面试加分项-HashMap源码中这些常量设计目的你知道吗

/**
 
 * The default initial capacity - MUST be a power of two.
 
 */
 
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; 
 

 final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
 
 boolean evict) {
 
 Node<K,V>[] tab; Node<K,V> p; int n, i;
 
 if ((tab = table) == null || (n = tab.length) == 0)
 
            n = (tab = resize()).length;
 
 if ((p = tab[i = (n - 1) & hash]) == null)
 
            tab[i] = newNode(hash, key, value, null);
 

 /**
 
     * The load factor used when none specified in constructor.
 
     */
 
 static final float DEFAULT_LOAD_FACTOR = 0.75f;
 

 * <p>As a general rule, the default load factor (.75) offers a good
 
 * tradeoff between time and space costs. Higher values decrease the
 
 * space overhead but increase the lookup cost (reflected in most of
 
 * the operations of the <tt>HashMap</tt> class, including
 
 * <tt>get</tt> and <tt>put</tt>). 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 bin count threshold for using a tree rather than list for a
 
     * bin.  Bins are converted to trees when adding an element to a
 
     * bin with at least this many nodes. The value must be greater
 
     * than 2 and should be at least 8 to mesh with assumptions in
 
     * tree removal about conversion back to plain bins upon
 
     * shrinkage.
 
     */
 
 static final int TREEIFY_THRESHOLD = 8;
 

* Ideally, under random hashCodes, the frequency of
 
* nodes in bins follows a Poisson distribution
 
* (http://en.wikipedia.org/wiki/Poisson_distribution) with a
 
* parameter of about 0.5 on average for the default resizing
 
* threshold of 0.75, although with a large variance because of
 
* resizing granularity. Ignoring variance, the expected
 
* occurrences of list size k are (exp(-0.5) * pow(0.5, k) /
 
* factorial(k)). The first values are:
 
*
 
* 0: 0.60653066
 
* 1: 0.30326533
 
* 2: 0.07581633
 
* 3: 0.01263606
 
* 4: 0.00157952
 
* 5: 0.00015795
 
* 6: 0.00001316
 
* 7: 0.00000094
 
* 8: 0.00000006
 
* more: less than 1 in ten million
 

/**
 
     * The bin count threshold for untreeifying a (split) bin during a
 
     * resize operation. Should be less than TREEIFY_THRESHOLD, and at
 
     * most 6 to mesh with shrinkage detection under removal.
 
     */
 
 static final int UNTREEIFY_THRESHOLD = 6;
 

/**
 
     * The maximum capacity, used if a higher value is implicitly specified
 
     * by either of the constructors with arguments.
 
     * MUST be a power of two <= 1<<30.
 
     */
 
 static final int MAXIMUM_CAPACITY = 1 << 30;
 

tab[i = (n - 1) & hash]
 

 System.out.println(1<<30);
 
 System.out.println(1<<31);
 
 System.out.println(1<<32);
 
 System.out.println(1<<33);
 
 System.out.println(1<<34);
 

1073741824
 
-2147483648
 
1
 
2
 
4
 

 /**
 
     * The smallest table capacity for which bins may be treeified.
 
     * (Otherwise the table is resized if too many nodes in a bin.)
 
     * Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
 
     * between resizing and treeification thresholds.
 
     */
 
 static final int MIN_TREEIFY_CAPACITY = 64;