HashMap中重要的成员变量
/**
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
//当无参构造创建 HashMap 时，不创建哈希桶；等到使用 put 方法向 hashMap 中插入 kv 时，
//调用 resize 方法，按需创建哈希桶，默认初始容量为 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.
*/
static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* The load factor used when none specified in constructor.
*/
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.
*/
static final int TREEIFY_THRESHOLD = 8;
//当链表长度大于 8 时，判断是否需要将链表转换为红黑树 (treeifyBin)
/**
* 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;
//当红黑树节点小于 6 时，将红黑树转换回链表
/**
* 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;
//当链表长度大于 8 时，判断哈希桶长度(length)是否大于 64 ，如果不大于则扩容哈希桶，将所有 kv rehash，如果大于则将链表转换为红黑树
/**
* The number of key-value mappings contained in this map.
*/
transient int size; //大小，hashMap中存放的kv个数
/**
* The number of times this HashMap has been structurally modified
* Structural modifications are those that change the number of mappings in
* the HashMap or otherwise modify its internal structure (e.g.,
* rehash). This field is used to make iterators on Collection-views of
* the HashMap fail-fast. (See ConcurrentModificationException).
*/
transient int modCount; //hashMap结构被修改次数
/**
* The next size value at which to resize (capacity * load factor).
*
* @serial
*/
// (The javadoc description is true upon serialization.
// Additionally, if the table array has not been allocated, this
// field holds the initial array capacity, or zero signifying
// DEFAULT_INITIAL_CAPACITY.)
int threshold; //阈值 = capacaty * loadFactor
/**
* The load factor for the hash table.
*
* @serial
*/
final float loadFactor; //负载因子，默认为0.75（一般不建议修改loadFactor的值）
//当存储的kv个数 = threshold = loadFactor * capacity 时，哈希桶进行2倍扩容
为什么要有负载因子？
loadFactor 与 hashMap 的扩容机制息息相关，loadFactor 表示了HashMap 满的程度，它决定了 hashMap 何时扩容。如果等到 HashMap 满了再扩容，那么在接近满的状态时插入新的 kv 时， 会造成很严重的哈希碰撞，大大降低了插入的效率，同时也会降低查询的效率（链表长度过长 / 红黑树深度过深？）。如果在 HashMap 未满时扩容，那么何时进行扩容，是最重要的问题，太早扩容浪费空间，太晚扩容浪费时间（rehash），所以 hashMap 源码将 loadFactor 设置为0.75.
loadFactor 为什么是0.75？
https://blog.csdn.net/qq_45401061/article/details/104479262
HashMap 有参构造
public HashMap(int initialCapacity, float loadFactor) {
//自定义初始容量以及负载因子
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;
// 根据初始化容量，计算出 KV 个数的阈值
this.threshold = tableSizeFor(initialCapacity);
}
/**
* 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.
*/
public HashMap(int initialCapacity) {
//自定义哈希桶初始容量，默认负载因子为0.75
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/**
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() {
//无参构造，不创建哈希桶（按需创建），默认负载因子为0.75
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
/**
* Constructs a new <tt>HashMap</tt> with the same mappings as the
* specified <tt>Map</tt>. The <tt>HashMap</tt> is created with
* default load factor (0.75) and an initial capacity sufficient to
* hold the mappings in the specified <tt>Map</tt>.
*
* @param m the map whose mappings are to be placed in this map
* @throws NullPointerException if the specified map is null
*/
public HashMap(Map<? extends K, ? extends V> m) {
//有参构造，下面详解
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
}
/**
* Returns a power of two size for the given target capacity.
*/
static final int tableSizeFor(int cap) {
int n = cap - 1;
n |= n >>> 1;// >>> 逻辑右移，当 n 为正数，相当于 n / (2 ^1)
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}
// | 按位或，& 按位与，^ 按位异或
// |= 先按位或再赋值
EG: tableSizeFor(2) = 4 tableSizeFor(25) = 32
final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
int s = m.size();
if (s > 0) {
//判断插入的 map 中 kv 个数是否大于0
if (table == null) { // pre-size
//如果原 map 为空，则计算加入新 map 后的预计大小
float ft = ((float)s / loadFactor) + 1.0F;
//判断预计大小是否大于 hashMap 最大容量，大于则将 map 大小设置为 MAXIMUM_CAPACITY
int t = ((ft < (float)MAXIMUM_CAPACITY) ?
(int)ft : MAXIMUM_CAPACITY);
//如果预计大小大于阈值，则阈值重新赋值，扩大到大于等于当前尺寸的最小2的幂次方
if (t > threshold)
threshold = tableSizeFor(t);
}
else if (s > threshold)
//如果原 map 不为空，如果插入的 map 大小大于阈值，则2倍扩容哈希桶
resize();
for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
//在 putVal 过程中，也有可能会使哈希桶扩容
K key = e.getKey();
V value = e.getValue();
putVal(hash(key), key, value, false, evict);
}
}
}