hashMap get put resize方法源码解析 - ToB企服应用市场:ToB评测及商务社交产业平台

/**
* The default initial capacity - MUST be a power of two.
* 默认初始容量-必须是2的幂。
* 初始容量
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
/**
* 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.
* 如果隐式指定了更高的值，则使用最大容量由带有参数的构造函数之一执行。
* 必须是2的幂<=1<<30。
* HashMap的最大容量
*/
static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* The load factor used when none specified in constructor.
* 构造函数中未指定时使用的负载系数
* 容量到达容器的0.75时HashMap将会进行扩容操作
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* 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.
* 对于bin使用树而不是列表的bin计数阈值。将元素添加到具有至少这么多节点的bin时，
* bin将转换为树。该值必须大于2，并且至少应为8，
* 以便与树木移除中关于收缩时转换回普通箱的假设相匹配
*
* 当链表长度超过8时HashMap会将链表转换为红黑树
* HashMap会优先将数组扩容到64时才会进行红黑树的转化
*/
static final int TREEIFY_THRESHOLD = 8;
/**
* 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.
* 在调整大小操作过程中取消筛选（拆分）垃圾箱的垃圾箱计数阈值。
* 应小于TREEIFY_THRESHOLD，最多为6，以便在移除时进行收缩检测
*
*/
static final int UNTREEIFY_THRESHOLD = 6;
/**
* 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.
* 可将箱子树化的最小工作台容量。
*（否则，如果bin中的节点过多，将调整表的大小。）
* 应至少为4*TREEIFY_THRESHOLD以避免冲突
* 在调整大小和树化阈值之间。
*
* 当数组长度到达64时且链表长度大于等于8时链表转换为红黑树
*/
static final int MIN_TREEIFY_CAPACITY = 64;

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//初始无参构造
//Constructs an empty HashMap with the default initial capacity (16) and the default load factor (0.75).
//使用默认初始容量（16）和默认负载因子（0.75）构建一个空的HashMap
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
/**
* 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.
*/
//使用指定的初始容量和默认负载因子（0.75）构造空HashMap。
//参数: initialCapacity–初始容量。
//抛出: IllegalArgumentException–如果初始容量为负数
//构造一个含有初始容量的HashMap
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and load factor.
*
* @param initialCapacity the initial capacity 初始化容量
* @param loadFactor the load factor 负载因子
* @throws IllegalArgumentException if the initial capacity is negative
* or the load factor is nonpositive
*/
//使用外部传入的HashMap的初始化容量和负载因子
public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 0) //判断外部传入的初始化容量是否小于0，小于0则抛出异常
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY) //判断初始化容量是否超过最大容量
initialCapacity = MAXIMUM_CAPACITY; //如果大于最大容量则使用最大容量
if (loadFactor <= 0 || Float.isNaN(loadFactor)) //判断负载因子是否小于0或者不是数字
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor); //抛出异常
this.loadFactor = loadFactor; //负载因子赋值
this.threshold = tableSizeFor(initialCapacity); //将初始化容量先赋给容器界限
}
//Returns a power of two size for the given target capacity.
//返回给定目标容量的两次幂。
//得到一个大于获等于他的2的幂并返回
static final int tableSizeFor(int cap) {
// >>>为无符号右移将数字转换为二进制向右移动高位补0
// |为或运算，两个数转化为二进制，有1则为1，全0则为0
// |= 与+=类似 a|=b 等价于 a=a|b
//防止传进来的数本身就是2^n,如果不减一则传出的值为2^n+1
int n = cap - 1;
//右移一位并与自己进行或运算，则原来是1则变成11。
//例如传进来数为100001001，右移一位得到0110000100，与自己进行或运算得到110001101
//一位1变成两位1，所以下面各右移1、2、4、8、16位
n |= n >>> 1;
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}

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