ArrayMap源码分析

ArrayMap比hashmap更高效，适合数据量比较小的情况下使用，效率更高。它是通过两个数组还保存数据，
一个数组保存key的hashcode，
另外一个保存k和v,K V的下标比较特殊。
k=index,v=index+1,k永远比v高出一个位置。

	// 最小增容的大小
 	private static final int BASE_SIZE = 4;

    /**
     * Maximum number of entries to have in array caches.
     * 可以缓存的最大数量
     */
    private static final int CACHE_SIZE = 10;

    /**
     * Special hash array value that indicates the container is immutable.
     */
    static final int[] EMPTY_IMMUTABLE_INTS = new int[0];

    /**
     * @hide Special immutable empty ArrayMap.
     */
    public static final ArrayMap EMPTY = new ArrayMap<>(-1);

    /**
     * Caches of small array objects to avoid spamming garbage.  The cache
     * Object[] variable is a pointer to a linked list of array objects.
     * The first entry in the array is a pointer to the next array in the
     * list; the second entry is a pointer to the int[] hash code array for it.
     */
    static Object[] mBaseCache;
    static int mBaseCacheSize;
    static Object[] mTwiceBaseCache;
    static int mTwiceBaseCacheSize;

    final boolean mIdentityHashCode;
    int[] mHashes;// 保存key的hashcode的数组
    Object[] mArray; // 保存k v的数组
    int mSize;//mHashes的大小
 public ArrayMap() {
        this(0, false);
    }

    /**
     * Create a new ArrayMap with a given initial capacity.
     */
    public ArrayMap(int capacity) {
        this(capacity, false);
    }

    /** {@hide} */
    public ArrayMap(int capacity, boolean identityHashCode) {
        mIdentityHashCode = identityHashCode;

        // If this is immutable, use the sentinal EMPTY_IMMUTABLE_INTS
        // instance instead of the usual EmptyArray.INT. The reference
        // is checked later to see if the array is allowed to grow.
        if (capacity < 0) {
            mHashes = EMPTY_IMMUTABLE_INTS;
            mArray = EmptyArray.OBJECT;
        } else if (capacity == 0) {
            mHashes = EmptyArray.INT;
            mArray = EmptyArray.OBJECT;
        } else {
       	//分配内存，该方法重要
            allocArrays(capacity);
        }
        mSize = 0;
    }

 

接下来看存入数据的流程

	  public V put(K key, V value) {
        final int osize = mSize;
        final int hash;
        int index;
        // key 为null hash=0 
        if (key == null) {
            hash = 0;
            //获取下标位置
            index = indexOfNull();
        } else {
        // 获取key的hashcode
            hash = mIdentityHashCode ? System.identityHashCode(key) : key.hashCode();
            // 计算应该存放的位置，也就是下标
            index = indexOf(key, hash);
        }
        // 如果>=0，证明该key已经存在，需要覆盖v
        if (index >= 0) {
            index = (index<<1) + 1;
            final V old = (V)mArray[index];
            mArray[index] = value;
            return old;
        }
		//取反
        index = ~index;
        if (osize >= mHashes.length) {
            final int n = osize >= (BASE_SIZE*2) ? (osize+(osize>>1))
                    : (osize >= BASE_SIZE ? (BASE_SIZE*2) : BASE_SIZE);

            if (DEBUG) Log.d(TAG, "put: grow from " + mHashes.length + " to " + n);

            final int[] ohashes = mHashes;
            final Object[] oarray = mArray;
            allocArrays(n);

            if (CONCURRENT_MODIFICATION_EXCEPTIONS && osize != mSize) {
                throw new ConcurrentModificationException();
            }

            if (mHashes.length > 0) {
                if (DEBUG) Log.d(TAG, "put: copy 0-" + osize + " to 0");
                System.arraycopy(ohashes, 0, mHashes, 0, ohashes.length);
                System.arraycopy(oarray, 0, mArray, 0, oarray.length);
            }

            freeArrays(ohashes, oarray, osize);
        }

        if (index < osize) {
            if (DEBUG) Log.d(TAG, "put: move " + index + "-" + (osize-index)
                    + " to " + (index+1));
            System.arraycopy(mHashes, index, mHashes, index + 1, osize - index);
            System.arraycopy(mArray, index << 1, mArray, (index + 1) << 1, (mSize - index) << 1);
        }

        if (CONCURRENT_MODIFICATION_EXCEPTIONS) {
            if (osize != mSize || index >= mHashes.length) {
                throw new ConcurrentModificationException();
            }
        }
        mHashes[index] = hash;
        mArray[index<<1] = key;
        mArray[(index<<1)+1] = value;
        mSize++;
        return null;
    }