JDK7HashMap

1、HashMap构造方法

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

    public HashMap() {
            this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
        }

空的构造方法默认调用有参构造

DEFAULT_INITIAL_CAPACITY 默认大小 16
DEFAULT_LOAD_FACTOR 默认加载因子 0.75

 public HashMap(int initialCapacity, float loadFactor) {
        //做参数校验 加载因子 容量大小 
        （大于0 小于等于MAXIMUM_CAPACITY）
        this.loadFactor = loadFactor;
        threshold = initialCapacity;
        init();
    }

2、put方法

map.put("k1", "v1");
//put方法
public V put(K key, V value) {
        if (table == EMPTY_TABLE) {
            inflateTable(threshold);
        }
        if (key == null)
            return putForNullKey(value);
        int hash = hash(key);
        int i = indexFor(hash, table.length);
        for (Entry<K,V> 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;
    }

2.1 inflateTable方法

    /**
     * Inflates the table.
     */
    private void inflateTable(int toSize) {
        // Find a power of 2 >= toSize
        //该方法用于寻找一个大于或等于该数的一个2的幂次方数，
        //为后期扩容或计算元素存放位置
        int capacity = roundUpToPowerOf2(toSize);

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

2.1.1 roundUpToPowerOf2方法

private static int roundUpToPowerOf2(int number) {
        // assert number >= 0 : "number must be non-negative";
        int rounded = number >= MAXIMUM_CAPACITY
                ? MAXIMUM_CAPACITY
                : (rounded = Integer.highestOneBit(number)) != 0
                    ? (Integer.bitCount(number) > 1) ? rounded << 1 : rounded
                    : 1;

        return rounded;
    }
  /**
   * 该方法用于返回一个小于等于i的一个2的幂次方数
   * 采用的的方法为最高为 1
  */
public static int highestOneBit(int i) {
        // HD, Figure 3-1
        i |= (i >>  1);
        i |= (i >>  2);
        i |= (i >>  4);
        i |= (i >>  8);
        i |= (i >> 16);
        //这里i得出为 00....0 1111
        //i >>> 1   00....0 0111
        //减去得到   00....0 1000
        return i - (i >>> 1);
    }

该方法就是讲hash值与对应的Entry数组的长度进行与运算
  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);
    }

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).
        //为了让数组元素存放的更加散列让高位也参与运算
        h ^= (h >>> 20) ^ (h >>> 12);
        return h ^ (h >>> 7) ^ (h >>> 4);
    }

2.2 addEntry方法

 void addEntry(int hash, K key, V value, int bucketIndex) {
        //当存放元素个数超过容量*加载因子 且当前下标为
        bucketIndex的table已经存放过元素则扩容
        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);
    }

2.2.1 createEntry() 方法

   void createEntry(int hash, K key, V value, int bucketIndex) {
        Entry<K,V> e = table[bucketIndex];
        //这里完成的逻辑就是将 新的Entry对象
        的next指向table[bucketIndex] 当前存放的元素，
        然后将table[bucketIndex] 指向新的Entry对象
        table[bucketIndex] = new Entry<>(hash, key, value, e);
        size++;
    }

2.2.2 resize() 方法

    // newCapacity为之前数组元素长度的两倍
    void resize(int newCapacity) {
    //转移元素到新数组
        transfer(newTable, initHashSeedAsNeeded(newCapacity));
    }

/**
     * Initialize the hashing mask value. We defer initialization until we
     * really need it.
     */
     //该方法就是为了重新生成hahs种子
    final boolean initHashSeedAsNeeded(int capacity) {
        //这里hashSeed之前默认为0 所以为false
        boolean currentAltHashing = hashSeed != 0;
        boolean useAltHashing = sun.misc.VM.isBooted() &&
                (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
        //所以这里只有当useAltHashing返回true
        //才会重新计算hashSeed
        boolean switching = currentAltHashing ^ useAltHashing;
        if (switching) {
            hashSeed = useAltHashing
                ? sun.misc.Hashing.randomHashSeed(this)
                : 0;
        }
        return switching;
    }

transfer 方法

    void transfer(Entry[] newTable, boolean rehash) {
        int newCapacity = newTable.length;
        //遍历当前Entry数组以及链表 即遍历所有元素
        //这里在并发环境下，会出现循环列表的情况
        //所以在get或put的时候就可能会导致死循环
        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);
                e.next = newTable[i];
                newTable[i] = e;
                e = next;
            }
        }
    }