public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable

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

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

/**
 * The number of key-value mappings contained in this map.
 */
transient int size;

transient int modCount;

static final Entry<?,?>[] EMPTY_TABLE = {};

/**
 * The table, resized as necessary. Length MUST Always be a power of two.
 * 这里也强调扩容时，长度必须是2的指数次幂
 */
transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;

static class Entry<K,V> implements Map.Entry<K,V> {
    final K key;
    V value;
    Entry<K,V> next;
    int hash;
}

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;
    threshold = initialCapacity;
    // 供子类实现
    init();
}

public V put(K key, V value) {
    // 1 如果table为空则需要初始化
    if (table == EMPTY_TABLE) {
        inflateTable(threshold);
    }
    // 2 如果key为空,则单独处理
    if (key == null)
        return putForNullKey(value);
     // 3 根据key获取hash值   
    int hash = hash(key);
    // 4 根据hash值和长度求取索引值。
    int i = indexFor(hash, table.length);
    // 5 根据索引值获取数组下的链表进行遍历，判断元素是否存在相同的key
    for (Entry<K,V> e = table[i]; e != null; e = e.next) {
        Object k;
        if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
            // 如果相等，则将新值替换旧值
            V oldValue = e.value;
            e.value = value;
            e.recordAccess(this);
            return oldValue;
        }
    }

    // 6 如果不存在重复的key, 则需要创建新的Entry，然后添加至链表中。
    // 先将修改次数加一
    modCount++;
    addEntry(hash, key, value, i);
    return null;
}

private void inflateTable(int toSize) {
    // Find a power of 2 >= toSize
    int capacity = roundUpToPowerOf2(toSize);

    // 其中阈值=容量*加载因子，然后再初始化数组。
    threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
    table = new Entry[capacity];
    initHashSeedAsNeeded(capacity);
}

private static int roundUpToPowerOf2(int number) {
    // assert number >= 0 : "number must be non-negative";
    return number >= MAXIMUM_CAPACITY
            ? MAXIMUM_CAPACITY
            : (number > 1) ? Integer.highestOneBit((number - 1) << 1) : 1;
}

private V putForNullKey(V value) {
    // 取数组下标为0的链表
    for (Entry<K,V> e = table[0]; e != null; e = e.next) {
        if (e.key == null) {
            V oldValue = e.value;
            e.value = value;
            e.recordAccess(this);
            return oldValue;
        }
    }
    modCount++;
    // 注意：索引值依然指定是0
    addEntry(0, null, value, 0);
    return null;
}

final int hash(Object k) {
    int h = hashSeed;
    if (0 != h && k instanceof String) {
        return sun.misc.Hashing.stringHash32((String) k);
    }
    
    h ^= k.hashCode();

    // This function ensures that hashCodes that differ only by
    // constant multiples at each bit position have a bounded
    // number of collisions (approximately 8 at default load factor).
    h ^= (h >>> 20) ^ (h >>> 12);
    return h ^ (h >>> 7) ^ (h >>> 4);
}

static int indexFor(int h, int length) {
    // assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";
    return h & (length-1);
}

void addEntry(int hash, K key, V value, int bucketIndex) {
    // 如果长度大于阈值，则需要进行扩容
    if ((size >= threshold) && (null != table[bucketIndex])) {
        // 进行2倍扩容
        resize(2 * table.length);
        hash = (null != key) ? hash(key) : 0;
        // 扩容之后因为长度变化了，需要重新计算下索引值。
        bucketIndex = indexFor(hash, table.length);
    }
    // 然后进行添加元素
    createEntry(hash, key, value, bucketIndex);
}

void createEntry(int hash, K key, V value, int bucketIndex) {
    Entry<K,V> e = table[bucketIndex];
    // 往表头插入
    table[bucketIndex] = new Entry<>(hash, key, value, e);
    size++;
}

void transfer(Entry[] newTable, boolean rehash) {
    int newCapacity = newTable.length;
    for (Entry<K,V> e : table) {
        while(null != e) {
            // 定一个next
            Entry<K,V> next = e.next;
            if (rehash) {
                e.hash = null == e.key ? 0 : hash(e.key);
            }
            // 重新计算索引下标。
            int i = indexFor(e.hash, newCapacity);
            // 头插法，
            e.next = newTable[i];
            newTable[i] = e;
            // 接着下个节点继续遍历
            e = next;
        }
    }
}

public V get(Object key) {
    if (key == null)
        return getForNullKey();
    Entry<K,V> entry = getEntry(key);

    return null == entry ? null : entry.getValue();
}

private V getForNullKey() {
        if (size == 0) {
            return null;
        }
        for (Entry<K,V> e = table[0]; e != null; e = e.next) {
            if (e.key == null)
                return e.value;
        }
        return null;
    }

    final Entry<K,V> getEntry(Object key) {
        if (size == 0) {
            return null;
        }

        int hash = (key == null) ? 0 : hash(key);
        for (Entry<K,V> e = table[indexFor(hash, table.length)];
             e != null;
             e = e.next) {
            Object k;
            if (e.hash == hash &&
                ((k = e.key) == key || (key != null && key.equals(k))))
                return e;
        }
        return null;
    }

Entry<K,V> next = e.next;

// 同理落位到2
int i = indexFor(e.hash, newCapacity);
e.next = newTable[i];
// 指向a
newTable[i] = e;
e = next;

e.next = newTable[i];