JDK7HashMap

一、HashMap構造方法

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

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

空的構造方法默認調用有參構造

DEFAULT_INITIAL_CAPACITY 默認大小 16
DEFAULT_LOAD_FACTOR 默認加載因子 0.75

public HashMap(int initialCapacity, float loadFactor) {
        //作參數校驗 加載因子 容量大小 
        （大於0 小於等於MAXIMUM_CAPACITY）
        this.loadFactor = loadFactor;
        threshold = initialCapacity;
        init();
    }

二、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
        //該方法用於尋找一個大於或等於該數的一個2的冪次方數，
        //爲後期擴容或計算元素存放位置
        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;
    }
  /**
   * 該方法用於返回一個小於等於i的一個2的冪次方數
   * 採用的的方法爲最高爲 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);
        //這裏i得出爲 00....0 1111
        //i >>> 1   00....0 0111
        //減去獲得   00....0 1000
        return i - (i >>> 1);
    }

該方法就是講hash值與對應的Entry數組的長度進行與運算
  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).
        //爲了讓數組元素存放的更加散列讓高位也參與運算
        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) {
        //當存放元素個數超過容量*加載因子 且當前下標爲
        bucketIndex的table已經存放過元素則擴容
        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];
        //這裏完成的邏輯就是將 新的Entry對象
        的next指向table[bucketIndex] 當前存放的元素，
        而後將table[bucketIndex] 指向新的Entry對象
        table[bucketIndex] = new Entry<>(hash, key, value, e);
        size++;
    }

2.2.2 resize() 方法

// newCapacity爲以前數組元素長度的兩倍
    void resize(int newCapacity) {
    //轉移元素到新數組
        transfer(newTable, initHashSeedAsNeeded(newCapacity));
    }

/**
     * Initialize the hashing mask value. We defer initialization until we
     * really need it.
     */
     //該方法就是爲了從新生成hahs種子
    final boolean initHashSeedAsNeeded(int capacity) {
        //這裏hashSeed以前默認爲0 因此爲false
        boolean currentAltHashing = hashSeed != 0;
        boolean useAltHashing = sun.misc.VM.isBooted() &&
                (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
        //因此這裏只有當useAltHashing返回true
        //纔會從新計算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;
        //遍歷當前Entry數組以及鏈表 即遍歷全部元素
        //這裏在併發環境下，會出現循環列表的狀況
        //因此在get或put的時候就可能會致使死循環
        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;
            }
        }
    }