上一章我们学习了HashMap的源码,这一节我们来讨论一下HashTable,HashTable和HashMap在某种程度上是类似的。我们依然遵循以下步骤:先对HashTable有个整体的认识,然后学习它的源码,深入剖析HashTable。
1.HashTable简介
首先看一下HashTable的继承关系
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- 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 { }
我们可以看出,HashTable不但继承了Dictionary,而且实现了Map、Cloneable和Serializable接口,所以HashTable也可以实例化。HashTable和hashMap不同,HashTable是线程安全的(等会我们在源码中就能看出)。下面我们先总览一下HashTable都有哪些API,然后我们详细分析它们。
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- 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()
从HashTable的API中可以看出,HashTable之所以是线程安全的,是因为方法上都加了synchronized关键字。
2. HashTable的数据结构
2.1 存储结构
和HashMap一样,HashTable内部也维护了一个数组,数组中存放的是Entry<K,V>实体,数组定义如下:
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- private transient Entry<K,V>[] table;
然后我们看看Entry实体的定义:
2.2 Entry实体
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- /**
- * Entry实体类的定义
- */
- private static class Entry<K,V> implements Map.Entry<K,V> {
- int hash; //哈希值
- final K key;
- V value;
- Entry<K,V> next; //指向的下一个Entry,即链表的下一个节点
- //构造方法
- protected Entry(int hash, K key, V value, Entry<K,V> next) {
- this.hash = hash;
- this.key = key;
- this.value = value;
- this.next = next;
- }
- //由于HashTable实现了Cloneable接口,所以支持克隆操作
- protected Object clone() {
- return new Entry<>(hash, key, value, (next==null ? null : (Entry<K,V>) next.clone()));
- }
- //下面对Map.Entry的具体操作了
- public K getKey() { //拿到key
- return key;
- }
- public V getValue() { //拿到value
- return value;
- }
- public V setValue(V value) { //设置value
- if (value == null) //从这里可以看出,HashTable中的value是不允许为空的!
- throw new NullPointerException();
- V oldValue = this.value;
- this.value = value;
- return oldValue;
- }
- //判断两个Entry是否相等
- public boolean equals(Object o) {
- if (!(o instanceof Map.Entry))
- return false;
- Map.Entry<?,?> e = (Map.Entry)o;
- //必须两个Entry的key和value均相等才行
- return key.equals(e.getKey()) && value.equals(e.getValue());
- }
- public int hashCode() { //计算hashCode
- return (Objects.hashCode(key) ^ Objects.hashCode(value));
- }
- public String toString() { //重写toString方法
- return key.toString()+"="+value.toString();
- }
- }
从Entry实体的源码中可以看出,HashTable其实就是个存储Entry的数组,Entry中包含了键值对以及下一个Entry(用来处理冲突的),形成链表。而且Entry中的value是不允许为nul的。好了,我们对HashTable整体上了解了后,下面开始详细分析HashTable中的源码。
3.HashTable源码分析(基于JDK1.7)
3.1 成员属性
首先我们看看HashTable都有哪些关键属性:
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- private transient Entry<K,V>[] table;
- private transient int count;//记录HashTable中有多少Entry实体
- //阈值,用于判断是否需要调整Hashtable的容量(threshold = 容量*加载因子)
- private int threshold;
- private float loadFactor; // 加载因子
- private transient int modCount = 0; // Hashtable被改变的次数,用于fail-fast
- // 序列版本号
- private static final long serialVersionUID = 1421746759512286392L;
- //最大的门限阈值,不能超过这个
- static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;
这写成员属性的功能和HashMap基本上都一样的,这里就不再赘述了,详细信息可以看下上一篇博文HashMap对应的该部分。下面看看HashTable的几个构造方法:
3.2 构造方法
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- //参数为数组容量和加载因子的构造方法
- 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]; //初始化数组
- //初始化门限 = 容量 * 加载因子
- threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
- initHashSeedAsNeeded(initialCapacity);
- }
- //参数为初始容量的构造方法
- public Hashtable(int initialCapacity) {
- this(initialCapacity, 0.75f); //我们可以看出,默认加载因子为0.75
- }
- //默认构造方法
- public Hashtable() { //可以看出,默认容量为11,加载因子为0.75
- this(11, 0.75f);
- }
- //包含“子Map”的构造函数
- public Hashtable(Map<? extends K, ? extends V> t) {
- this(Math.max(2*t.size(), 11), 0.75f);//先比较容量,如果Map的2倍容量大于11,则使用新的容量
- putAll(t);
- }
我们可以看到,如果我们不指定数组容量和加载因子,HashTable会自动初始化容量为11,加载因子为0.75。加载因子和HashMap是相同的。
3.3 存取方法
