常量和属性

/*************************常量********************************/
//默认初始化容量，容量必须是2的幂
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
//最大容量，2^31
static final int MAXIMUM_CAPACITY = 1 << 30;
//默认的负载因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;
//树化的临界值。大于8，由链表转红黑树
static final int TREEIFY_THRESHOLD = 8;
//拆分红黑树临界值。小于6，由红黑树转链表
static final int UNTREEIFY_THRESHOLD = 6;
//桶中结构转化为红黑树对应的table的最小数量
static final int MIN_TREEIFY_CAPACITY = 64;
/**************************属性*******************************/
//桶，存储元素的数组，总是2的幂次倍
transient Node<K,V>[] table;
//存放具体元素的集
transient Set<Map.Entry<K,V>> entrySet;
// 元素的个数，包含链表和红黑树中的元素
transient int size;
// 每次扩容和更改map结构的计数器
transient int modCount;  
// 临界值 当实际大小(容量*填充因子)超过临界值时，会进行扩容
int threshold;
// 填充因子
final float loadFactor;

构造方法

//无参构造方法，只对loadFactor属性进行了赋值
public HashMap() {
    this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
//构造方法调用
public HashMap(int initialCapacity) {
    this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
//真正的双参数构造方法
public HashMap(int initialCapacity, float loadFactor) {
    // 初始容量不能小于0，否则报错
    if (initialCapacity < 0)
        throw new IllegalArgumentException("Illegal initial capacity: " +
                                            initialCapacity);
    // 初始容量不能大于最大值，否则为最大值
    if (initialCapacity > MAXIMUM_CAPACITY)
        initialCapacity = MAXIMUM_CAPACITY;
    // 填充因子不能小于或等于0，不能为非数字
    if (loadFactor <= 0 || Float.isNaN(loadFactor))
        throw new IllegalArgumentException("Illegal load factor: " +
                                            loadFactor);
    // 初始化填充因子                                        
    this.loadFactor = loadFactor;
    // 初始化threshold大小
    this.threshold = tableSizeFor(initialCapacity);    
}

 内置类

Node<K,V>

    /**
     * hashMap子节点
     * Basic hash bin node, used for most entries.  (See below for
     * TreeNode subclass, and in LinkedHashMap for its Entry subclass.)
     */
    static class Node<K,V> implements Map.Entry<K,V> {
        final int hash;
        final K key;
        V value;
        Node<K,V> next;

        Node(int hash, K key, V value, Node<K,V> next) {
            this.hash = hash;
            this.key = key;
            this.value = value;
            this.next = next;
        }

        public final K getKey()        { return key; }
        public final V getValue()      { return value; }
        public final String toString() { return key + "=" + value; }

        public final int hashCode() {
            return Objects.hashCode(key) ^ Objects.hashCode(value);
        }

        public final V setValue(V newValue) {
            V oldValue = value;
            value = newValue;
            return oldValue;
        }

        public final boolean equals(Object o) {
            if (o == this)
                return true;
            if (o instanceof Map.Entry) {
                Map.Entry<?,?> e = (Map.Entry<?,?>)o;
                if (Objects.equals(key, e.getKey()) &&
                    Objects.equals(value, e.getValue()))
                    return true;
            }
            return false;
        }
    }

方法

    /**
     * Returns a power of two size for the given target capacity.
     */
    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;
    }