Android ArrayMap源代码分析

int indexOf(Object key, int hash) {
    final int N = mSize;

    // Important fast case: if nothing is in here, nothing to look for.
    //数组为空，直接返回
    if (N == 0) {
      return ~0;
    }

    //二分查找，不细说了
    int index = ContainerHelpers.binarySearch(mHashes, N, hash);

    // If the hash code wasn't found, then we have no entry for this key.
    //没找到hashCode，返回index，一个负数
    if (index < 0) {
      return index;
    }

    // If the key at the returned index matches, that's what we want.
    //对比key值，相同则返回index
    if (key.equals(mArray[index<<1])) {
      return index;
    }

    // Search for a matching key after the index.
    //如果返回的index对应的key值，与传入的key值不等，则可能对应的key在index后面
    int end;
    for (end = index + 1; end < N && mHashes[end] == hash; end++) {
      if (key.equals(mArray[end << 1])) return end;
    }

    // Search for a matching key before the index.
    //接上句，后面没有，那一定在前面。
    for (int i = index - 1; i >= 0 && mHashes[i] == hash; i--) {
      if (key.equals(mArray[i << 1])) return i;
    }

    // Key not found -- return negative value indicating where a
    // new entry for this key should go. We use the end of the
    // hash chain to reduce the number of array entries that will
    // need to be copied when inserting.
    //毛都没找到，那肯定是没有了，返回个负数
    return ~end;
  }

public V put(K key, V value) {
    final int hash;
    int index;
    //key是空，则通过indexOfNull查找对应的index；如果不为空，通过indexOf查找对应的index
    if (key == null) {
      hash = 0;
      index = indexOfNull();
    } else {
      hash = key.hashCode();
      index = indexOf(key, hash);
    }
    
    //index大于或等于0，一定是之前put过相同的key，直接替换对应的value。因为mArray中不只保存了value，还保存了key。
    //其结构为[key1,value1,key2,value2,key3,value3,......]
    //所以，需要将index乘2对应key，index乘2再加1对应value
    if (index >= 0) {
      index = (index<<1) + 1;
      final V old = (V)mArray[index];
      mArray[index] = value;
      return old;
    }

    //取正数
    index = ~index;
    //mSize的大小，即已经保存的数据量与mHashes的长度相同了，需要扩容啦
    if (mSize >= mHashes.length) {
      //扩容后的大小，有以下几个档位，BASE_SIZE（4），BASE_SIZE的2倍（8），mSize+(mSize>>1)（比之前的数据量扩容1/2）
      final int n = mSize >= (BASE_SIZE*2) ? (mSize+(mSize>>1))
          : (mSize >= 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;
      //扩容方法的实现
      allocArrays(n);

      //扩容后，需要把原来的数据拷贝到新数组中
      if (mHashes.length > 0) {
        if (DEBUG) Log.d(TAG, "put: copy 0-" + mSize + " to 0");
        System.arraycopy(ohashes, 0, mHashes, 0, ohashes.length);
        System.arraycopy(oarray, 0, mArray, 0, oarray.length);
      }

      //看看被废弃的数组是否还有利用价值
      //如果被废弃的数组的数据量为4或8，说明可能利用价值，以后用到的时候可以直接用。
      //如果被废弃的数据量太大，扔了算了，要不太占内存。如果浪费内存了，还费这么大劲，加了类干啥。
      freeArrays(ohashes, oarray, mSize);
    }

    //这次put的key对应的hashcode排序没有排在最后（index没有指示到数组结尾），因此需要移动index后面的数据
    if (index < mSize) {
      if (DEBUG) Log.d(TAG, "put: move " + index + "-" + (mSize-index)
          + " to " + (index+1));
      System.arraycopy(mHashes, index, mHashes, index + 1, mSize - index);
      System.arraycopy(mArray, index << 1, mArray, (index + 1) << 1, (mSize - index) << 1);
    }

    //把数据保存到数组中。看到了吧，key和value都在mArray中；hashCode放到mHashes
    mHashes[index] = hash;
    mArray[index<<1] = key;
    mArray[(index<<1)+1] = value;
    mSize++;
    return null;
  }

