HashMap 构造函数

HashMap总共提供了三个构造函数

1  /**
2      * Constructs an empty <tt>HashMap</tt> with the default initial capacity
3      * (16) and the default load factor (0.75).
4      */
5     public HashMap() {
6         this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
7     }

解读：这是无参构造函数,构造了一个初始容量16和负载因子为0.75的HashMap实例。
　负载因子介绍请参考本人博客："HashMap负载因子"

 1 /**
 2      * Constructs an empty <tt>HashMap</tt> with the specified initial
 3      * capacity and the default load factor (0.75).
 4      *
 5      * @param  initialCapacity the initial capacity.
 6      * @throws IllegalArgumentException if the initial capacity is negative.
 7      */
 8     public HashMap(int initialCapacity) {
 9         this(initialCapacity, DEFAULT_LOAD_FACTOR);
10     }

解读：构造一个指定初始容量的HashMap实例,此时负载因子还是0.75.

 1     /**
 2      * Constructs an empty <tt>HashMap</tt> with the specified initial
 3      * capacity and load factor.
 4      *
 5      * @param  initialCapacity the initial capacity
 6      * @param  loadFactor      the load factor
 7      * @throws IllegalArgumentException if the initial capacity is negative
 8      *         or the load factor is nonpositive
 9      */
10     public HashMap(int initialCapacity, float loadFactor) {
11         if (initialCapacity < 0)
12             throw new IllegalArgumentException("Illegal initial capacity: " +
13                                                initialCapacity);
14         if (initialCapacity > MAXIMUM_CAPACITY)
15             initialCapacity = MAXIMUM_CAPACITY;
16         if (loadFactor <= 0 || Float.isNaN(loadFactor))
17             throw new IllegalArgumentException("Illegal load factor: " +
18                                                loadFactor);
19         this.loadFactor = loadFactor;
20         this.threshold = tableSizeFor(initialCapacity);
21     }

解读：指定初始容量和负载因子构造一个HashMap实例.