HashMap和HashTable
1.继承父类不同
Hashtable继承自Dictionary类,而HashMap继承自AbstractMap类。但二者都实现了Map接口。
public class HashMap<K,V> extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable
public class Hashtable<K,V>
extends Dictionary<K,V>
implements Map<K,V>, Cloneable, java.io.Serializable
2.是否允许空值
HashMap可以存储null键和null值;
HashTable不能存储null键和null值;
public synchronized V put(K key, V value) {
// Make sure the value is not null
if (value == null) {
throw new NullPointerException();
}
// Makes sure the key is not already in the hashtable.
Entry<?,?> tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
@SuppressWarnings("unchecked")
Entry<K,V> entry = (Entry<K,V>)tab[index];
for(; entry != null ; entry = entry.next) {
if ((entry.hash == hash) && entry.key.equals(key)) {
V old = entry.value;
entry.value = value;
return old;
}
}
addEntry(hash, key, value, index);
return null;
}
3.线程安全问题
HashMap是非synchronized,而Hashtable是synchronized,这意味着Hashtable是线程安全的,多个线程可以共享一个Hashtable;而如果没有正确的同步的话,多个线程是不能共享HashMap的。
4.速度问题
因为HashMap是线程不安全的,所以在速度上 会比HashTable要快一些。
5.hash值不同
Hashtable计算hash值,直接用key的hashCode(),而HashMap重新计算了key的hash值,Hashtable在求hash值对应的位置索引时,用取模运算,而HashMap在求位置索引时,则用与运算
Hashtable.java
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;
//直接调用key的hashCode方法
int hash = key.hashCode();
//计算索引的时候是使用取模运算
int index = (hash & 0x7FFFFFFF) % tab.length;
@SuppressWarnings("unchecked")
Entry<K,V> entry = (Entry<K,V>)tab[index];
for(; entry != null ; entry = entry.next) {
if ((entry.hash == hash) && entry.key.equals(key)) {
V old = entry.value;
entry.value = value;
return old;
}
}
addEntry(hash, key, value, index);
return null;
}
HashMap.java
public V put(K key, V value) {
//重新计算key的hashCode
return putVal(hash(key), key, value, false, true);
}
static final int hash(Object key) {
int h;
//计算方式为key的hashCode与其无符号右移2^16的异或操作
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
//计算索引的时候是使用与运算
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
6.内部初始化及扩容方式不同
HashTable中hash数组默认大小是11,增加的方式是 old*2+1。而HashMap扩容时,将容量变为原来的2倍。
Hashtable.java
public Hashtable() {
this(11, 0.75f);
}
HashMap.java
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
7.是否提供contains方法
Hashtable则保留了contains,containsValue和containsKey三个方法,其中contains和containsValue功能相同。
HashMap把Hashtable的contains方法去掉了,改成containsValue和containsKey,因为contains方法容易让人引起误解。