HashMap源码注解 之 常量定义（一）

注意 ，本文基于JDK 1.8

1.默认容量

/** * The default initial capacity - MUST be a power of two. */
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

默认初始化的容量为16，必须是2的幂。
2.最大容量

   /** * 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;

最大容量是2^30
3.装载因子

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

默认的装载因子是0.75
4.由链表转换成树的阈值TREEIFY_THRESHOLD

    /**
     * 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;

一个桶中bin（箱子）的存储方式由链表转换成树的阈值。即当桶中bin的数量超过TREEIFY_THRESHOLD时使用树来代替链表。默认值是8
5.由树转换成链表的阈值UNTREEIFY_THRESHOLD

    /** * 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;

当执行resize操作时，当桶中bin的数量少于UNTREEIFY_THRESHOLD时使用链表来代替树。默认值是6
6.MIN_TREEIFY_CAPACITY

    /** * 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;

当桶中的bin被树化时最小的hash表容量。（如果没有达到这个阈值，即hash表容量小于MIN_TREEIFY_CAPACITY，当桶中bin的数量太多时会执行resize扩容操作）这个MIN_TREEIFY_CAPACITY的值至少是TREEIFY_THRESHOLD的4倍。