/**
* The default initial capacity - MUST be a power of two.
* 默认初始化容量大小16,容量大小必须是2的次方
*/
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.
* 最大的容量为 2^30
*/
static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* The load factor used when none specified in constructor.
* 负载因子,一旦超过就会进行扩容
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* 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).
* 用于快速失败，由于HashMap非线程安全，在对HashMap进行迭代时，如果期间其他线程的参与导致HashMap的结构发生变化了
*（比如put，remov*e等操作），需要抛出异常ConcurrentModificationException
*/
transient int modCount;
/**
* The next size value at which to resize (capacity * load factor).
*
* @serial
*阈值，当table == {}时，该值为初始容量（初始容量默认为16）；当table被填充了，也就是为table分配内存空间后，
*threshold一般为capacity*loadFactory。HashMap在进行扩容时需要参考threshold
*/
int 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.
*当一个bucket是一个链表，链表个数大于等于8时，就要树状化，也就是要从链表结构变成红黑树结构
*/
static final int TREEIFY_THRESHOLD = 8;
HashMap构造函数：有4个构造器，其他构造器如果用户没有传入initialCapacity 和loadFactor这两个参数，会使用默认值initialCapacity默认为16，loadFactory默认为0.75
//指定初始容量，负载因子
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);
}
//指定初始容量
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
//无参
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
//将已存在的map放进去进行初始化，若为空则抛null异常
public HashMap(Map<? extends K, ? extends V> m) {
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
}
在常规构造器HashMap()中，没有为数组table分配内存空间，而是在执行put操作的时候才真正构建table数组。以下是put方法：