 /**
   * Remove the key/value mapping at the given index.
   * @param index The desired index, must be between 0 and {@link #size()}-1.
   * @return Returns the value that was stored at this index.
   */
  public V removeAt(int index) {
    final Object old = mArray[(index << 1) + 1];
    //如果数据量小于等于1，说明删除该元素后，没有数组为空，清空两个数组。
    if (mSize <= 1) {
      // Now empty.
      if (DEBUG) Log.d(TAG, "remove: shrink from " + mHashes.length + " to 0");
      //put中已有说明
      freeArrays(mHashes, mArray, mSize);
      mHashes = EmptyArray.INT;
      mArray = EmptyArray.OBJECT;
      mSize = 0;
    } else {
      //如果当初申请的数组最大容纳数据个数大于BASE_SIZE的2倍（8），并且现在存储的数据量只用了申请数量的1/3，
      //则需要重新分配空间，已减少对内存的占用
      if (mHashes.length > (BASE_SIZE*2) && mSize < mHashes.length/3) {
        // Shrunk enough to reduce size of arrays. We don't allow it to
        // shrink smaller than (BASE_SIZE*2) to avoid flapping between
        // that and BASE_SIZE.
        //新数组的大小
        final int n = mSize > (BASE_SIZE*2) ? (mSize + (mSize>>1)) : (BASE_SIZE*2);

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

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

        mSize--;
        //index之前的数据拷贝到新数组中
        if (index > 0) {
          if (DEBUG) Log.d(TAG, "remove: copy from 0-" + index + " to 0");
          System.arraycopy(ohashes, 0, mHashes, 0, index);
          System.arraycopy(oarray, 0, mArray, 0, index << 1);
        }
        //将index之后的数据拷贝到新数组中，和（index>0）的分支结合，就将index位置的数据删除了
        if (index < mSize) {
          if (DEBUG) Log.d(TAG, "remove: copy from " + (index+1) + "-" + mSize
              + " to " + index);
          System.arraycopy(ohashes, index + 1, mHashes, index, mSize - index);
          System.arraycopy(oarray, (index + 1) << 1, mArray, index << 1,
              (mSize - index) << 1);
        }
      } else {
        mSize--;
        //将index后的数据向前移位
        if (index < mSize) {
          if (DEBUG) Log.d(TAG, "remove: move " + (index+1) + "-" + mSize
              + " to " + index);
          System.arraycopy(mHashes, index + 1, mHashes, index, mSize - index);
          System.arraycopy(mArray, (index + 1) << 1, mArray, index << 1,
              (mSize - index) << 1);
        }
        //移位后最后一个数据清空
        mArray[mSize << 1] = null;
        mArray[(mSize << 1) + 1] = null;
      }
    }
    return (V)old;
  }

　　private static void freeArrays(final int[] hashes, final Object[] array, final int size) {
    //已经废弃的数组个数为BASE_SIZE的2倍（8），则用mTwiceBaseCache保存废弃的数组；
    //如果个数为BASE_SIZE（4），则用mBaseCache保存废弃的数组
    if (hashes.length == (BASE_SIZE*2)) {
      synchronized (ArrayMap.class) {
        if (mTwiceBaseCacheSize < CACHE_SIZE) {
          //array为刚刚废弃的数组，mTwiceBaseCache如果有内容，则放入array[0]位置，
          //在allocArrays中会从array[0]取出，放回mTwiceBaseCache
          array[0] = mTwiceBaseCache;
          //array[1]存放hash数组。因为array中每个元素都是Object对象，所以每个元素都可以存放数组
          array[1] = hashes;
          //清除index为2和之后的数据
          for (int i=(size<<1)-1; i>=2; i--) {
            array[i] = null;
          }
          mTwiceBaseCache = array;
          mTwiceBaseCacheSize++;
          if (DEBUG) Log.d(TAG, "Storing 2x cache " + array
              + " now have " + mTwiceBaseCacheSize + " entries");
        }
      }
    } else if (hashes.length == BASE_SIZE) {
      synchronized (ArrayMap.class) {
        if (mBaseCacheSize < CACHE_SIZE) {
          //代码的注释可以参考上面，不重复说明了
          array[0] = mBaseCache;
          array[1] = hashes;
          for (int i=(size<<1)-1; i>=2; i--) {
            array[i] = null;
          }
          mBaseCache = array;
          mBaseCacheSize++;
          if (DEBUG) Log.d(TAG, "Storing 1x cache " + array
              + " now have " + mBaseCacheSize + " entries");
        }
      }
    }
  }