从Java源码的角度来分析HashMap与HashTable的区别
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由于HashMap与HashTable都是用来存储Key-Value的键值对，所以经常拿来对比二者的区别，下面就从源码的角度来分析一下HashMap与HashTable的区别，
首先介绍一下两者的区别，然后再从源码分析。
HahMap与HahTable两者主要区别：
1、继承的父类不同
<span style="font-size:18px;">public class HashMap<K, V> extends AbstractMap<K, V> implements Cloneable, Serializable</span><span style="font-size:18px;"><pre name="code" class="java">public class Hashtable<K, V> extends Dictionary<K, V> implements Map<K, V>, Cloneable, Serializable</span>3、HashTable中Key与Value都不允许出现NULL值，而HashMap中Key可以为
NULL,由于HashMap是Map中实现类，Map集合中要求每个Key唯一，所以HashMap
中之多只能出现一个Key为NULL的情况，但HashMap中可以出现多个Value为NULL
的情况。
4、遍历方式不同
Hashtable、HashMap都使用了 Iterator。而由于历史原因，Hashtable还使用了
Enumeration的方式 。
然后接下来看看HashMap的源码。
HashMap中一些属性Field如下：
<span style="font-size:18px;"> /**
* Min capacity (other than zero) for a HashMap. Must be a power of two
* greater than 1 (and less than 1 << 30).
* HashMap的最小容量
*/
private static final int MINIMUM_CAPACITY = 4;
/**
* Max capacity for a HashMap. Must be a power of two >= MINIMUM_CAPACITY.
* HashMap的最大容量
*/
private static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* An empty table shared by all zero-capacity maps (typically from default
* constructor). It is never written to, and replaced on first put. Its size
* is set to half the minimum, so that the first resize will create a
* minimum-sized table.
*
* HashMapEntry数组，HashMapEntry条目数组,HashMapEntry数组大小为MINIMUM_CAPACITY/2
*/
private static final Entry[] EMPTY_TABLE
= new HashMapEntry[MINIMUM_CAPACITY >>> 1];
/**
* The default load factor. Note that this implementation ignores the
* load factor, but cannot do away with it entirely because it's
* mentioned in the API.
*
* <p>Note that this constant has no impact on the behavior of the program,
* but it is emitted as part of the serialized form. The load factor of
* .75 is hardwired into the program, which uses cheap shifts in place of
* expensive division.
* 默认的负载因子
*/
static final float DEFAULT_LOAD_FACTOR = .75F;
/**
* The hash table. If this hash map contains a mapping for null, it is
* not represented this hash table.
* HashMap容器数组，用于存放HashMapEntry对象的数组
*/
transient HashMapEntry<K, V>[] table;
/**
* The entry representing the null key, or null if there's no such mapping.
* 空键值对应的HashMapEntry条目对象
*/
transient HashMapEntry<K, V> entryForNullKey;
/**
* The number of mappings in this hash map.
* HashMap中元素的数量(大小)
*/
transient int size;
/**
* Incremented by "structural modifications" to allow (best effort)
* detection of concurrent modification.
* 用于确保使用迭代器的时候，HashMap并未进行更改
*/
transient int modCount;
/**
* The table is rehashed when its size exceeds this threshold.
* The value of this field is generally .75 * capacity, except when
* the capacity is zero, as described in the EMPTY_TABLE declaration
* above.
* 负载因子(阈值)
*/
private transient int threshold;
// Views - lazily initialized
/**由K键组成的Set集合*/
private transient Set<K> keySet;
/**由K-Value组成的Set集合,Set集合中存放了Entry<K, V>元素*/
private transient Set<Entry<K, V>> entrySet;
private transient Collection<V> values;</span>HashMap的构造函数主要有以下几个：
HashMap()Constructs a new empty HashMap instance. |
HashMap(int capacity)Constructs a new HashMap instance with the specified capacity. |
HashMap(int capacity, float loadFactor)Constructs a new HashMap instance with the specified capacity and load factor. |
HashMap(Map<? extendsK,? extends V> map)Constructs a new HashMap instance containing the mappings from the specified map. |
<span style="font-size:18px;"> /**
* Constructs a new {@code HashMap} instance with the specified capacity.
*
* @param capacity
* the initial capacity of this hash map.
* @throws IllegalArgumentException
* when the capacity is less than zero.
*通过一个指定的容量来构造一个HashMap实例对象
*@param capacity：HashMap初始化容量
*
*/
public HashMap(int capacity) {
if (capacity < 0) {
throw new IllegalArgumentException("Capacity: " + capacity);
}
if (capacity == 0) {
@SuppressWarnings("unchecked")
HashMapEntry<K, V>[] tab = (HashMapEntry<K, V>[]) EMPTY_TABLE;
table = tab;
threshold = -1; // Forces first put() to replace EMPTY_TABLE
return;
}
if (capacity < MINIMUM_CAPACITY) {
capacity = MINIMUM_CAPACITY;
} else if (capacity > MAXIMUM_CAPACITY) {
capacity = MAXIMUM_CAPACITY;
} else {
capacity = Collections.roundUpToPowerOfTwo(capacity);
}
makeTable(capacity);
}</span>
<span style="font-size:18px;"> /**
* Allocate a table of the given capacity and set the threshold accordingly.
* 根据指定容量来分配table数组的大小，并设置负载因子(阈值)
* @param newCapacity must be a power of two
*
*/
private HashMapEntry<K, V>[] makeTable(int newCapacity) {
@SuppressWarnings("unchecked") HashMapEntry<K, V>[] newTable
= (HashMapEntry<K, V>[]) new HashMapEntry[newCapacity];
table = newTable;
threshold = (newCapacity >> 1) + (newCapacity >> 2); // 3/4 capacity
return newTable;
}</span>
<span style="font-size:18px;"> /**
* Maps the specified key to the specified value.