和HashMap的分析一样,HashTable的存取部分重点分析put和get方法,其他的方法我放到代码中分析。首先看看HashTable是如何存储数据的:
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- public synchronized V put(K key, V value) {
- //确保value不为空
- if (value == null) {
- throw new NullPointerException();
- }
- Entry tab[] = table;
- int hash = hash(key); //计算哈希值
- int index = (hash & 0x7FFFFFFF) % tab.length; //根据哈希值计算在数组中的索引
- for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
- if ((e.hash == hash) && e.key.equals(key)) { //如果对应的key已经存在
- V old = e.value;
- e.value = value; //替换掉原来的value
- return old;
- }
- }
- //否则新添加一个Entry
- modCount++;
- if (count >= threshold) { //判断数组中的Entry数量是否已经达到阈值
- rehash(); //如果达到了,扩容
- tab = table;
- hash = hash(key); //重新计算哈希值
- index = (hash & 0x7FFFFFFF) % tab.length; //重新计算在新的数组中的索引
- }
- //创建一个新的Entry
- Entry<K,V> e = tab[index];
- //存到对应的位置,并将其next置为原来该位置的Entry,这样就与原来的连上了
- tab[index] = new Entry<>(hash, key, value, e);
- count++;
- return null;
- }
put方法中,首先检测value是否为null,如果为null则会抛出NullPointerException异常。然后往下走,跟HashMap的过程一样,先计算哈希值,再根据哈希值计算在数组中的索引位置,不过这里计算索引位置的方法和HashMap不同,HashMap里使用的是 hash & (length-1)的方法,其实本质上跟这里用的(hash & 0x7FFFFFFF) % table.length一样的效果,但是HashMap中的方法效率要高,至于它们两为啥本质一样的,可以参见我的上一博客:HashMap,那里分析的很详细。HashTable中的很好理解,直接取余就是索引值,地球人都知道~
然后便开始往数组中存数据了,如果当前的key已经在里面了,那么直接替换原来旧的value,如果不存在,先判断数组中的Entry数量有没有达到门限值,达到了就要调用rehash方法进行扩容,然后重新计算当前key在新的数组中的索引值,然后在该位置添加进去即可。下面我们看一下rehash方法:
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- private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
- protected void rehash() {
- int oldCapacity = table.length;
- Entry<K,V>[] oldMap = table; //保存旧数组
- int newCapacity = (oldCapacity << 1) + 1; //新数组容量 = 2 * 旧容量 + 1
- if (newCapacity - MAX_ARRAY_SIZE > 0) {
- if (oldCapacity == MAX_ARRAY_SIZE)
- return;
- newCapacity = MAX_ARRAY_SIZE; //不能超出最大值
- }
- Entry<K,V>[] newMap = new Entry[newCapacity];
- modCount++;
- threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
- boolean rehash = initHashSeedAsNeeded(newCapacity);
- 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;
- if (rehash) {
- e.hash = hash(e.key);
- }
- int index = (e.hash & 0x7FFFFFFF) % newCapacity;//重新计算在新的数组中的索引
- //第一次newMap[index]为空,后面每次的nex都是当前的Entry,这样才能连上
- e.next = newMap[index];
- newMap[index] = e;//然后将该Entry放到当前位置
- }
- }
- }
到这里put方法就分析完了,还有个putAll方法,是将整个Map加到当前HashTable中,内部也是遍历每个Entry,然后调用上面的put方法而已,简单看一下吧:
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- 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());
- }
到这里,是不是感觉HashTable其实很简单,比HashMap简单多了。下面来看看get方法,也很简单,我觉得已经不用再分析了……
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- public synchronized V get(Object key) {
- Entry tab[] = table;
- int hash = hash(key); //哈希值
- int index = (hash & 0x7FFFFFFF) % tab.length; //索引值
- for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
- if ((e.hash == hash) && e.key.equals(key)) {
- return e.value; //拿到value
- }
- }
- return null;
- }
3.4 其他方法
上面分析完了存取方法,剩下来的其他方法我放到代码里分析了,也很简单:
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- //返回数组中Entry数
- public synchronized int size() {
- return count;
- }
- //判断是否为空
- public synchronized boolean isEmpty() {
- return count == 0;
- }
- //返回所有key的枚举对象
- public synchronized Enumeration<K> keys() {
- return this.<K>getEnumeration(KEYS);
- }
- //返回所有value的枚举对象
- public synchronized Enumeration<V> elements() {
- return this.<V>getEnumeration(VALUES);
- }
- //内部私有方法,返回枚举对象
- private <T> Enumeration<T> getEnumeration(int type) {
- if (count == 0) {
- return Collections.emptyEnumeration();
- } else {
- return new Enumerator<>(type, false); //new一个Enumeration对象,见下面:
- }
- }
- // Types of Enumerations/Iterations
