1 使用方法
HashMap是散清單，存儲的内容為key-value鍵值對，key的值是唯一的，可以為null。
public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable {}
HashMap繼承了AbstractMap并實作了Map、Cloneable以及Serializable接口，是以HashMap支援clone和序列化。
1.1 方法介紹
HashMap提供的API主要如下：
void clear() //清空HashMap
Object clone() //複制HashMap
boolean containsKey(Object key) //判斷是否存在key
boolean containsValue(Object value) //判斷是否存在Value
Set<Entry<K, V>> entrySet() //傳回HashMap的Entry組成的set集合
V get(Object key) //擷取鍵為key的元素值
boolean isEmpty() //判空
Set<K> keySet() //擷取HashMap的key組成的set集合
V put(K key, V value) //加入HashMap
void putAll(Map<? extends K, ? extends V> map) //批量加入
V remove(Object key) //删除鍵為key的Entry
int size() //擷取大小
Collection<V> values() //擷取HashMap的value集合
1.2 使用示例
public void testHashMap() {
//建立hashMap
HashMap hashMap = new HashMap(); //建立hashMap
//添加元素
hashMap.put(, "one");
hashMap.put(, "two");
hashMap.put(, "three");
hashMap.put(, "four");
//列印元素
this.printMapByEntrySet(hashMap);
//擷取大小
System.out.println("hashMap的大小為: " + hashMap.size());
//是否包含key為4的元素
System.out.println("hashMap是否包含key為4的元素: " + hashMap.containsKey());
//是否包含值為5的元素
System.out.println("hashMap是否包含value為two的元素: " + hashMap.containsValue("two"));
hashMap.put(, "five");
hashMap.put(, "six");
//删除元素
System.out.println("删除key為2的元素: " + hashMap.remove());
//列印元素
this.printMapByKeySet(hashMap);
//clone
HashMap cloneMap = (HashMap) hashMap.clone();
//列印克隆map
System.out.println("cloneMap的元素為: " + cloneMap);
//清空map
hashMap.clear();
//判空
System.out.println("hashMap是否為空: " + hashMap.isEmpty());
}
/**
* 根據entrySet()擷取Entry集合,然後周遊Set集合擷取鍵值對
* @param map
*/
private void printMapByEntrySet(HashMap map) {
Integer key = null;
String value = null;
Iterator iterator = map.entrySet().iterator(); //
System.out.print("hashMap中含有的元素有: ");
while (iterator.hasNext()) {
Map.Entry entry = (Map.Entry) iterator.next();
key = (Integer) entry.getKey();
value = (String) entry.getValue();
System.out.print("key/value : " + key + "/" + value + " ");
}
System.out.println();
}
/**
* 使用keySet擷取key的Set集合,利用key擷取值
* @param map
*/
private void printMapByKeySet(HashMap map) {
Integer key = null;
String value = null;
Iterator iterator = map.keySet().iterator();
System.out.print("hashMap中含有的元素有: ");
while (iterator.hasNext()) {
key = (Integer) iterator.next();
value = (String) map.get(key);
System.out.print("key/value : " + key + "/" + value + " ");
}
System.out.println();
}
運作結果如下：
hashMap中含有的元素有: key/value : /one key/value : /two key/value : /three key/value : /four
hashMap的大小為:
hashMap是否包含key為的元素: true
hashMap是否包含value為two的元素: true
删除key為的元素: two
hashMap中含有的元素有: key/value : /one key/value : /three key/value : /four key/value : /five key/value : /six
cloneMap的元素為: {=one, =three, =four, =five, =six}
hashMap是否為空: true
2 源碼分析
2.1構造函數
HashMap有四個構造函數，每個構造函數的不同之處在于初始容量和加載因子不同。初始容量為申請的HashMap初始大小，當加入元素後的容量大于加載因子和目前容量的乘積是，HashMap需要再hash增大容量。
/**
* 申請初始容量為initialCapacity, 加載因子為loadFactor
* @param initialCapacity 初始容量
* @param loadFactor 加載因子
* @throws IllegalArgumentException 非法參數異常
*/
public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < )
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY) //最大容量為2^30
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <= || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
this.loadFactor = loadFactor; //加載因子
this.threshold = tableSizeFor(initialCapacity); //容量大小, >=initialCapacity的最小的2的倍數
}
/**
* 初始容量大小為initialCapacity, 加載因子為預設0.75
* @param initialCapacity the initial capacity.
