用java源代码学数据结构<六>: HashSet HashMap 详解(二)
ze;
//阈值，当size超过threshold时，table将会扩容.
//threshold = capacity * loadFactor
int threshold;
//加载因子
final float loadFactor;
//修改次数，用于检查线程是否同步
transient int modCount;
//默认的阀值
static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;
private static class Holder {
static final int ALTERNATIVE_HASHING_THRESHOLD;
static {
//获取jdk内置的阀值
String altThreshold = java.security.AccessController.doPrivileged(
new sun.security.action.GetPropertyAction(
"jdk.map.althashing.threshold"));
int threshold;
try {
//设置当前阀值
threshold = (null != altThreshold)
Integer.parseInt(altThreshold)
: ALTERNATIVE_HASHING_THRESHOLD_DEFAULT;
// disable alternative hashing if -1
if (threshold == -1) {
threshold = Integer.MAX_VALUE;
}
if (threshold < 0) {
throw new IllegalArgumentException("value must be positive integer.");
}
} catch(IllegalArgumentException failed) {
throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed);
}
ALTERNATIVE_HASHING_THRESHOLD = threshold;
}
}
//使用初始化容量和加载因子初始化HashMap
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();
}
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
public HashMap() {
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
}
/*
* Constructs a new HashMap with the same mappings as the
* specified Map. The HashMap is created with
* default load factor (0.75) and an initial capacity sufficient to
* hold the mappings in the specified Map.
*/
public HashMap(Map< extends K, extends V> m) {
this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
inflateTable(threshold);
putAllForCreate(m);
}
/**
* A randomizing value associated with this instance that is applied to
* hash code of keys to make hash collisions harder to find.
If 0 then alternative hashing is disabled.
*/
transient int hashSeed = 0;
//工具函数,将number扩展成2的倍数
private static int roundUpToPowerOf2(int number) {
// assert number >= 0 : "number must be non-negative";
int rounded = number >= MAXIMUM_CAPACITY
MAXIMUM_CAPACITY
: (rounded = Integer.highestOneBit(number)) != 0
(Integer.bitCount(number) > 1) rounded << 1 : rounded
: 1;
return rounded;