HashMap
hashmap会将key和value生成一个entry对象。
将entry放在数组中。放的位置，将key进行hashcode后取余，获取数组下标。
链表：头插法、尾插法。头插法更快。
1.7创建HashMap的底层源码
- 1.创建HashMap
HashMap map = new HashMap();
- 2.查看底层调用方法
/**
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (0.75).
* 生成一个长度为16的数组，扩容因子是0.75
*/
public HashMap() {
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
}
- 3.this方法调用.
该方法中，只做参数验证和数据赋值，init方法是没有用的。
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and load factor.
*
* @param initialCapacity the initial capacity
* @param loadFactor the load factor
* @throws IllegalArgumentException if the initial capacity is negative
* or the load factor is nonpositive
*/
public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
this.loadFactor = loadFactor;
threshold = initialCapacity;
init();
}