* @throws IllegalArgumentException if the initial capacity is negative.
*/
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/**
* 初始容量大小為0, 加載因子為0.75
*/
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
/**
* 申請一個HashMap并且用m初始化
*
* @param m the map whose mappings are to be placed in this map
* @throws NullPointerException if the specified map is null
*/
public HashMap(Map<? extends K, ? extends V> m) {
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
}
2.2 put方法
/**
* 為HashMap插入一個鍵為key,值為value的元素
* @param key
* @param value
* @return
*/
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
/**
* Implements Map.put and related methods
*
* @param hash hash for key
* @param key the key
* @param value the value to put
* @param onlyIfAbsent if true, don't change existing value
* @param evict if false, the table is in creation mode.
* @return previous value, or null if none
*/
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) == ) //hash數組為null或者長度為0
n = (tab = resize()).length; //初始化數組
if ((p = tab[i = (n - ) & hash]) == null) //下标不存在,則這個下表所對應的元素為一個新節點
tab[i] = newNode(hash, key, value, null);
else { //将元素節點連結到連結清單最後
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k)))) //鍵已經存在
e = p;
else if (p instanceof TreeNode) //TreeNode節點
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = ; ; ++binCount) {
if ((e = p.next) == null) { //将元素節點連結到最後
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - ) // -1 for 1st
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k)))) //鍵存在
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) //超過容量值
resize();
afterNodeInsertion(evict);
return null;
}
2.3 get方法
/**
* 擷取鍵為key的鍵值對的值
* @param key
* @return
*/
public V get(Object key) {
Node<K,V> e;
return (e = getNode(hash(key), key)) == null ? null : e.value;
}
/**
* Implements Map.get and related methods
*
* @param hash hash for key
* @param key the key
* @return the node, or null if none
*/
final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
if ((tab = table) != null && (n = tab.length) > &&
(first = tab[(n - ) & hash]) != null) {
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null) {
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
remove方法
/**
* 删除鍵為key的鍵值對
* @param key
* @return
*/
public V remove(Object key) {
Node<K,V> e;
return (e = removeNode(hash(key), key, null, false, true)) == null ?
null : e.value;
}
/**
* Implements Map.remove and related methods
*
* @param hash hash for key
* @param key the key
* @param value the value to match if matchValue, else ignored
* @param matchValue if true only remove if value is equal
* @param movable if false do not move other nodes while removing
* @return the node, or null if none
*/
final Node<K,V> removeNode(int hash, Object key, Object value,
boolean matchValue, boolean movable) {
Node<K, V>[] tab;
Node<K, V> p;
int n, index;
if ((tab = table) != null && (n = tab.length) > &&
(p = tab[index = (n - ) & hash]) != null) { //hash表不為空,長度 > 0,下标對應的元素存在
Node<K, V> node = null, e;
K k;
V v;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k)))) //判斷第一個元素
node = p;
else if ((e = p.next) != null) { //同一下标有多個元素,周遊連結清單
if (p instanceof TreeNode)
node = ((TreeNode<K, V>) p).getTreeNode(hash, key);
else {
do {
if (e.hash == hash &&
((k = e.key) == key ||
(key != null && key.equals(k)))) {
node = e;
break;
}
p = e;
} while ((e = e.next) != null);
}
}
if (node != null && (!matchValue || (v = node.value) == value ||
(value != null && value.equals(v)))) { //删除元素
if (node instanceof TreeNode)
((TreeNode<K, V>) node).removeTreeNode(this, tab, movable);
else if (node == p)
tab[index] = node.next;
else
p.next = node.next;
++modCount;
--size;
afterNodeRemoval(node);
return node;
}
}
}
3 HashMap和Hashtable差別
HashMap和Hashtable從功能上來說幾乎完全相同，主要差別在于Hashtable是線程安全的而HashMap不是。
1）HashMap的key和Value可以接受null，Hashtable不行；
2）Hashtable除了構造函數外幾乎所有的方法都加上了synchronized保證線程安全，HashMap沒有線程安全保證；
3） Hashtable由于使用了synchronized導緻在單線程情況下速度較慢；
4） Hashtable構造時預設大小為11，HashMap為16；
參考：
[1] http://www.cnblogs.com/skywang12345/p/3310835.html
[2] http://blog.csdn.net/mazhimazh/article/details/17876641
[3] http://blog.csdn.net/ns_code/article/details/36034955
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