static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;static final int MAXIMUM_CAPACITY = 1 << 30;static final float DEFAULT_LOAD_FACTOR = 0.75f;/** An empty table instance to share when the table is not inflated. */static final Entry
    [] EMPTY_TABLE = {}; // 懒初始化使用的空数组，// 一个默认阈值。高于该值，且以String作为Key时，使用备用哈希hash函数（sun.misc.Hashing.stringHash32），这个备用hash函数能减少String的hash冲突。// 此值是最大int型，表明hash表再也不能扩容了，继续put下去，hash冲突会越来越多。// 至于为什么是String型才有这种特殊待遇，因为String是最常见的Key的类型。Integer虽然也很常见，但是Integer有范围限制，并且它的hashCode设计得就非常好（就是自身的数值）。// 可以通过定义系统属性jdk.map.althashing.threshold来覆盖这个值。// 值为1时，总是会对String类型的key使用备用的hash函数；值为-1，则一定不使用备用hash函数。static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;

/** The table, resized as necessary. Length MUST Always be a power of two. */transient Entry
     
      [] table = (Entry
      
       []) EMPTY_TABLE; // 懒初始化，因此给一个空数组 ，其实空数组也没什么用，用null也行transient int size;final float loadFactor;transient int modCount;// 通常的扩容阈值，另外在懒初始化中，真正初始化时会使用这个值作为第一次的容量int threshold;// 一个hash种子，用于参与hash函数中的运算，获得更好的hash值来减少hash冲突// 值为0时会禁止掉备用hash函数transient int hashSeed = 0;private transient Set
       
        
         > entrySet = null;// keySet values继承使用AbstractMap的父类的属性
        
       
      
     

// Entry就一个变化，hash值不再是final的static class Entry
    
      implements Map.Entry
     
       {    final K key;    V value;    Entry
      
        next;    int hash; // 就这一个改变，hash不再是final的，扩容时可以重新计算hash值    // 后面的跟1.6的一样，不说了}/** holds values which can't be initialized until after VM is booted.  */// 这个类就是用来持有 ALTERNATIVE_HASHING_THRESHOLD的，影响String在极端情况下的hash值的计算，不影响HashMap基本的实现private static class Holder {    /**  Table capacity above which to switch to use alternative hashing. */    static final int ALTERNATIVE_HASHING_THRESHOLD;    static {        String altThreshold = java.security.AccessController.doPrivileged(            new sun.security.action.GetPropertyAction( "jdk.map.althashing.threshold"));         int threshold;        try {            threshold = (null != altThreshold) ? Integer.parseInt(altThreshold) : ALTERNATIVE_HASHING_THRESHOLD_DEFAULT;            // disable alternative hashing if -1            if (threshold == -1) {                threshold = Integer.MAX_VALUE;            }            if (threshold < 0) {                throw new IllegalArgumentException("value must be positive integer.");            }        } catch(IllegalArgumentException failed) {            throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed);        }        ALTERNATIVE_HASHING_THRESHOLD = threshold;    }}
      
     
    

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; // 初始容量保存在threshold中，真正初始化后threshold才是阈值    init(); // 这个方法什么也没做    // 初始化后数组table是默认值空数组，没有真正进行初始化}public HashMap(int initialCapacity) {    this(initialCapacity, DEFAULT_LOAD_FACTOR);}public HashMap() {    this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);}// loadFactor使用默认值0.75f，因为m是接口类型，可能没有loadFactor这个属性public HashMap(Map
     m) {    this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);    inflateTable(threshold);    putAllForCreate(m);}

// 使用比较高大上的算法求不小于number的满足2^n的数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;}// 真正初始化HashMapprivate void inflateTable(int toSize) {    // Find a power of 2 >= toSize    int capacity = roundUpToPowerOf2(toSize);    threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);    table = new Entry[capacity];    initHashSeedAsNeeded(capacity);}// 初始化hash种子// hashseed相关的可以不用理解太多，它只影响hash值的生成，以及扩容时是否需要重新计算hash值（rehash），本身对HashMap的基本的实现没影响final boolean initHashSeedAsNeeded(int capacity) {    boolean currentAltHashing = hashSeed != 0;    boolean useAltHashing = sun.misc.VM.isBooted() && (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);    boolean switching = currentAltHashing ^ useAltHashing;    if (switching) {        hashSeed = useAltHashing ? sun.misc.Hashing.randomHashSeed(this) : 0;    }    return switching;}

// 多了个hashSeed，以及对String的特别处理// null 依然视为hash = 0，总是放在index = 0的hash桶中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);}

