SparseArray是android.util包中提供的類，用于建立整數對對象的映射，比HashMap性能更佳，因為它避免了自動裝箱并且內部數據結構不依賴額外實體對象。因為使用二分搜索來尋找鍵，所以不適合存儲數量比較大（數百以上）的情況。

    //已刪除值通用的標記對象
    private static final Object DELETED = new Object();
    //是否需要進行gc
    private boolean mGarbage = false;

    //存儲鍵的數組
    private int[] mKeys;
    //存儲值的數組
    private Object[] mValues;
    //存儲大小
    private int mSize;

    /**
     * Creates a new SparseArray containing no mappings.
     */
    public SparseArray() {
        this(10);
    }

    /**
     * Creates a new SparseArray containing no mappings that will not
     * require any additional memory allocation to store the specified
     * number of mappings.  If you supply an initial capacity of 0, the
     * sparse array will be initialized with a light-weight representation
     * not requiring any additional array allocations.
     */
    public SparseArray(int initialCapacity) {
        if (initialCapacity == 0) {
            mKeys = EmptyArray.INT;
            mValues = EmptyArray.OBJECT;
        } else {
            mValues = ArrayUtils.newUnpaddedObjectArray(initialCapacity);
            mKeys = new int[mValues.length];
        }
        mSize = 0;
    }

構造函數。默認容量為10。

    /**
     * Adds a mapping from the specified key to the specified value,
     * replacing the previous mapping from the specified key if there
     * was one.
     */
    public void put(int key, E value) {
        int i = ContainerHelpers.binarySearch(mKeys, mSize, key);

        if (i >= 0) {
            mValues[i] = value;
        } else {
            i = ~i;

            if (i < mSize && mValues[i] == DELETED) {
                mKeys[i] = key;
                mValues[i] = value;
                return;
            }

            if (mGarbage && mSize >= mKeys.length) {
                gc();

                // Search again because indices may have changed.
                i = ~ContainerHelpers.binarySearch(mKeys, mSize, key);
            }

            mKeys = GrowingArrayUtils.insert(mKeys, mSize, i, key);
            mValues = GrowingArrayUtils.insert(mValues, mSize, i, value);
            mSize++;
        }
    }

上面是增。順便附上ContainerHelpers類的binarySearch方法的代碼：

    // This is Arrays.binarySearch(), but doesn't do any argument validation.
    static int binarySearch(int[] array, int size, int value) {
        int lo = 0;
        int hi = size - 1;

        while (lo <= hi) {
            final int mid = (lo + hi) >>> 1;
            final int midVal = array[mid];

            if (midVal < value) {
                lo = mid + 1;
            } else if (midVal > value) {
                hi = mid - 1;
            } else {
                return mid;  // value found
            }
        }
        return ~lo;  // value not present
    }

這是相當標準的二分搜索實現。final int mid = (lo + hi) >>> 1，這里用無符號右移符>>>避免了溢出。當查找不到指定值時，返回的是lo的按位取反值，為負數。因此put方法中可以用返回值是否大于等于0來判斷是否查找成功。
回頭看put方法。若二分搜索成功則直接替換對應值，否則將i按位取反得到非負數，然后看對應值是否已刪除，是則替換對應鍵值，否則先需要gc則gc，再將新鍵值插入數組。GrowingArrayUtils的insert方法中，若數組已滿，則擴容為2倍。

    /**
     * Puts a key/value pair into the array, optimizing for the case where
     * the key is greater than all existing keys in the array.
     */
    public void append(int key, E value) {
        if (mSize != 0 && key <= mKeys[mSize - 1]) {
            put(key, value);
            return;
        }

        if (mGarbage && mSize >= mKeys.length) {
            gc();
        }

        mKeys = GrowingArrayUtils.append(mKeys, mSize, key);
        mValues = GrowingArrayUtils.append(mValues, mSize, value);
        mSize++;
    }

增的另一個方法。優化新鍵比現有鍵都大的情況，免去二分搜索，直接插入到末位。

    /**
     * Alias for {@link #delete(int)}.
     */
    public void remove(int key) {
        delete(key);
    }

    /**
     * Removes the mapping from the specified key, if there was any.
     */
    public void delete(int key) {
        int i = ContainerHelpers.binarySearch(mKeys, mSize, key);

        if (i >= 0) {
            if (mValues[i] != DELETED) {
                mValues[i] = DELETED;
                mGarbage = true;
            }
        }
    }

刪。（同樣的功能硬是給兩個方法入口總覺得有點尷尬。）二分搜索，若查找成功且對應值未被刪除則將該值標為已刪除并記錄待gc。這里的刪除并不是馬上移動數組元素，而是將刪除對象標記，待插入相同鍵時重用或者在之后需要gc時才調整數組。這是考慮了性能優化的方法。

    /**
     * Gets the Object mapped from the specified key, or <code>null</code>
     * if no such mapping has been made.
     */
    public E get(int key) {
        return get(key, null);
    }

    /**
     * Gets the Object mapped from the specified key, or the specified Object
     * if no such mapping has been made.
     */
    @SuppressWarnings("unchecked")
    public E get(int key, E valueIfKeyNotFound) {
        int i = ContainerHelpers.binarySearch(mKeys, mSize, key);

        if (i < 0 || mValues[i] == DELETED) {
            return valueIfKeyNotFound;
        } else {
            return (E) mValues[i];
        }
    }

查。使用二分搜索。

     private void gc() {
        // Log.e("SparseArray", "gc start with " + mSize);

        int n = mSize;
        int o = 0;
        int[] keys = mKeys;
        Object[] values = mValues;

        for (int i = 0; i < n; i++) {
            Object val = values[i];

            if (val != DELETED) {
                if (i != o) {
                    keys[o] = keys[i];
                    values[o] = val;
                    values[i] = null;
                }

                o++;
            }
        }

        mGarbage = false;
        mSize = o;

        // Log.e("SparseArray", "gc end with " + mSize);
    }

gc方法。遍歷值數組，將標記為刪除的對象置空并前移鍵。

    /**
     * Returns the number of key-value mappings that this SparseArray
     * currently stores.
     */
    public int size() {
        if (mGarbage) {
            gc();
        }

        return mSize;
    }

gc前的mSize不是準確的存儲大小，所以獲取size需要先進行gc。