Hashtable源码分析
类结构
public class Hashtable<K,V>
extends Dictionary<K,V>
implements Map<K,V>, Cloneable, java.io.Serializable
Hashtable继承自Dictionary实现了Map接口。
Hashtable实现了Cloneable可以进行克隆。
Hashtable实现了Serializable可以进行序列化。
属性
//保存节点的数组bucket
private transient Entry<?,?>[] table;
//Hashtable中存放元素的个数
private transient int count;
//Hashtable进行扩容的阈值
private int threshold;
//用于计算阈值的加载因子
private float loadFactor;
//进行破坏结构的修改次数，与遍历时的快速失败有关
private transient int modCount = 0;
//最大容量，2的31次方-9
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
节点
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();
}
}
构造器
//指定初始化容量和加载因子的构造器
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);
}
//指定初始化大小的构造器
public Hashtable(int initialCapacity) {
//默认加载因子为0.75
this(initialCapacity, 0.75f);
}
//默认初始化大小为11，加载因子为0.75
public Hashtable() {
this(11, 0.75f);
}
//使用集合初始化的构造器
public Hashtable(Map<? extends K, ? extends V> t) {
this(Math.max(2*t.size(), 11), 0.75f);
putAll(t);
}
方法
Hashtable的所有方法都加了synchronized关键字，所以是线程安全的
contains(Object)方法判断Hashtable中是否包含某个元素
public synchronized boolean contains(Object value) {
if (value == null) {
throw new NullPointerException();
}
Entry<?,?> tab[] = table;
for (int i = tab.length ; i-- > 0 ;) {//遍历保存元素数组
for (Entry<?,?> e = tab[i] ; e != null ; e = e.next) {//有后继就遍历链表
if (e.value.equals(value)) {//使用保存节点的V的equals方法判断是否相等
return true;
}
}
}
return false;
}
containsKey(Object）判断Hashtable中是否包含这个key
public synchronized boolean containsKey(Object key) {
Entry<?,?> tab[] = table;
int hash = key.hashCode();
//计算下标的公式，如果包含这个key那么一定在数组的这个位置上或后继节点上
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {//hash值要相等并且key要equals
return true;
}
}
return false;
}
get(Object）方法根据key获取值，不能使用null的key，会抛出NullPointerException
public synchronized V get(Object key) {
Entry<?,?> tab[] = table;
//key为null会抛出NullPointerException
int hash = key.hashCode();
//计算下标
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;
}
put(K，V）向Hashtable中添加元素,可以看到Hashtable不允许key或value为null，抛出NullPointerException
public synchronized V put(K key, V value) {
// 确保添加的value不空
if (value == null) {
throw new NullPointerException();
}
// Makes sure the key is not already in the hashtable.
Entry<?,?> tab[] = table;
//key为null会抛出NullPointerException
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
@SuppressWarnings("unchecked")
Entry<K,V> entry = (Entry<K,V>)tab[index];//通过key计算下标，如果该key已经使用，那么一定在该节点或该节点的后继
for(; entry != null ; entry = entry.next) {//遍历
if ((entry.hash == hash) && entry.key.equals(key)) {
V old = entry.value;
entry.value = value;
return old;
}
}
//如果没找到key对应的节点，说明key没有使用过，那么新增节点保存<K,V>
addEntry(hash, key, value, index);
return null;
}
private void addEntry(int hash, K key, V value, int index) {
modCount++;
Entry<?,?> tab[] = table;
if (count >= threshold) {//达到了阈值要进行扩容
// 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];
//头插法
tab[index] = new Entry<>(hash, key, value, e);
count++;
}
protected void rehash() {
int oldCapacity = table.length;
Entry<?,?>[] oldMap = table;
// 新容量为老容量的2倍+1
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 ;) {//遍历原来的保存节点的数组，把原来的节点全部添加到新数组
for (Entry<K,V> old = (Entry<K,V>)oldMap[i] ; old != null ; ) {
Entry<K,V> e = old;
old = old.next;
//重新计算节点保存的下标
int index = (e.hash & 0x7FFFFFFF) % newCapacity;
//头插法
//把新计算下标的元素挂到当前元素的next
e.next = (Entry<K,V>)newMap[index];
//把当前元素添加到新下标位置
newMap[index] = e;
}
}
}
remove(Object）方法根据key移除Hashtable中的节点
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)) {//找到要移除的节点
modCount++;
if (prev != null) {//如果有前驱把前驱的next指向移除节点的next
prev.next = e.next;
} else {//没有前驱说明是保存在table数组中的元素，直接把该元素的后继保存到数组
tab[index] = e.next;
}
count--;
V oldValue = e.value;
e.value = null;
return oldValue;
}
}
return null;
}
remove(Object，Object）根据key和value移除节点，也需要key和value都不为null
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)) {//通过key和value一起判断
modCount++;
if (prev != null) {
prev.next = e.next;
} else {
tab[index] = e.next;
}
count--;
e.value = null;
return true;
}
}
return false;
}
replace(K，V，V）根据K,V把找到节点把原value替换为新value
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)) {//根据key找到节点
if (e.value.equals(oldValue)) {//判断原value是不是和参数一致
//替换
e.value = newValue;
return true;
} else {
return false;
}
}
}
return false;
}
replace(K，V）根据key找到节点使用新value替换
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)) {//使用key判断
V oldValue = e.value;
e.value = value;
return oldValue;
}
}
return null;
}