- private static final int KEYS = 0;
- private static final int VALUES = 1;
- private static final int ENTRIES = 2;
- //私有内部类,实现了Enumeration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;
- //该字段用来决定是使用iterator还是Enumeration
- boolean iterator; //false表示使用Enumeration
- //fail-fast
- protected int expectedModCount = modCount;
- Enumerator(int type, boolean iterator) {
- this.type = type;
- this.iterator = iterator;
- }
- public boolean hasMoreElements() { //判断是否含有下一个元素
- Entry<K,V> e = entry;
- int i = index;
- Entry[] t = table;
- /* Use locals for faster loop iteration */
- while (e == null && i > 0) {
- e = t[--i];
- }
- entry = e;
- index = i;
- return e != null;
- }
- public T nextElement() { //获得下一个元素
- Entry<K,V> 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<K,V> e = lastReturned = entry;
- entry = e.next;
- //根据传进来的关键字决定返回什么
- 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) { //保证了线程安全
- 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();
- }
- }
- }
- //判断HashTable中是否包含value值
- public synchronized boolean contains(Object value) {
- if (value == null) { //value不能为空
- throw new NullPointerException();
- }
- Entry tab[] = table;
- //从后向前遍历table数组中的元素(Entry)
- 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);
- }
- //判断HashTable中是否包含key
- public synchronized boolean containsKey(Object key) {
- Entry tab[] = table;
- int hash = hash(key);
- 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;
- }
- //删除HashTable中键为key的Entry,并返回value
- public synchronized V remove(Object key) {
- Entry tab[] = table;
- int hash = hash(key);
- int index = (hash & 0x7FFFFFFF) % tab.length;
- //找到key对应的Entry,然后在链表中找到要删除的节点,删除之。
- 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;
- }
- //清空HashTable
- public synchronized void clear() {
- Entry tab[] = table;
- modCount++;
- for (int index = tab.length; --index >= 0; )
- tab[index] = null; //将HashTable中数组值全部设置为null
- count = 0;
- }
- //克隆一个HashTable,并以Object的形式返回
- 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) {
- // this shouldn't happen, since we are Cloneable
- throw new InternalError();
- }
- }
- //重写toString方法:{, ,}
- 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(", ");
- }
- }
- // Hashtable的“key的集合”。它是一个Set,意味着没有重复元素
- private transient volatile Set<K> keySet = null;
- // Hashtable的“key-value的集合”。它是一个Set,意味着没有重复元素
- private transient volatile Set<Map.Entry<K,V>> entrySet = null;
- // Hashtable的“value的集合”。它是一个Collection,意味着可以有重复元素
- private transient volatile Collection<V> values = null;
- //返回一个被synchronizedSet封装后的keySet对象
- //synchronizedSet封装的目的是对keySet的所有方法都添加synchronized,实现多线程同步
- 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); //返回一个迭代器,装有HashTable的信息
- //从这里也可以看出,获取到了key的Set集合后,要想取数据,只能通过迭代器
- }
- 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();
- }
- }
- // 获取Hashtable的迭代器
- // 若Hashtable的实际大小为0,则返回“空迭代器”对象;
- // 否则,返回正常的Enumerator的对象。(由上面代码可知,Enumerator实现了迭代器和枚举两个接口)
- private <T> Iterator<T> getIterator(int type) {
- if (count == 0) {
- return Collections.emptyIterator();
- } else {
- return new Enumerator<>(type, true);
- }
- }
- //返回一个被synchronizedSet封装后的entrySet对象
- public Set<Map.Entry<K,V>> entrySet() {
- if (entrySet==null)
- entrySet = Collections.synchronizedSet(new EntrySet(), this);
- return entrySet;
- }
- //跟keySet类似
- 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);
- }
- // 查找EntrySet中是否包含Object(o)
- // 首先,在table中找到o对应的Entry(Entry是一个单向链表)
- // 然后,查找Entry链表中是否存在Object
- 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 = hash(key);
- 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;
- }
- // 删除元素Object(o)
- // 首先,在table中找到o对应的Entry(Entry是一个单向链表)
