Android特有的数据结构分析

android为了减少内存的使用和装箱拆箱损耗的性能，提供一些特有的数据接口，在 android.util包下面，都是使用数据进行保存，适当的使用这些对象可以优化我们的应用

• ArrayMap
• ArraySet
• SparseArray
• SparseIntArray
• SparseBooleanArray
• SparseLongArray

ArrayMap可代替Map

//保存hash
int[] mHashes;
//保存key和value
Object[] mArray;

 public V put(K key, V value) {
        final int hash;
        int index;
        //先计算index取反的值
        if (key == null) {
            hash = 0;
            index = indexOfNull();
        } else {
            hash = mIdentityHashCode ? System.identityHashCode(key) : key.hashCode();
            index = indexOf(key, hash);
        }
        if (index >= 0) {
            index = (index<<1) + 1;
            final V old = (V)mArray[index];
            mArray[index] = value;
            return old;
        }

        index = ~index;
        if (mSize >= mHashes.length) {
            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;
            allocArrays(n);

            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);
            }

            freeArrays(ohashes, oarray, mSize);
        }

        //扩展数组
        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);
        }

        mHashes[index] = hash;
        //key value连续保存
        mArray[index<<1] = key;
        mArray[(index<<1)+1] = value;
        mSize++;
        return null;
    }

ArraySet
跟ArrayMap数据结构一样，只保存value，value唯一

int[] mHashes;
    Object[] mArray;

 public boolean add(E value) {
        final int hash;
        int index;
        if (value == null) {
            hash = 0;
            index = indexOfNull();
        } else {
            hash = mIdentityHashCode ? System.identityHashCode(value) : value.hashCode();
            index = indexOf(value, hash);
        }
        if (index >= 0) {
            return false;
        }

        index = ~index;
        if (mSize >= mHashes.length) {
            final int n = mSize >= (BASE_SIZE*2) ? (mSize+(mSize>>1))
                    : (mSize >= BASE_SIZE ? (BASE_SIZE*2) : BASE_SIZE);

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

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

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

            freeArrays(ohashes, oarray, mSize);
        }

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

        mHashes[index] = hash;
        mArray[index] = value;
        mSize++;
        return true;
    }

SparseIntArray
key-value映射类型，使用基础类型int，原来替换Map<>Map

private int[] mKeys;
private int[] mValues;

    public void put(int key, int value) {
        int i = ContainerHelpers.binarySearch(mKeys, mSize, key);

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

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

    //GrowingArrayUtils.java
     public static <T> T[] insert(T[] array, int currentSize, int index, T element) {
        assert currentSize <= array.length;

        //数组长度足够，插入数据
        if (currentSize + 1 <= array.length) {
            System.arraycopy(array, index, array, index + 1, currentSize - index);
            array[index] = element;
            return array;
        }

        //数字长度不足，一般扩展2倍空间
        @SuppressWarnings("unchecked")
        T[] newArray = ArrayUtils.newUnpaddedArray((Class<T>)array.getClass().getComponentType(),
                growSize(currentSize));
        //复制index前面部分数据
        System.arraycopy(array, 0, newArray, 0, index);
        newArray[index] = element;
         //复制index后面部分数据
        System.arraycopy(array, index, newArray, index + 1, array.length - index);
        return newArray;
    }

        public static int growSize(int currentSize) {
        return currentSize <= 4 ? 8 : currentSize * 2;
    }

SparseLongArray 可代替Map

private int[] mKeys;
private long[] mValues;

SparseBooleanArray 可代替Map

 private int[] mKeys;
private boolean[] mValues;

SparseArray Map

private int[] mKeys;
    private Object[] mValues;