Java集合：HashMap使用详解及源码分析

1 使用方法

　　HashMap是散列表，存储的内容为key-value键值对，key的值是唯一的，可以为null。

public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable {}

　　HashMap继承了AbstractMap并实现了Map、Cloneable以及Serializable接口，所以HashMap支持clone和序列化。

1.1 方法介绍

　　HashMap提供的API主要如下：

void                 clear() //清空HashMap
Object               clone() //复制HashMap
boolean              containsKey(Object key) //判断是否存在key
boolean              containsValue(Object value) //判断是否存在Value
Set<Entry<K, V>>     entrySet() //返回HashMap的Entry组成的set集合
V                    get(Object key) //获取键为key的元素值
boolean              isEmpty() //判空
Set<K>               keySet() //获取HashMap的key组成的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) //删除键为key的Entry
int                  size() //获取大小
Collection<V>        values() //获取HashMap的value集合

1.2 使用示例

public void testHashMap() {
//新建hashMap
HashMap hashMap = new HashMap(); //新建hashMap
//添加元素
hashMap.put(1, "one");
hashMap.put(2, "two");
hashMap.put(3, "three");
hashMap.put(4, "four");
//打印元素
this.printMapByEntrySet(hashMap);
//获取大小
System.out.println("hashMap的大小为: " + hashMap.size());
//是否包含key为4的元素
System.out.println("hashMap是否包含key为4的元素: " + hashMap.containsKey(4));
//是否包含值为5的元素
System.out.println("hashMap是否包含value为two的元素: " + hashMap.containsValue("two"));

hashMap.put(5, "five");
hashMap.put(6, "six");

//删除元素
System.out.println("删除key为2的元素: " + hashMap.remove(2));
//打印元素
this.printMapByKeySet(hashMap);
//clone
HashMap cloneMap = (HashMap) hashMap.clone();
//打印克隆map
System.out.println("cloneMap的元素为: " + cloneMap);
//清空map
hashMap.clear();
//判空
System.out.println("hashMap是否为空: " + hashMap.isEmpty());
}

/**
* 根据entrySet()获取Entry集合,然后遍历Set集合获取键值对
* @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获取key的Set集合,利用key获取值
* @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();
}

　　运行结果如下：

hashMap中含有的元素有: key/value : 1/one key/value : 2/two key/value : 3/three key/value : 4/four
hashMap的大小为: 4
hashMap是否包含key为4的元素: true
hashMap是否包含value为two的元素: true
删除key为2的元素: two
hashMap中含有的元素有: key/value : 1/one key/value : 3/three key/value : 4/four key/value : 5/five key/value : 6/six
cloneMap的元素为: {1=one, 3=three, 4=four, 5=five, 6=six}
hashMap是否为空: true

2 源码分析

2.1构造函数

　　HashMap有四个构造函数，每个构造函数的不同之处在于初始容量和加载因子不同。初始容量为申请的HashMap初始大小，当加入元素后的容量大于加载因子和当前容量的乘积是，HashMap需要再hash增大容量。

/**
* 申请初始容量为initialCapacity, 加载因子为loadFactor
* @param initialCapacity 初始容量
* @param loadFactor 加载因子
* @throws IllegalArgumentException 非法参数异常
*/
public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY) //最大容量为2^30
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
this.loadFactor = loadFactor; //加载因子
this.threshold = tableSizeFor(initialCapacity); //容量大小, >=initialCapacity的最小的2的倍数
}

/**
* 初始容量大小为initialCapacity, 加载因子为默认0.75
* @param  initialCapacity the initial capacity.
* @throws IllegalArgumentException if the initial capacity is negative.
*/
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}

/**
* 初始容量大小为0, 加载因子为0.75
*/
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}

/**
* 申请一个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

/**
* 为HashMap插入一个键为key,值为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) == 0) //hash数组为null或者长度为0
n = (tab = resize()).length; //初始化数组
if ((p = tab[i = (n - 1) & hash]) == null) //下标不存在,则这个下表所对应的元素为一个新节点
tab[i] = newNode(hash, key, value, null);
else { //将元素节点链接到链表最后
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k)))) //键已经存在
e = p;
else if (p instanceof TreeNode) //TreeNode节点
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) { //将元素节点链接到最后
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)))) //键存在
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) //超过容量值
resize();
afterNodeInsertion(evict);
return null;
}

2.3 get方法

/**
* 获取键为key的键值对的值
* @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) > 0 &&
(first = tab[(n - 1) & 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;
}
2.4 remove方法
/**
* 删除键为key的键值对
* @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) > 0 &&
(p = tab[index = (n - 1) & hash]) != null) { //hash表不为空,长度 > 0,下标对应的元素存在
Node<K, V> node = null, e;
K k;
V v;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k)))) //判断第一个元素
node = p;
else if ((e = p.next) != null) { //同一下标有多个元素,遍历链表
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区别

　　HashMap和Hashtable从功能上来说几乎完全相同，主要区别在于Hashtable是线程安全的而HashMap不是。

　　1）HashMap的key和Value可以接受null，Hashtable不行；

　　2）Hashtable除了构造函数外几乎所有的方法都加上了synchronized保证线程安全，HashMap没有线程安全保证；

　　3） Hashtable由于使用了synchronized导致在单线程情况下速度较慢；

　　4） Hashtable构造时默认大小为11，HashMap为16；

参考：

[1] /content/3625839.html

[2] /article/1382138.html

[3] /article/1333663.html