- // 然后,删除链表中的元素Object
- 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 = hash(key);
- 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();
- }
- }
- // 返回一个被synchronizedCollection封装后的ValueCollection对象
- // synchronizedCollection封装的目的是对ValueCollection的所有方法都添加synchronized,实现多线程同步
- 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); //同上
- }
- public int size() {
- return count;
- }
- public boolean contains(Object o) {
- return containsValue(o);
- }
- public void clear() {
- Hashtable.this.clear();
- }
- }
- //重写equals()方法
- // 若两个Hashtable的所有key-value键值对都相等,则判断它们两个相等
- 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;
- }
- //计算哈希值
- //若HashTable的实际大小为0或者加载因子<0,则返回0
- //否则返回“HashTable中的每个Entry的key和value的异或值的总和”
- public synchronized int hashCode() {
- int h = 0;
- if (count == 0 || loadFactor < 0)
- return h; // Returns zero
- loadFactor = -loadFactor; // Mark hashCode computation in progress
- Entry[] tab = table;
- for (Entry<K,V> entry : tab)
- while (entry != null) {
- h += entry.hashCode();
- entry = entry.next;
- }
- loadFactor = -loadFactor; // Mark hashCode computation complete
- return h;
- }
- // java.io.Serializable的写入函数
- // 将Hashtable的“总的容量,实际容量,所有的Entry”都写入到输出流中
- private void writeObject(java.io.ObjectOutputStream s)
- throws IOException {
- Entry<K, V> entryStack = null;
- synchronized (this) {
- // Write out the length, threshold, loadfactor
- s.defaultWriteObject();
- // Write out length, 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<K,V> 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;
- }
- }
- // java.io.Serializable的读取函数: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 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<K,V>[] newTable = new Entry[length];
- threshold = (int) Math.min(length * loadFactor, MAX_ARRAY_SIZE + 1);
- count = 0;
- initHashSeedAsNeeded(length);
- // 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(newTable, key, value);
- }
- this.table = newTable;
- }
- private void reconstitutionPut(Entry<K,V>[] 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 = hash(key);
- 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();
- }
- }
- // Creates the new entry.
- Entry<K,V> e = tab[index];
- tab[index] = new Entry<>(hash, key, value, e);
- count++;
- }
4.HashTable的遍历方式
Hashtable的遍历方式比较简单,一般分两步:
1. 获得Entry或key或value的集合;
2. 通过Iterator迭代器或者Enumeration遍历此集合。
4.1 遍历HashTable的Entry (效率高)
[java] view plain copy
- // 假设table是HashTable对象
- // table中的key是String类型,value是Integer类型
- Integer value = null;
- Iterator iter = table.entrySet().iterator();
- while(iter.hasNext()) {
- Map.Entry entry = (Map.Entry)iter.next();
- // 获取key
- key = (String)entry.getKey();
- // 获取value
- value = (Integer)entry.getValue();
- }
4.2 遍历HashTable的key
[java] view plain copy
- String key = null;
- Integer value = null;
- Iterator iter = table.keySet().iterator();
- while (iter.hasNext()) {
- // 获取key
- key = (String)iter.next();
- // 根据key,获取value
- value = (Integer)table.get(key);
- }
4.3 遍历HashTable的value
[java] view plain copy
- Integer value = null;
- Collection c = table.values();
- Iterator iter= c.iterator();
- while (iter.hasNext()) {
- value = (Integer)iter.next();
- }
4.5 通过Enumeration遍历HashTable的key(效率高)
[java] view plain copy
- Enumeration enu = table.keys();
- while(enu.hasMoreElements()) {
- System.out.println(enu.nextElement());
- }
4.6 通过Enumeration遍历HashTable的value (效率高)
[java] view plain copy
- Enumeration enu = table.elements();
- while(enu.hasMoreElements()) {
- System.out.println(enu.nextElement());
- }
HashTable的遍历就介绍到这吧,至此,HashTable的源码就讨论完了,如有错误之处,欢迎留言指正~
转载:http://blog.csdn.net/eson_15/article/details/51208166