// resize 修复了threshold过早地变为Integer.MAX_VALUE的问题，其余跟1.6一致void resize(int newCapacity) {    Entry[] oldTable = table;    int oldCapacity = oldTable.length;    if (oldCapacity == MAXIMUM_CAPACITY) { // 此时数组才是真正不能扩容了        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);    // 上面这行代码，可以避免1.6可能发生的因为newCapacity * LoadFacotr大于Integer.MAX_VALUE，导致后续本来还可以扩容，但是无法进入resize方法的问题}// 多一个真正的rehash的判断，其余跟1.6的一致void transfer(Entry[] newTable, boolean rehash) {    int newCapacity = newTable.length;    for (Entry
    
      e : table) {        while(null != e) {            Entry
     
       next = e.next;            if (rehash) { // 为true就要进行真正的rehash                e.hash = null == e.key ? 0 : hash(e.key);            }            int i = indexFor(e.hash, newCapacity);            e.next = newTable[i];            newTable[i] = e;            e = next;        }    }}
     
    

public int size() {    return size;}public boolean isEmpty() {    return size == 0;}public V get(Object key) {    if (key == null)        return getForNullKey();    Entry
    
      entry = getEntry(key);    return null == entry ? null : entry.getValue();}private V getForNullKey() {    if (size == 0) {        return null;    }    for (Entry
     
       e = table[0]; e != null; e = e.next) {        if (e.key == null)            return e.value;    }    return null;}public boolean containsKey(Object key) {    return getEntry(key) != null;}final Entry
      
        getEntry(Object key) {    if (size == 0) {        return null;    }    int hash = (key == null) ? 0 : hash(key);    for (Entry
       
         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;}public boolean containsValue(Object value) {    if (value == null)        return containsNullValue();    Entry[] tab = table;    for (int i = 0; i < tab.length ; i++)        for (Entry e = tab[i] ; e != null ; e = e.next)            if (value.equals(e.value))                return true;    return false;}private boolean containsNullValue() {    Entry[] tab = table;    for (int i = 0; i < tab.length ; i++)        for (Entry e = tab[i] ; e != null ; e = e.next)            if (e.value == null)                return true;    return false;}
       
      
     
    

public V put(K key, V value) {    if (table == EMPTY_TABLE) { // 考虑还未真正初始化的情况        inflateTable(threshold);    }    if (key == null)        return putForNullKey(value);    int hash = hash(key);    int i = indexFor(hash, table.length);    for (Entry
    
      e = table[i]; e != null; e = e.next) {        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++;    addEntry(hash, key, value, i);    return null;}private V putForNullKey(V value) {    for (Entry
     
       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++;    addEntry(0, null, value, 0);    return null;}// 这个跟1.6有些区别// 1、走实用主义，扩容多了个条件。当添加的节点是hash桶的第一个节点时，一定不扩容，所以会出现size > threshold的情况。// 2、几步的操作顺序不一样。jdk1.6的是先把节点添加到链表中，再判断是否扩容；1.7这里是先判断是否扩容，扩容完再把节点添加到链表中。void addEntry(int hash, K key, V value, int bucketIndex) {    if ((size >= threshold) && (null != table[bucketIndex])) {        resize(2 * table.length);        hash = (null != key) ? hash(key) : 0;        bucketIndex = indexFor(hash, table.length);    }    createEntry(hash, key, value, bucketIndex);}void createEntry(int hash, K key, V value, int bucketIndex) {    Entry
      
        e = table[bucketIndex];    table[bucketIndex] = new Entry<>(hash, key, value, e);    size++;}public void putAll(Map
        m) {    int numKeysToBeAdded = m.size();    if (numKeysToBeAdded == 0)        return;    if (table == EMPTY_TABLE) { // 考虑还未真正初始化的情况        inflateTable((int) Math.max(numKeysToBeAdded * loadFactor, threshold));    }    // 跟1.6的一样，稍微保守些，多判断下    if (numKeysToBeAdded > threshold) {        int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);        if (targetCapacity > MAXIMUM_CAPACITY)            targetCapacity = MAXIMUM_CAPACITY;        int newCapacity = table.length;        while (newCapacity < targetCapacity)            newCapacity <<= 1;        if (newCapacity > table.length)            resize(newCapacity);    }    for (Map.Entry
        e : m.entrySet())        put(e.getKey(), e.getValue());}public V remove(Object key) {    Entry
       
         e = removeEntryForKey(key);    return (e == null ? null : e.value);}final Entry
        
          removeEntryForKey(Object key) {    if (size == 0) { // 考虑还未真正初始化的情况        return null;    }    int hash = (key == null) ? 0 : hash(key);    int i = indexFor(hash, table.length);    Entry
         
           prev = table[i]; Entry
          
            e = prev; while (e != null) { Entry
           
             next = e.next; Object k; if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) { modCount++; size--; if (prev == e) table[i] = next; else prev.next = next; e.recordRemoval(this); return e; } prev = e; e = next; } return e;}public void clear() { modCount++; Arrays.fill(table, null); // 就是循环赋值 size = 0;}