)
return ;
Collection c = table.values();
Iterator iter= c.iterator();
while (iter.hasNext()) {
System.out.println(iter.next());
}
}
}
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第5部分 Hashtable示例
下面通过一个实例来学习如何使用Hashtable。
import java.util.*;
public class HashtableTest {
public static void main(String[] args) {
testHashtableAPIs();
}
private static void testHashtableAPIs() {
Random r = new Random();
Hashtable table = new Hashtable();
table.put("one", r.nextInt(10));
table.put("two", r.nextInt(10));
table.put("three", r.nextInt(10));
System.out.println("table:"+table );
Iterator iter = table.entrySet().iterator();
while(iter.hasNext()) {
Map.Entry entry = (Map.Entry)iter.next();
System.out.println("next : "+ entry.getKey() +" - "+entry.getValue());
}
System.out.println("size:"+table.size());
System.out.println("contains key two : "+table.containsKey("two"));
System.out.println("contains key five : "+table.containsKey("five"));
System.out.println("contains value 0 : "+table.containsValue(new Integer(0)));
table.remove("three");
System.out.println("table:"+table );
table.clear();
System.out.println((table.isEmpty()?"table is empty":"table is not empty") );
}
}
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(某一次)运行结果:
table:{two=5, one=0, three=6}
next : two - 5
next : one - 0
next : three - 6
size:3
contains key two : true
contains key five : false
contains value 0 : true
table:{two=5, one=0}
table is empty