Hashmap源码解析-构造函数

//最大容量 2的30次方

static final int MAXIMUM_CAPACITY = 1 << 30;

//默认的加载因子

static final float DEFAULT_LOAD_FACTOR = 0.75f;

//哈希桶，存放链表。 长度是2的N次方，或者初始化时为0.

transient Node<K,V>[] table;

//加载因子，用于计算哈希表元素数量的阈值。  threshold = 哈希桶.length * loadFactor;

final float loadFactor;

//哈希表内元素数量的阈值，当哈希表内元素数量超过阈值时，会发生扩容resize()。

int threshold;

public HashMap() {

    //默认构造函数，赋值加载因子为默认的0.75f

    this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted

}

public HashMap(int initialCapacity) {

    //指定初始化容量的构造函数

    this(initialCapacity, DEFAULT_LOAD_FACTOR);

}

//同时指定初始化容量 以及 加载因子， 用的很少，一般不会修改loadFactor

public HashMap(int initialCapacity, float loadFactor) {

    //边界处理

    if (initialCapacity < 0)

        throw new IllegalArgumentException("Illegal initial capacity: " +

                                            initialCapacity);

    //初始容量最大不能超过2的30次方

    if (initialCapacity > MAXIMUM_CAPACITY)

        initialCapacity = MAXIMUM_CAPACITY;

    //显然加载因子不能为负数

    if (loadFactor <= 0 || Float.isNaN(loadFactor))

        throw new IllegalArgumentException("Illegal load factor: " +

                                            loadFactor);

    this.loadFactor = loadFactor;

    //设置阈值为  >= 初始化容量的最近的2的n次方的值

    this.threshold = tableSizeFor(initialCapacity);

}

//新建一个哈希表，同时将另一个map m 里的所有元素加入表中

public HashMap(Map<? extends K, ? extends V> m) {

    this.loadFactor = DEFAULT_LOAD_FACTOR;

    putMapEntries(m, false);

}

static final int tableSizeFor(int cap) {

    int n = cap - 1;

    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;

}

int n = cap - 1;