转载于:http://blog.youkuaiyun.com/gaopu12345/article/details/50831631
有几个重要的常量:
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
static final int MAXIMUM_CAPACITY = 1 << 30;
static final float DEFAULT_LOAD_FACTOR = 0.75f;
(一)调用HashMap()构造方法
这个构造方法长这样:
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR;
}
这就完了,然后用于hash的桶数组为null,当我们第一次put的时候,会到达这段代码:
Node<K,V>[] tab; int n;
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
所以会进入resize()方法,到达这段代码:
else {
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
newCap的意思是:即将用于hash的桶数组的长度,这里的默认值是16
newThr的意思是:当桶中的键值对的个数超过这个值时就进行扩容,这时候上面提到的负载因子就起作用了,所以这里的newThr为0.75×16 = 12
所以经过这个默认构造方法,并且进行第一次put后,桶数组建立了,桶容量为16,极限值为12。
(二)调用HashMap(int initialCapacity)构造方法
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
(三)调用HashMap(int initialCapacity, float loadFactor)构造方法
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);
}
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所以经过这个方法之后,桶容量确定了,极限值也确定了,但是桶数组还是null。
当第一次put时,触发如下代码:
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
接着看resize()中触发了哪些代码:
Node<K,V>[] oldTab = table;
int oldCap = (oldTab == null) ? 0 : oldTab.length;
int oldThr = threshold;
int newCap, newThr = 0;
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1;
}
else if (oldThr > 0)
newCap = oldThr;
然后还会进入下面这个分支:
if (newThr == 0) {
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
这个分支会按负载因子设置极限值
(四)超过极限值后的扩容
在每次put之后,会有下面这个判断:
if (++size > threshold)
resize();
就是说超过极限值时会进行扩容,扩容方式如下:
Node<K,V>[] oldTab = table;
int oldCap = (oldTab == null) ? 0 : oldTab.length;
int oldThr = threshold;
int newCap, newThr = 0;
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1;
}
if (newThr == 0) {
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
threshold = newThr;
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接上面这个例子,下一次扩容会如何?
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1;
}
直到…………
当桶容量加倍到最大值会怎么样?
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
桶容量变为最大值时,紧接着的一次扩容只是简单的将极限值修改为Integer.MAX_VALUE。桶数组大小并不会继续加倍
(五)如果HashMap中的键值对数量超过Integer.MAX_VALUE了呢?
我们仔细看一下这段代码:
这个size类型是int,那么最大就是Integer.MAX_VALUE,所以不会有++size > threshold的情况,往后都不会进行扩容了。
if (++size > threshold)
resize();