HashMap

官方介绍

Hash table based implementation of the Map interface. This implementation provides all of the optional map operations, and permits null values and the null key. (The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.)

• HashMap 和Hashtable 差不多
• HashMap 是线程不安全的 – it is unsynchronized
• HashMap 允许key与value为null – permits nulls

As a general rule, the default load factor (.75) offers a good tradeoff between time and space costs.
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.
/**
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
static final int MAXIMUM_CAPACITY = 1 << 30;

• 默认初始大小 – default initial capacity : 16
• 负载因子 – load factor : 0.75

/**
* Constructs a new, empty hashtable with a default initial capacity (11)
* and load factor (0.75).
*/
public Hashtable() {
this(11, 0.75f);
}
//此处做对比
//Hashtable 的 初始大小为11,负载因子也是0.75

未完待续…