看SparseArray源码

SparseArray 的性能

对于 HashMap<Integer, Object> ，Google 推荐使用 SparseArray<Object>，因为性能更好，至于为什么，看一下源码就知道了。

源码解读

package android.util;

import com.android.internal.util.ArrayUtils;
import com.android.internal.util.GrowingArrayUtils;

import libcore.util.EmptyArray;

/**
 * SparseArrays map integers to Objects.  Unlike a normal array of Objects,
 * there can be gaps in the indices.  It is intended to be more memory efficient
 * than using a HashMap to map Integers to Objects, both because it avoids
 * auto-boxing keys and its data structure doesn't rely on an extra entry object
 * for each mapping.
 *
 * SparseArrays 映射了 Integer 到 Object。和普通的数组不同，SparseArrays 没有空位，每个位置上都有元素。
 * SparseArrays 避免了键的自动装箱和它的数据结构决定了每个映射不需要依赖于额外的对象，因此 SparseArrays
 * 的内存效率比 HashMap<Integer, Object> 要好。
 *
 * <p>Note that this container keeps its mappings in an array data structure,
 * using a binary search to find keys.  The implementation is not intended to be appropriate for
 * data structures
 * that may contain large numbers of items.  It is generally slower than a traditional
 * HashMap, since lookups require a binary search and adds and removes require inserting
 * and deleting entries in the array.  For containers holding up to hundreds of items,
 * the performance difference is not significant, less than 50%.</p>
 *
 * SparseArrays 使用二分查找来查找键，因此 SparseArrays 不适合大数据量的情况。
 * 因为查找是使用二分查找，增加和删除时涉及到数组的挪动，因此 SparseArrays 通常会比 HashMap 慢。
 * 数据量为上百个的时候，性能差异不是很明显，少于 50%。
 *
 * <p>To help with performance, the container includes an optimization when removing
 * keys: instead of compacting its array immediately, it leaves the removed entry marked
 * as deleted.  The entry can then be re-used for the same key, or compacted later in
 * a single garbage collection step of all removed entries.  This garbage collection will
 * need to be performed at any time the array needs to be grown or the the map size or
 * entry values are retrieved.</p>
 * 
 * 为了优化性能，在移除元素时，SparseArrays 做了一些优化：移除元素时，SparseArray 不是马上压缩数组，
 * 而是标记为 DELETE。这样元素就可以重复利用，或者在多次删除元素之后通过一次 gc 操作将数组压缩。
 * gc 操作会在下列几种情况执行：数组需要增长、要获取 SparseArrays 键值对个数、获取 SparseArrays 的值。
 *
 * <p>It is possible to iterate over the items in this container using
 * {@link #keyAt(int)} and {@link #valueAt(int)}. Iterating over the keys using
 * <code>keyAt(int)</code> with ascending values of the index will return the
 * keys in ascending order, or the values corresponding to the keys in ascending
 * order in the case of <code>valueAt(int)</code>.</p>
 *
 * 可以使用 keyAt(int)、valueAt(int) 遍历 SparseArrays 中的元素。
 */
public class SparseArray<E> implements Cloneable {
    // 要删除的元素被标记为 DELETE
    // 有元素要被删除时，将该元素标记为 DELETE，并设置 mGarbage 为 true
    private static final Object DELETED = new Object();
    private boolean mGarbage = false;

    // 存储键
    private int[] mKeys;
    // 存储值
    private Object[] mValues;
    // 元素的个数
    private int mSize;

    /**
     * Creates a new SparseArray containing no mappings.
     */
    public SparseArray() {
        this(10);  // 默认容量是10个元素
    }

    /**
     * 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;  // EmptyArray.INT = new int[0]
            mValues = EmptyArray.OBJECT;  // EmptyArray.OBJECT = new Object[0]
        } else {
            mValues = ArrayUtils.newUnpaddedObjectArray(initialCapacity);
            mKeys = new int[mValues.length];
        }
        mSize = 0;
    }

    @Override
    @SuppressWarnings("unchecked")
    public SparseArray<E> clone() {
        SparseArray<E> clone = null;
        try {
            clone = (SparseArray<E>) super.clone();
            clone.mKeys = mKeys.clone();
            clone.mValues = mValues.clone();
        } catch (CloneNotSupportedException cnse) {
            /* ignore */
        }
        return clone;
    }

    /**
     * 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) {
        // 二分查找
        int i = ContainerHelpers.binarySearch(mKeys, mSize, key);

        // 小于0或者标记为 DELETE，就是没有找到元素
        if (i < 0 || mValues[i] == DELETED) {
            return valueIfKeyNotFound;
        } else {
            return (E) mValues[i];
        }
    }

    /**
     * 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) { // 大于等于 0 就是找到该元素了
            if (mValues[i] != DELETED) {
                // 标记为 DELETE
                mValues[i] = DELETED;
                // 将 gc 标志设为 true
                mGarbage = true;
            }
        }
    }

