public HashMap() {
    // 默认容量 16
    this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}

public HashMap(int initialCapacity) {
    // 自定义最大容量
    this(initialCapacity, DEFAULT_LOAD_FACTOR);
}

/** 自定义最大容量与负载因子 **/
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);
}

public HashMap(Map<? extends K, ? extends V> m) {
    this.loadFactor = DEFAULT_LOAD_FACTOR;
    putMapEntries(m, false);
}

public static void main(String[] args) {
    Map<String, String> map = new HashMap<>();
    map.put("Banana", "香蕉");
    map.put("Orange", "橘子");
    map.put("Apple", "苹果");
    map.entrySet().forEach(System.out::println);
}
// Apple=苹果
// Orange=橘子
// Banana=香蕉

/** 默认初始容量 = 16, 比如为 2 的幂 **/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
/** 最大容量 如果没有被构造方法指定的话将会使用这个数值, 必须是 2 的幂并且 <= 1 << 30 **/
static final int MAXIMUM_CAPACITY = 1 << 30;
/** 默认负载因子, 也就是HashMap中存储的实际Node数量超过总容量的这个百分比之后将会扩容
* 可以等于 1, 也可以大于 1, 只是冲突太多, 降低效率。
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/** 将链表转化为红黑树的阈值. 下面的 bin 指的是 bucket, 如下图中的2开始的一串, 而不是从 0-7 的bucket数组 **/
/**
 * 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;
/** 红黑树转化为链表的阈值 **/
static final int UNTREEIFY_THRESHOLD = 6;
/** 树化时的最小容量 **/
/**
* 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;

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 V setValue(V newValue) { ... }
    
    /** 重写 hashCode 一定要重写 equals */
    public final int hashCode() { ... }
    public final boolean equals(Object o) { ... }
}

public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}

/**
 * Implements Map.put and related methods.
 *
 * @param hash hash for key
 * @param key the key
 * @param value the value to put
 * @param onlyIfAbsent if true, don't change existing value
 * @param evict if false, the table is in creation mode.
 * @return previous value, or null if none
 */
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    // 判断数组是否为空或者为 null 若是则进行扩容并将扩容后的长度赋值给 n
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    // 根据 key 值计算得到的 hash 值寻找数组索引 i
    // 如果 tab[i] 为空 则创建并添加新的结点
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    else {
        // 如果 tab[i] 不空 说明当前位置有元素
        Node<K,V> e; K k;
        // 判断 tab[i] 的首个元素是否和key一样，如果相同直接覆盖value
        if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        // 如果 p 是红黑树的结点 如果是红黑树 则在红黑树中添加键值对
        else if (p instanceof TreeNode)
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else { // 不是红黑树，则是普通的链表
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    // 链表长度大于等于 8 转换为红黑树进行处理
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        // 树化
                        treeifyBin(tab, hash);
                    break;
                }
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                // key 已经存在直接覆盖value
                p = e;
            }
        }
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount; // 扩容次数
    // 超过最大容量 就扩容
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
}

static final int hash(Object key) {   //jdk1.8 & jdk1.7
     int h;
     // h = key.hashCode() 为第一步 取hashCode值
     // h ^ (h >>> 16)  为第二步 无符号右移 16 位 高位参与运算
     return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

static int indexFor(int h, int length) {  //jdk1.7的源码，jdk1.8没有这个方法，但是实现原理一样的
     return h & (length-1);  //第三步 取模运算
    // 这个方法非常巧妙，它通过h & (table.length -1)来得到该对象的保存位
    // 而HashMap底层数组的长度总是 2 的n次方(见下方)，这是HashMap在速度上的优化
    // 当length总是2的n次方时，h& (length-1)运算等价于对length取模，也就是h%length
    // 但是&比%具有更高的效率
}

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;
}

public class HashMap<K,V> extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable {
    
    int threshold;  // 所能容纳的 key-value 阈值 
    // 如果内存空间很多而又对时间效率要求很高，可以降低负载因子Load factor的值；
    // 相反，如果内存空间紧张而对时间效率要求不高，可以增加负载因子loadFactor的值，这个值可以大于1。
    final float loadFactor;    // 负载因子 
    int modCount;  // 用来记录HashMap内部结构发生变化的次数，主要用于迭代的快速失败。
    int size; // 实际存在的键值对数量
    
