HashMap - 类注释

了解到Java8以后HashMap的实现换了，也看了很多博客一直在向我这个小菜鸡说HashMap的重要。因此我决定洗心革面，好好正视HashMap

• 基于jdk 8

先从类注释开始入手学习，顺便提高提高英文水平。如果有任何错误欢迎指出，thanks

 * Hash table based implementation of the <tt>Map</tt> interface.  This
 * implementation provides all of the optional map operations, and permits
 * <tt>null</tt> values and the <tt>null</tt> key.  (The <tt>HashMap</tt>
 * class is roughly equivalent to <tt>Hashtable</tt>, except that it is
 * unsynchronized and permits nulls.)  This class makes no guarantees as to
 * the order of the map; in particular, it does not guarantee that the order
 * will remain constant over time.

这段注释说，Hashmap基于hash表实现了map接口，并且提供了Map中的所有方法。key,value 允许为null.Hashmap可以粗略的等同与HashTable，但是不同的是HashMap是线程不安全的，并且允许为null.Hashmap并不保证map的顺序.

知识点

• HashMap的<key,value>都允许为null
  • 额，这一点，思考了一下还是觉得写段代码验证验证,毕竟代码不会骗我

  • @Test
        public void TestnullValue() {
            Map<String, String> map = new HashMap();
            map.put(null,null);
            map.put("2", "tony");
            System.out.println(map.toString());
        }

    结果

    {null=null, 2=tony}

    好吧，看来编写HashMap的前辈，木有骗我

• HashMap线程不安全
• HashMap无序

 * <p>This implementation provides constant-time performance for the basic
 * operations (<tt>get</tt> and <tt>put</tt>), assuming the hash function
 * disperses the elements properly among the buckets.  Iteration over
 * collection views requires time proportional to the "capacity" of the
 * <tt>HashMap</tt> instance (the number of buckets) plus its size (the number
 * of key-value mappings).  Thus, it's very important not to set the initial
 * capacity too high (or the load factor too low) if iteration performance is
 * important.

emem...说实话读到这一段的时候，我有点难受...要不是我天生聪慧（英文太烂）,我可能会被拍死在注释中

如果hash函数把元素正确的放在容器中，HashMap提供了耗费常量时间的操作, put 和get。迭代整个集合试图耗费的时间与容器的大小有关。因此不要为HashMap设置一个太高的初始值（或者太低的负荷系数）

负荷系数是扩容的时候需要用到的

知识点:

• 理想情况下HashMap的get, put的时间复杂度是O(1)

什么情况下不是O(1)呢？当发生Hash冲突的时候，时间复杂度就不是O(1)了

 * <p>An instance of <tt>HashMap</tt> has two parameters that affect its
 * performance: <i>initial capacity</i> and <i>load factor</i>.  The
 * <i>capacity</i> is the number of buckets in the hash table, and the initial
 * capacity is simply the capacity at the time the hash table is created.  The
 * <i>load factor</i> is a measure of how full the hash table is allowed to
 * get before its capacity is automatically increased.  When the number of
 * entries in the hash table exceeds the product of the load factor and the
 * current capacity, the hash table is <i>rehashed</i> (that is, internal data
 * structures are rebuilt) so that the hash table has approximately twice the
 * number of buckets.

影响HashMap性能的因素有两个,容量和负荷因子。

初始容量是创建hash表的时候就确定的，负荷因子用来判断是否允许自动扩容。当hash表中的数量超过容器的大小*负荷因子的时候.Hash表要再做一次内部构建，扩容为原来的两倍.

 * <p>As a general rule, the default load factor (.75) offers a good
 * tradeoff between time and space costs.  Higher values decrease the
 * space overhead but increase the lookup cost (reflected in most of
 * the operations of the <tt>HashMap</tt> class, including
 * <tt>get</tt> and <tt>put</tt>).  The expected number of entries in
 * the map and its load factor should be taken into account when
 * setting its initial capacity, so as to minimize the number of
 * rehash operations.  If the initial capacity is greater than the
 * maximum number of entries divided by the load factor, no rehash
 * operations will ever occur.

一般负荷因子的数值是0.75 它平衡了时间和空间复杂度。 负载因子值越大空间开销越大，但是查找的开销就越低。HashMap的绝大部分操作都是Get Put,

当哈希表中entry的总数少于负载因子和初始容量乘积时， 就不会发生rehash（内部重新构建）动作

 * <p>If many mappings are to be stored in a <tt>HashMap</tt>
 * instance, creating it with a sufficiently large capacity will allow
 * the mappings to be stored more efficiently than letting it perform
 * automatic rehashing as needed to grow the table.  Note that using
 * many keys with the same {@code hashCode()} is a sure way to slow
 * down performance of any hash table. To ameliorate impact, when keys
 * are {@link Comparable}, this class may use comparison order among
 * keys to help break ties.
 *

如果有大量的数值需要被存储到HashMap中，那么要确保初始够大，防止map rehash自动扩容增大开销。

使用大量keys转换后的hash地址相同会降低hash表的效率，因此当keys支持java.lang.Comparable的时候，可以利用排序降低影响。

 * <p><strong>Note that this implementation is not synchronized.</strong>
 * If multiple threads access a hash map concurrently, and at least one of
 * the threads modifies the map structurally, it <i>must</i> be
 * synchronized externally.  (A structural modification is any operation
 * that adds or deletes one or more mappings; merely changing the value
 * associated with a key that an instance already contains is not a
 * structural modification.)  This is typically accomplished by
 * synchronizing on some object that naturally encapsulates the map.

HashMap是线程不安全的，如果多个线程同时访问hash map，至少有一个线程修改了表结构，那么必须加锁。

（改变表结构指的是，添加或删除一个或多个映射,如果是改变values值不是修改表结构)

当多个线程操作hashmap的时候，一般使用同步对象锁锁住map上

 * If no such object exists, the map should be "wrapped" using the
 * {@link Collections#synchronizedMap Collections.synchronizedMap}
 * method.  This is best done at creation time, to prevent accidental
 * unsynchronized access to the map:<pre>
 *   Map m = Collections.synchronizedMap(new HashMap(...));</pre>

如果不存在这样的对象时，应该调用Collections.synchronizedMap把HashMap转化成线程安全的SynchronizedMap

  public static <K,V> Map<K,V> synchronizedMap(Map<K,V> m) {
        return new SynchronizedMap<>(m);
    }

 *
 * <p>The iterators returned by all of this class's "collection view methods"
 * are <i>fail-fast</i>: if the map is structurally modified at any time after
 * the iterator is created, in any way except through the iterator's own
 * <tt>remove</tt> method, the iterator will throw a
 * {@link ConcurrentModificationException}.  Thus, in the face of concurrent
 * modification, the iterator fails quickly and cleanly, rather than risking
 * arbitrary, non-deterministic behavior at an undetermined time in the
 * future.

iterators会返回所的集合视图方法。如果iterator迭代器被创建了以后，map修改了表结构（除了iterator自身的remove），iterator会抛出ConcurrentModificationException异常。因此在并发修改的时候修改表结构操作会失败

@Test
    public void test() {
        Map<String, String> map = new HashMap();
        map.put("1", "tony");
        map.put("2", "tony");
        Set<Map.Entry<String, String>> entries =  map.entrySet();
        Iterator<Map.Entry<String, String>> iterator = entries.iterator();
        while (iterator.hasNext()) {
            if (iterator.next().getKey().equals("1")) {
                //不报错
                iterator.remove();
                //报错
                map.put("3", "tony");
            }
        }
        System.out.println(map.toString());
    }