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] 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