HashMap 构造函数

HashMap总共提供了三个构造函数

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

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"

 /**
      * Constructs an empty <tt>HashMap</tt> with the specified initial
      * capacity and the default load factor (0.75).
      *
      * @param  initialCapacity the initial capacity.
      * @throws IllegalArgumentException if the initial capacity is negative.
      */
     public HashMap(int initialCapacity) {
         this(initialCapacity, DEFAULT_LOAD_FACTOR);
     }

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.

     /**
      * 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;
         this.threshold = tableSizeFor(initialCapacity);
     }

解读：指定初始容量和负载因子构造一个HashMap实例.