/**
     * 默認(rèn)的初始大小，16，值必須是 2 的冪值
     */
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

    /**
     * 最大值，必須是 2 冪值且 小于等于 2 的30 次冪
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;
    /**
     * 默認(rèn)加載因子，0.75，就是map里的元素值超過(guò) 75% 就會(huì)觸發(fā)擴(kuò)容
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    //下面還有三個(gè)樹(shù)化的參數(shù)
    //如果哈希值相同的元素超過(guò) 8 個(gè)，則樹(shù)化
    static final int TREEIFY_THRESHOLD = 8;
    //樹(shù)的節(jié)點(diǎn)小于 6 個(gè)，則轉(zhuǎn)成鏈表
    static final int UNTREEIFY_THRESHOLD = 6;
    //如果 map 的元素小于 64，即便是 哈希值相同的元素超過(guò) 8 個(gè)了，也不樹(shù)化，會(huì)擴(kuò)容
    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;

        //... ... 

}

	//求 key 的哈希值
    static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

	static Class<?> myComparableClassFor(Object x) {
        if (x instanceof Comparable) {
            Class<?> c = x.getClass();
            // 獲取 x 實(shí)現(xiàn)的所有接口
            Type[] ts = c.getGenericInterfaces();
            Type[] as;
            Type t;
            ParameterizedType p;
            //如果 x  是 String 類型 直接返回
            if (c == String.class) {
                return c;
            }
            if (ts != null) {
                // 遍歷實(shí)現(xiàn)的所有接口
                for (int i = 0; i < ts.length; ++i) {
                    t = ts[i];
                    // 如果接口有泛型
                    if (t instanceof ParameterizedType) {
                        p = (ParameterizedType) t;
                        // 如果泛型對(duì)象是 Comparable
                        if (p.getRawType() == Comparable.class) {
                            as = p.getActualTypeArguments();
                            // 只有一個(gè)泛型參數(shù)，且參數(shù)類型是 x.class
                            if (as != null && as.length == 1 && as[0] == c) {
                                return c;
                            }
                        }
                    }
                }
            }
        }
        return null;
    }

class MyTest implements Comparable<MyTest> {}

會(huì)返回 MyTest.class

class MyTest implements Comparable<Integer> {}

會(huì)返回 null

    @SuppressWarnings({"rawtypes","unchecked"}) // for cast to Comparable
    static int compareComparables(Class<?> kc, Object k, Object x) {
        return (x == null || x.getClass() != kc ? 0 :
                ((Comparable)k).compareTo(x));
    }

    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;
    }

    transient Node<K,V>[] table;

    int threshold;

    final float loadFactor;

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

final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
        int s = m.size();
        if (s <= 0){
            return;
        }
        if (table == null) {
            float ft = ((float)s / loadFactor) + 1.0F;
            int t;
            if (ft < (float)MAXIMUM_CAPACITY){
                t = (int)ft;
            }else {
                t = MAXIMUM_CAPACITY
            }
            if (t > threshold){
                threshold = tableSizeFor(t);
            }

        }
        else if (s > threshold){
            resize();
        }


        for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
            K key = e.getKey();
            V value = e.getValue();
            putVal(hash(key), key, value, false, evict);
        }
    }

    final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) {
        HashMap.Node<K, V>[] tab;
        HashMap.Node<K, V> p;
        int n, i;

        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;

        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            HashMap.Node<K, V> e;
            K k;
            if (p.hash == hash) {
                k = p.key;
                if (k == key || key != null && key.equals(k)) {
                    e = p;
                }
            } else {
                if (p instanceof HashMap.TreeNode) {
                    e = ((HashMap.TreeNode<K, V>) p).putTreeVal(this, tab, hash, key, value);
                } else {
                    for (int binCount = 0; ; ++binCount) {
                        if ((e = p.next) == null) {
                            p.next = newNode(hash, key, value, null);
                            if (binCount >= TREEIFY_THRESHOLD - 1) {
                                treeifyBin(tab, hash);
                            }
                            break;
                        }
                        if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k))))
                            break;

                        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;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

final HashMap.Node<K, V>[] resize() {
    HashMap.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"})
    HashMap.Node<K, V>[] newTab = (HashMap.Node<K, V>[]) new HashMap.Node[newCap];
    table = newTab;
    if (oldTab != null) {
        for (int j = 0; j < oldCap; ++j) {
            HashMap.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 HashMap.TreeNode)
                    ((HashMap.TreeNode<K, V>) e).split(this, newTab, j, oldCap);
                else { // preserve order
                    HashMap.Node<K, V> loHead = null, loTail = null;
                    HashMap.Node<K, V> hiHead = null, hiTail = null;
                    HashMap.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;
}

Map<String, String> map = new HashMap<>(40);

    public V get(Object key) {
        Node<K,V> e;
        return (e = getNode(hash(key), key)) == null ? null : e.value;
    }

 public native int hashCode();