public class HashMapExam {
    public static void main(String[] args) {

        Map<Integer, String> map = new HashMap<>();
        for (int i = 0; i < 15; i++) {
            map.put(i, new String(new char[]{(char) ('A'+ i)}));
        }


        System.out.println("======Key和Value=======");
        for (Integer key:map.keySet()) {
            System.out.println(key);
        }
        for (String value:map.values()) {
            System.out.println(value);
        }

        System.out.println("======Iterator迭代器=======");
        Iterator<Map.Entry<Integer, String>> iterator = map.entrySet().iterator();
        while (iterator.hasNext()) {
            Map.Entry<Integer, String> mapEntry = iterator.next();
            System.out.println(mapEntry.getKey()+ "====" + mapEntry.getValue());
        }

        System.out.println("======Get的方式=======");
        Set<Integer> keySet = map.keySet();
        for (Integer key : keySet) {
            System.out.println(key + "====" + map.get(key));
        }
        
        System.out.println("======forEach=======");
        map.forEach((key,value) -> System.out.println(key+ "----" + value));
    }
}

// 序列化版本號(hào)
private static final long serialVersionUID = 362498820763181265L;

// 默認(rèn)的初始容量是16 --> 1<<4 相當(dāng)于 1*2的4次方也就是16
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; 

// 最大容量（傳入容量過(guò)大將被這個(gè)替換）
static final int MAXIMUM_CAPACITY = 1 << 30;

// 默認(rèn)加載因子為0.75，通過(guò)這個(gè)來(lái)算出臨界值
static final float DEFAULT_LOAD_FACTOR = 0.75f;

// 當(dāng)桶（bucket）上的結(jié)點(diǎn)數(shù)大于這個(gè)值時(shí)會(huì)轉(zhuǎn)換成紅黑樹(shù)
static final int TREEIFY_THRESHOLD = 8;

// 當(dāng)桶(bucket)上的結(jié)點(diǎn)數(shù)小于這個(gè)值時(shí)樹(shù)轉(zhuǎn)鏈表
static final int UNTREEIFY_THRESHOLD = 6;

// 桶中結(jié)構(gòu)轉(zhuǎn)化為紅黑樹(shù)對(duì)應(yīng)的數(shù)組長(zhǎng)度最小的值 
static final int MIN_TREEIFY_CAPACITY = 64;

//存儲(chǔ)元素的數(shù)組 
transient Node<K,V>[] table;

//存放具體元素的集合
transient Set<Map.Entry<K,V>> entrySet;

//存放元素的個(gè)數(shù)，注意這個(gè)不等于數(shù)組的長(zhǎng)度。
 transient int size;

// 每次擴(kuò)容和更改map結(jié)構(gòu)的計(jì)數(shù)器
 transient int modCount;  

// 臨界值 當(dāng)實(shí)際大小(容量*負(fù)載因子)超過(guò)臨界值時(shí)，會(huì)進(jìn)行擴(kuò)容
int threshold;

// 負(fù)載因子實(shí)際大小
final float loadFactor;

public HashMap() {
   this.loadFactor = DEFAULT_LOAD_FACTOR; // 將默認(rèn)的加載因子0.75賦值給loadFactor，并沒(méi)有創(chuàng)建數(shù)組
}

// 指定“容量大小”的構(gòu)造函數(shù)
public HashMap(int initialCapacity) {
    this(initialCapacity, DEFAULT_LOAD_FACTOR);
}

public HashMap(int initialCapacity, float loadFactor) {
    //判斷初始化容量initialCapacity是否小于0
    if (initialCapacity < 0)
        //如果小于0，則拋出非法的參數(shù)異常IllegalArgumentException
        throw new IllegalArgumentException("Illegal initial capacity: " +
                                            initialCapacity);
    //判斷初始化容量initialCapacity是否大于集合的最大容量MAXIMUM_CAPACITY-》2的30次冪
    if (initialCapacity > MAXIMUM_CAPACITY)
        //如果超過(guò)MAXIMUM_CAPACITY，會(huì)將MAXIMUM_CAPACITY賦值給initialCapacity
        initialCapacity = MAXIMUM_CAPACITY;
    //判斷負(fù)載因子loadFactor是否小于等于0或者是否是一個(gè)非數(shù)值
    if (loadFactor <= 0 || Float.isNaN(loadFactor))
        //如果滿(mǎn)足上述其中之一，則拋出非法的參數(shù)異常IllegalArgumentException
        throw new IllegalArgumentException("Illegal load factor: " +
                                            loadFactor);
     //將指定的加載因子賦值給HashMap成員變量的負(fù)載因子loadFactor
    this.loadFactor = loadFactor;
    /*
    	tableSizeFor(initialCapacity) 判斷指定的初始化容量是否是2的n次冪，如果不是那么會(huì)變?yōu)楸戎?		定初始化容量大的最小的2的n次冪。這點(diǎn)上述已經(jīng)講解過(guò)。
    	但是注意，在tableSizeFor方法體內(nèi)部將計(jì)算后的數(shù)據(jù)返回給調(diào)用這里了，并且直接賦值給threshold邊			界值了。有些人會(huì)覺(jué)得這里是一個(gè)bug,應(yīng)該這樣書(shū)寫(xiě)：
    	this.threshold = tableSizeFor(initialCapacity) * this.loadFactor;
    	這樣才符合threshold的意思（當(dāng)HashMap的size到達(dá)threshold這個(gè)閾值時(shí)會(huì)擴(kuò)容）。
		但是，請(qǐng)注意，在jdk8以后的構(gòu)造方法中，并沒(méi)有對(duì)table這個(gè)成員變量進(jìn)行初始化，table的初始化被推			 遲到了put方法中，在put方法中會(huì)對(duì)threshold重新計(jì)算，put方法的具體實(shí)現(xiàn)我們下面會(huì)進(jìn)行講解
    */
    this.threshold = tableSizeFor(initialCapacity);
}
最后調(diào)用了tableSizeFor，來(lái)看一下方法實(shí)現(xiàn)：
/**
 * Returns a power of two size for the given target capacity.
   返回比指定初始化容量大的最小的2的n次冪
 */
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;
}