*
* @param key
* the key.
* @param value
* the value.
* @return the value of any previous mapping with the specified key or
* {@code null} if there was no such mapping.
*/
@Override public V put(K key, V value) {
if (key == null) {
return putValueForNullKey(value);
}
int hash = secondaryHash(key);
HashMapEntry<K, V>[] tab = table;
int index = hash & (tab.length - 1);
for (HashMapEntry<K, V> e = tab[index]; e != null; e = e.next) {
if (e.hash == hash && key.equals(e.key)) {
preModify(e);
V oldValue = e.value;
e.value = value;
return oldValue;
}
}
// No entry for (non-null) key is present; create one
modCount++;
if (size++ > threshold) {
tab = doubleCapacity();
index = hash & (tab.length - 1);
}
addNewEntry(key, value, hash, index);
return null;
}</span>
<span style="font-size:18px;"> /**
* Doubles the capacity of the hash table. Existing entries are placed in
* the correct bucket on the enlarged table. If the current capacity is,
* MAXIMUM_CAPACITY, this method is a no-op. Returns the table, which
* will be new unless we were already at MAXIMUM_CAPACITY.
* 将数组容量扩大
*/
private HashMapEntry<K, V>[] doubleCapacity() {
/**旧的table数组*/
HashMapEntry<K, V>[] oldTable = table;
int oldCapacity = oldTable.length;
if (oldCapacity == MAXIMUM_CAPACITY) {
return oldTable;
}
int newCapacity = oldCapacity * 2;
HashMapEntry<K, V>[] newTable = makeTable(newCapacity);
if (size == 0) {
return newTable;
}
for (int j = 0; j < oldCapacity; j++) {
/*
* Rehash the bucket using the minimum number of field writes.
* This is the most subtle and delicate code in the class.
*/
HashMapEntry<K, V> e = oldTable[j];
if (e == null) {
continue;
}
int highBit = e.hash & oldCapacity;
HashMapEntry<K, V> broken = null;
newTable[j | highBit] = e;
for (HashMapEntry<K, V> n = e.next; n != null; e = n, n = n.next) {
int nextHighBit = n.hash & oldCapacity;
if (nextHighBit != highBit) {
if (broken == null)
newTable[j | nextHighBit] = n;
else
broken.next = n;
broken = e;
highBit = nextHighBit;
}
}
if (broken != null)
broken.next = null;
}
return newTable;
}</span>
<span style="font-size:18px;"> /**
* Returns the value of the mapping with the specified key.
*
* @param key
* the key.
* @return the value of the mapping with the specified key, or {@code null}
* if no mapping for the specified key is found.
*/
public V get(Object key) {
if (key == null) {
HashMapEntry<K, V> e = entryForNullKey;
return e == null ? null : e.value;
}
// Doug Lea's supplemental secondaryHash function (inlined).
// Replace with Collections.secondaryHash when the VM is fast enough (http://b/8290590).
int hash = key.hashCode();
hash ^= (hash >>> 20) ^ (hash >>> 12);
hash ^= (hash >>> 7) ^ (hash >>> 4);
HashMapEntry<K, V>[] tab = table;
for (HashMapEntry<K, V> e = tab[hash & (tab.length - 1)];
e != null; e = e.next) {
K eKey = e.key;
if (eKey == key || (e.hash == hash && key.equals(eKey))) {
return e.value;
}
}
return null;
}</span>与此同时，我们再看看HashTable的源码，可以发现HashTable与HashMap有一部分方式的底层实现差不多，在此就不详述，主要看下两者在底层实现中的一些区别。
java.util.Hashtable.put(K key, V value)源码：
<span style="font-size:18px;"> /**
* Associate the specified value with the specified key in this
* {@code Hashtable}. If the key already exists, the old value is replaced.
* The key and value cannot be null.
*
* @param key
* the key to add.
* @param value
* the value to add.
* @return the old value associated with the specified key, or {@code null}
* if the key did not exist.
* @see #elements
* @see #get
* @see #keys
* @see java.lang.Object#equals
*/
public synchronized V put(K key, V value) {
if (key == null) {
throw new NullPointerException("key == null");
} else if (value == null) {
throw new NullPointerException("value == null");
}
int hash = Collections.secondaryHash(key);
HashtableEntry<K, V>[] tab = table;
int index = hash & (tab.length - 1);
HashtableEntry<K, V> first = tab[index];
for (HashtableEntry<K, V> e = first; e != null; e = e.next) {
if (e.hash == hash && key.equals(e.key)) {
V oldValue = e.value;
e.value = value;
return oldValue;
}
}
// No entry for key is present; create one
modCount++;
if (size++ > threshold) {
rehash(); // Does nothing!!
tab = doubleCapacity();
index = hash & (tab.length - 1);
first = tab[index];
}
tab[index] = new HashtableEntry<K, V>(key, value, hash, first);
return null;
}</span>下面给出java.util.Hashtable.get(Object key)源码，亦可以发现相似结果，故就不具体分析。
<span style="font-size:18px;"> /**
* Returns the value associated with the specified key in this
* {@code Hashtable}.
*
* @param key
* the key of the value returned.
* @return the value associated with the specified key, or {@code null} if
* the specified key does not exist.
* @see #put
*/
public synchronized V get(Object key) {
int hash = Collections.secondaryHash(key);
HashtableEntry<K, V>[] tab = table;
for (HashtableEntry<K, V> e = tab[hash & (tab.length - 1)];
e != null; e = e.next) {
K eKey = e.key;
if (eKey == key || (e.hash == hash && key.equals(eKey))) {
return e.value;
}
}
return null;
}</span>
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