    /**
     * @hide
     * Removes the mapping from the specified key, if there was any, returning the old value.
     */
    public E removeReturnOld(int key) {
        int i = ContainerHelpers.binarySearch(mKeys, mSize, key);

        if (i >= 0) {
            if (mValues[i] != DELETED) {
                final E old = (E) mValues[i];
                mValues[i] = DELETED;
                mGarbage = true;
                return old;
            }
        }
        return null;
    }

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

    /**
     * Removes the mapping at the specified index.
     *
     * <p>For indices outside of the range <code>0...size()-1</code>,
     * the behavior is undefined.</p>
     */
    public void removeAt(int index) {
        // 删除指定位置的值
        if (mValues[index] != DELETED) {
            mValues[index] = DELETED;
            mGarbage = true;
        }
    }

    /**
     * Remove a range of mappings as a batch.
     *
     * @param index Index to begin at
     * @param size Number of mappings to remove
     *
     * <p>For indices outside of the range <code>0...size()-1</code>,
     * the behavior is undefined.</p>
     */
    public void removeAtRange(int index, int size) {
        final int end = Math.min(mSize, index + size);
        for (int i = index; i < end; i++) {
            removeAt(i);
        }
    }

    // 把元素前移
    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);
    }

    /**
     * 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++;
        }
    }

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

        return mSize;
    }

    /**
     * Given an index in the range <code>0...size()-1</code>, returns
     * the key from the <code>index</code>th key-value mapping that this
     * SparseArray stores.
     *
     * <p>The keys corresponding to indices in ascending order are guaranteed to
     * be in ascending order, e.g., <code>keyAt(0)</code> will return the
     * smallest key and <code>keyAt(size()-1)</code> will return the largest
     * key.</p>
     *
     * <p>For indices outside of the range <code>0...size()-1</code>,
     * the behavior is undefined.</p>
     */
    public int keyAt(int index) {
        if (mGarbage) {
            gc();
        }

        return mKeys[index];
    }

    /**
     * Given an index in the range <code>0...size()-1</code>, returns
     * the value from the <code>index</code>th key-value mapping that this
     * SparseArray stores.
     *
     * <p>The values corresponding to indices in ascending order are guaranteed
     * to be associated with keys in ascending order, e.g.,
     * <code>valueAt(0)</code> will return the value associated with the
     * smallest key and <code>valueAt(size()-1)</code> will return the value
     * associated with the largest key.</p>
     *
     * <p>For indices outside of the range <code>0...size()-1</code>,
     * the behavior is undefined.</p>
     */
    @SuppressWarnings("unchecked")
    public E valueAt(int index) {
        if (mGarbage) {
            gc();
        }

        return (E) mValues[index];
    }

    /**
     * Given an index in the range <code>0...size()-1</code>, sets a new
     * value for the <code>index</code>th key-value mapping that this
     * SparseArray stores.
     *
     * <p>For indices outside of the range <code>0...size()-1</code>, the behavior is undefined.</p>
     */
    public void setValueAt(int index, E value) {
        if (mGarbage) {
            gc();
        }

        mValues[index] = value;
    }

    /**
     * Returns the index for which {@link #keyAt} would return the
     * specified key, or a negative number if the specified
     * key is not mapped.
     */
    public int indexOfKey(int key) {
        if (mGarbage) {
            gc();
        }

        return ContainerHelpers.binarySearch(mKeys, mSize, key);
    }

    /**
     * Returns an index for which {@link #valueAt} would return the
     * specified key, or a negative number if no keys map to the
     * specified value.
     * <p>Beware that this is a linear search, unlike lookups by key,
     * and that multiple keys can map to the same value and this will
     * find only one of them.
     * <p>Note also that unlike most collections' {@code indexOf} methods,
     * this method compares values using {@code ==} rather than {@code equals}.
     */
    public int indexOfValue(E value) {
        if (mGarbage) {
            gc();
        }

        for (int i = 0; i < mSize; i++)
            if (mValues[i] == value)
                return i;

        return -1;
    }

    /**
     * Removes all key-value mappings from this SparseArray.
     */
    public void clear() {
        int n = mSize;
        Object[] values = mValues;

        for (int i = 0; i < n; i++) {
            values[i] = null;
        }

        mSize = 0;
        mGarbage = false;
    }

    /**
     * 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++;
    }

    /**
     * {@inheritDoc}
     *
     * <p>This implementation composes a string by iterating over its mappings. If
     * this map contains itself as a value, the string "(this Map)"
     * will appear in its place.
     */
    @Override
    public String toString() {
        if (size() <= 0) {
            return "{}";
        }

        StringBuilder buffer = new StringBuilder(mSize * 28);
        buffer.append('{');
        for (int i=0; i<mSize; i++) {
            if (i > 0) {
                buffer.append(", ");
            }
            int key = keyAt(i);
            buffer.append(key);
            buffer.append('=');
            Object value = valueAt(i);
            if (value != this) {
                buffer.append(value);
            } else {
                buffer.append("(this Map)");
            }
        }
        buffer.append('}');
        return buffer.toString();
    }
}

参考资料

http://blog.csdn.net/u013493809/article/details/21699121
https://github.com/cheyiliu/All-in-One/wiki/SparseArray
http://www.tuicool.com/articles/Ej6FJff