//HashMap的主干數(shù)組，可以看到就是一個Entry數(shù)組，初始值為空數(shù)組{}，主干數(shù)組的長度一定是2的次冪，至于為什么這么做，后面會有詳細(xì)分析。
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;//存儲指向下一個Entry的引用，單鏈表結(jié)構(gòu)
        int hash;//對key的hashcode值進(jìn)行hash運(yùn)算后得到的值，存儲在Entry，避免重復(fù)計算

        /**
         * Creates new entry.
         */
        Entry(int h, K k, V v, Entry<K,V> n) {
            value = v;
            next = n;
            key = k;
            hash = h;
        }

//實際存儲的key-value鍵值對的個數(shù)
transient int size;
//閾值，當(dāng)table == {}時，該值為初始容量（初始容量默認(rèn)為16）；當(dāng)table被填充了，也就是為table分配內(nèi)存空間后，threshold一般為 capacity*loadFactory。HashMap在進(jìn)行擴(kuò)容時需要參考threshold，后面會詳細(xì)談到
int threshold;
//負(fù)載因子，代表了table的填充度有多少，默認(rèn)是0.75
final float loadFactor;
//用于快速失敗，由于HashMap非線程安全，在對HashMap進(jìn)行迭代時，如果期間其他線程的參與導(dǎo)致HashMap的結(jié)構(gòu)發(fā)生變化了（比如put，remove等操作），需要拋出異常ConcurrentModificationException
transient int modCount;

public HashMap(int initialCapacity, float loadFactor) {
　　　　　//此處對傳入的初始容量進(jìn)行校驗，最大不能超過MAXIMUM_CAPACITY = 1<<30(230)
        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;
　　　　　
        init();//init方法在HashMap中沒有實際實現(xiàn)，不過在其子類如 linkedHashMap中就會有對應(yīng)實現(xiàn)
    }

public V put(K key, V value) {
        //如果table數(shù)組為空數(shù)組{}，進(jìn)行數(shù)組填充（為table分配實際內(nèi)存空間），入?yún)閠hreshold，此時threshold為initialCapacity 默認(rèn)是1<<4(24=16)
        if (table == EMPTY_TABLE) {
            inflateTable(threshold);
        }
       //如果key為null，存儲位置為table[0]或table[0]的沖突鏈上
        if (key == null)
            return putForNullKey(value);
        int hash = hash(key);//對key的hashcode進(jìn)一步計算，確保散列均勻
        int i = indexFor(hash, table.length);//獲取在table中的實際位置
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {
        //如果該對應(yīng)數(shù)據(jù)已存在，執(zhí)行覆蓋操作。用新value替換舊value，并返回舊value
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }
        modCount++;//保證并發(fā)訪問時，若HashMap內(nèi)部結(jié)構(gòu)發(fā)生變化，快速響應(yīng)失敗
        addEntry(hash, key, value, i);//新增一個entry
        return null;
    }

private void inflateTable(int toSize) {
        int capacity = roundUpToPowerOf2(toSize);//capacity一定是2的次冪
        threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);//此處為threshold賦值，取capacity*loadFactor和MAXIMUM_CAPACITY+1的最小值，capaticy一定不會超過MAXIMUM_CAPACITY，除非loadFactor大于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;
    }

//這是一個神奇的函數(shù)，用了很多的異或，移位等運(yùn)算，對key的hashcode進(jìn)一步進(jìn)行計算以及二進(jìn)制位的調(diào)整等來保證最終獲取的存儲位置盡量分布均勻
final int hash(Object k) {
        int h = hashSeed;
        if (0 != h && k instanceof String) {
            return sun.misc.Hashing.stringHash32((String) k);
        }

        h ^= k.hashCode();

        h ^= (h >>> 20) ^ (h >>> 12);
        return h ^ (h >>> 7) ^ (h >>> 4);
    }

/**
     * 返回數(shù)組下標(biāo)
     */
    static int indexFor(int h, int length) {
        return h & (length-1);
    }

void addEntry(int hash, K key, V value, int bucketIndex) {
        if ((size >= threshold) && (null != table[bucketIndex])) {
            resize(2 * table.length);//當(dāng)size超過臨界閾值threshold，并且即將發(fā)生哈希沖突時進(jìn)行擴(kuò)容
            hash = (null != key) ? hash(key) : 0;
            bucketIndex = indexFor(hash, table.length);
        }

        createEntry(hash, key, value, bucketIndex);
    }

void resize(int newCapacity) {
        Entry[] oldTable = table;
        int oldCapacity = oldTable.length;
        if (oldCapacity == MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return;
        }

        Entry[] newTable = new Entry[newCapacity];
        transfer(newTable, initHashSeedAsNeeded(newCapacity));
        table = newTable;
        threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
    }

void transfer(Entry[] newTable, boolean rehash) {
        int newCapacity = newTable.length;
　　　　　//for循環(huán)中的代碼，逐個遍歷鏈表，重新計算索引位置，將老數(shù)組數(shù)據(jù)復(fù)制到新數(shù)組中去（數(shù)組不存儲實際數(shù)據(jù)，所以僅僅是拷貝引用而已）
        for (Entry<K,V> e : table) {
            while(null != e) {
                Entry<K,V> next = e.next;
                if (rehash) {
                    e.hash = null == e.key ? 0 : hash(e.key);
                }
                int i = indexFor(e.hash, newCapacity);
　　　　　　　　　 //將當(dāng)前entry的next鏈指向新的索引位置,newTable[i]有可能為空，有可能也是個entry鏈，如果是entry鏈，直接在鏈表頭部插入。
                e.next = newTable[i];
                newTable[i] = e;
                e = next;
            }
        }
    }

public V get(Object key) {
　　　　 //如果key為null,則直接去table[0]處去檢索即可。
        if (key == null)
            return getForNullKey();
        Entry<K,V> entry = getEntry(key);
        return null == entry ? null : entry.getValue();
 }

final Entry<K,V> getEntry(Object key) {
            
        if (size == 0) {
            return null;
        }
        //通過key的hashcode值計算hash值
        int hash = (key == null) ? 0 : hash(key);
        //indexFor (hash&length-1) 獲取最終數(shù)組索引，然后遍歷鏈表，通過equals方法比對找出對應(yīng)記錄
        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;
    }

/**
 * Created by chengxiao on 2016/11/15.
 */
public class MyTest {
    private static class Person{
        int idCard;
        String name;

        public Person(int idCard, String name) {
            this.idCard = idCard;
            this.name = name;
        }
        @Override
        public boolean equals(Object o) {
            if (this == o) {
                return true;
            }
            if (o == null || getClass() != o.getClass()){
                return false;
            }
            Person person = (Person) o;
            //兩個對象是否等值，通過idCard來確定
            return this.idCard == person.idCard;
        }

    }
    public static void main(String []args){
        HashMap<Person,String> map = new HashMap<Person, String>();
        Person person = new Person(1234,"喬峰");
        //put到hashmap中去
        map.put(person,"天龍八部");
        //get取出，從邏輯上講應(yīng)該能輸出“天龍八部”
        System.out.println("結(jié)果:"+map.get(new Person(1234,"蕭峰")));
    }
}