Java 8 HashMap
* 1. Hash tables based implementation of the Map interface. This implementation provides all of the
* 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. this class makes no guarantees(v.保证;n.保证)
* as to the order of the map; in particular , it does not guarantee that the order will remain constant over time.
* 2. This implementation provides constant-time performance for the basic operations get and put, assuming
* the hash function disperses(v.分配) the elements properly among the bucket. Interation over collection
* view requires time proportional to(与什么成比例) the "capacity" of the HashMap instance plus its size.
* thus , its very importance not to set the initial capacity to hight (or the load factory to low) if interation
* performance is importance
* 3. An instance of HashMap has two parameters that affect its performance: initial capacity and load
* factor. The capacity is the number of bucket in the hash table , and the initial capacity is simply the