//構(gòu)造一個(gè)映射關(guān)系與指定 Map 相同的新 HashMap。
public HashMap(Map<? extends K, ? extends V> m) {
    //負(fù)載因子loadFactor變?yōu)槟J(rèn)的負(fù)載因子0.75
    this.loadFactor = DEFAULT_LOAD_FACTOR;
    putMapEntries(m, false);
}
// 最后調(diào)用了putMapEntries，來(lái)看一下方法實(shí)現(xiàn)：
final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
    //獲取參數(shù)集合的長(zhǎng)度
    int s = m.size();
    if (s > 0)
    {
        //判斷參數(shù)集合的長(zhǎng)度是否大于0，說(shuō)明大于0
        if (table == null)  // 判斷table是否已經(jīng)初始化
        { // pre-size
                // 未初始化，s為m的實(shí)際元素個(gè)數(shù)
                float ft = ((float)s / loadFactor) + 1.0F;
                int t = ((ft < (float)MAXIMUM_CAPACITY) ?
                        (int)ft : MAXIMUM_CAPACITY);
                // 計(jì)算得到的t大于閾值，則初始化閾值
                if (t > threshold)
                    threshold = tableSizeFor(t);
        }
        // 已初始化，并且m元素個(gè)數(shù)大于閾值，進(jìn)行擴(kuò)容處理
        else if (s > threshold)
            resize();
        // 將m中的所有元素添加至HashMap中
        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);
        }
    }
}

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;
    }
}

static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {
    TreeNode<K,V> parent;  
    TreeNode<K,V> left;
    TreeNode<K,V> right;
    TreeNode<K,V> prev;   
    boolean red;
    TreeNode(int hash, K key, V val, Node<K,V> next) {
        super(hash, key, val, next);
    }

    final TreeNode<K,V> root() {
        for (TreeNode<K,V> r = this, p;;) {
            if ((p = r.parent) == null)
                return r;
            r = p;
        }
    }

    static <K,V> void moveRootToFront(Node<K,V>[] tab, TreeNode<K,V> root) {
        ....
    }

    final TreeNode<K,V> find(int h, Object k, Class<?> kc) {
        ....
    }

    final TreeNode<K,V> getTreeNode(int h, Object k) {
        return ((parent != null) ? root() : this).find(h, k, null);
    }

    static int tieBreakOrder(Object a, Object b) {
        ....
    }

    final void treeify(Node<K,V>[] tab) {
        ....
    }

    final Node<K,V> untreeify(HashMap<K,V> map) {
        ....
    }

    final TreeNode<K,V> putTreeVal(HashMap<K,V> map, Node<K,V>[] tab,
                                       int h, K k, V v) {
        ....
    }

    final void removeTreeNode(HashMap<K,V> map, Node<K,V>[] tab,
                                  boolean movable) {
        ....
    }

    final void split(HashMap<K,V> map, Node<K,V>[] tab, int index, int bit) {
        ....
    }

    static <K,V> TreeNode<K,V> rotateLeft(TreeNode<K,V> root,
                                              TreeNode<K,V> p) {
        ....
    }

    static <K,V> TreeNode<K,V> rotateRight(TreeNode<K,V> root,
                                               TreeNode<K,V> p) {
        ....
    }

    static <K,V> TreeNode<K,V> balanceInsertion(TreeNode<K,V> root,
                                                    TreeNode<K,V> x) {
        ....
    }

    static <K,V> TreeNode<K,V> balanceDeletion(TreeNode<K,V> root,
                                                   TreeNode<K,V> x) {
        ....
    }

    static <K,V> boolean checkInvariants(TreeNode<K,V> t) {
        ....
    }
}

static final int hash(Object key) {
    int h;
    // (h = key.hashCode()) ^ (h >>> 16)：異或Hash右移16位算法
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

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

final Node<K,V> getNode(int hash, Object key) {
    // 當(dāng)前 HashMap 的散列表的引用
    Node<K,V>[] tab; 
    // first：桶頭元素
    // e：用于存儲(chǔ)臨時(shí)元素
    Node<K,V> first, e;
    // n：table 數(shù)組的長(zhǎng)度
    int n; 
    // 元素中的 k
    K k;
    // 將 table 賦值給 tab，不等于 null 說(shuō)明有數(shù)據(jù)，（n = table.length）> 0 同理說(shuō)明 table 中有值
    if ((tab = table) != null && (n = tab.length) > 0 &&
        // 同時(shí)將 該位置的元素賦值為 first
        (first = tab[(n - 1) & hash]) != null) {
        // 定位到了桶的到的位置就是想要獲取的 key 對(duì)應(yīng)的，直接返回該元素
        if (first.hash == hash && 
            ((k = first.key) == key || (key != null && key.equals(k))))
            return first;
        // 到這一步說(shuō)明定位到的元素不是想要的，且該位置不僅僅有一個(gè)元素，想要判斷是鏈表還是樹(shù)
        if ((e = first.next) != null) {
            // 是否已經(jīng)樹(shù)化
            if (first instanceof TreeNode)
                return ((TreeNode<K,V>)first).getTreeNode(hash, key);
            // 處理鏈表的情況
            do {
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    return e;
            } while ((e = e.next) != null);
        }
    }
    // 遍歷不到返回null
    return null;
}

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