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16 默认初始容量
    static final float DEFAULT_LOAD_FACTOR = 0.75f; // 默认负载因子
    static final int TREEIFY_THRESHOLD = 8; // 树化门槛
    static final int UNTREEIFY_THRESHOLD = 6; // 反树化 即转化为链表门槛
}

 1 void resize(int newCapacity) {   //传入新的容量
 2     Entry[] oldTable = table;    //引用扩容前的Entry数组
 3     int oldCapacity = oldTable.length;         
 4     if (oldCapacity == MAXIMUM_CAPACITY) {  //扩容前的数组大小如果已经达到最大(2^30)了
 5         threshold = Integer.MAX_VALUE; //修改阈值为int的最大值(2^31-1)，这样以后就不会扩容了
 6         return;
 7     }
 8  
 9     Entry[] newTable = new Entry[newCapacity];  //初始化一个新的Entry数组
10     transfer(newTable);                         //！！将数据转移到新的Entry数组里
11     table = newTable;                           //HashMap的table属性引用新的Entry数组
12     threshold = (int)(newCapacity * loadFactor);//修改阈值
13 }

 1 void transfer(Entry[] newTable) {
 2     Entry[] src = table;                   //src引用了旧的Entry数组
 3     int newCapacity = newTable.length;
 4     for (int j = 0; j < src.length; j++) { //遍历旧的Entry数组
 5         Entry<K,V> e = src[j];             //取得旧Entry数组的每个元素
 6         if (e != null) {
 7             src[j] = null;//释放旧Entry数组的对象引用（for循环后，旧的Entry数组不再引用任何对象）
 8             do {
 9                 Entry<K,V> next = e.next;
10                 int i = indexFor(e.hash, newCapacity); //！！重新计算每个元素在数组中的位置
11                 e.next = newTable[i]; //标记[1]
12                 newTable[i] = e;      //将元素放在数组上
13                 e = next;             //访问下一个Entry链上的元素
14             } while (e != null);
15         }
16     }
17 }

final Node<K,V>[] resize() {
    Node<K,V>[] oldTab = table;
    int oldCap = (oldTab == null) ? 0 : oldTab.length;
    int oldThr = threshold;
    int newCap, newThr = 0;
    if (oldCap > 0) {
        if (oldCap >= MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return oldTab;
        }
        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                 oldCap >= DEFAULT_INITIAL_CAPACITY)
            newThr = oldThr << 1; // double threshold
    }
    else if (oldThr > 0) // initial capacity was placed in threshold
        newCap = oldThr;
    else {               // zero initial threshold signifies using defaults
        newCap = DEFAULT_INITIAL_CAPACITY;
        newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
    }
    if (newThr == 0) {
        float ft = (float)newCap * loadFactor;
        newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                  (int)ft : Integer.MAX_VALUE);
    }
    threshold = newThr;
    @SuppressWarnings({"rawtypes","unchecked"})
    Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
    table = newTab;
    if (oldTab != null) {
        for (int j = 0; j < oldCap; ++j) {
            Node<K,V> e;
            if ((e = oldTab[j]) != null) {
                oldTab[j] = null;
                if (e.next == null)
                    newTab[e.hash & (newCap - 1)] = e;
                else if (e instanceof TreeNode)
                    ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                else { // preserve order
                    Node<K,V> loHead = null, loTail = null;
                    Node<K,V> hiHead = null, hiTail = null;
                    Node<K,V> next;
                    do {
                        next = e.next;
                        if ((e.hash & oldCap) == 0) {
                            if (loTail == null)
                                loHead = e;
                            else
                                loTail.next = e;
                            loTail = e;
                        }
                        else {
                            if (hiTail == null)
                                hiHead = e;
                            else
                                hiTail.next = e;
                            hiTail = e;
                        }
                    } while ((e = next) != null);
                    if (loTail != null) {
                        loTail.next = null;
                        newTab[j] = loHead;
                    }
                    if (hiTail != null) {
                        hiTail.next = null;
                        newTab[j + oldCap] = hiHead;
                    }
                }
            }
        }
    }
    return newTab;
}