SparseArray

• 2013 年 08 月 01 日
• android

最近在重构one的项目，其中用HashMap来缓存ActivityGroup加载过的View，Eclipse给出了一个警告，之前考虑项目进度没怎么在意，这次仔细看了下提示，如下：

Use new SparseArray<View> (...) instead for better performance

意思就是说用SparseArray来替代，以获取更好的性能。对SparseArray根本不熟悉，甚至都没听过，第一感觉应该是Android提供的类，于是F3进入SparseArray的源码，果不其然，确实是Android提供的一个工具类,部分源码如下：

* Copyright (C) 2006 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * *      http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */package android.util;import com.android.internal.util.ArrayUtils;/** * SparseArrays map integers to Objects.  Unlike a normal array of Objects, * there can be gaps in the indices.  It is intended to be more efficient * than using a HashMap to map Integers to Objects. */public class SparseArray<E> implements Cloneable {    private static final Object DELETED = new Object();    private boolean mGarbage = false;    private int[] mKeys;    private Object[] mValues;    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.     */    public SparseArray(int initialCapacity) {        initialCapacity = ArrayUtils.idealIntArraySize(initialCapacity);        mKeys = new int[initialCapacity];        mValues = new Object[initialCapacity];        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) {        int i = binarySearch(mKeys, 0, mSize, key);        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 = binarySearch(mKeys, 0, mSize, key);        if (i >= 0) {            if (mValues[i] != DELETED) {                mValues[i] = DELETED;                mGarbage = true;            }        }    }    /**     * Alias for {@link #delete(int)}.     */    public void remove(int key) {        delete(key);    }    /**     * Removes the mapping at the specified index.     */    public void removeAt(int index) {        if (mValues[index] != DELETED) {            mValues[index] = DELETED;            mGarbage = true;        }    }    ...

mSize++;单纯从字面上来理解，SparseArray指的是稀疏数组(Sparse array)，所谓稀疏数组就是数组中大部分的内容值都未被使用（或都为零），在数组中仅有少部分的空间使用。因此造成内存空间的浪费，为了节省内存空间，并且不影响数组中原有的内容值，我们可以采用一种压缩的方式来表示稀疏数组的内容。

继续阅读SparseArray的源码，从构造方法我们可以看出，它和一般的List一样，可以预先设置容器大小，默认的大小是10：

public SparseArray() {    this(10);}public SparseArray(int initialCapacity) {    initialCapacity = ArrayUtils.idealIntArraySize(initialCapacity);    mKeys = new int[initialCapacity];    mValues = new Object[initialCapacity];    mSize = 0;}

再来看看它对数据的“增删改查”。

public void put(int key, E value) {}public void append(int key, E value){}

修改数据起初以为只有setValueAt(int index, E value)可以修改数据，但后来发现put(int key, E value)也可以修改数据，我们查看put(int key, E value)的源码可知，在put数据之前，会先查找要put的数据是否已经存在，如果存在就是修改，不存在就添加。

public void put(int key, E value) {    int i = binarySearch(mKeys, 0, 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 = ~binarySearch(mKeys, 0, mSize, key);        }        if (mSize >= mKeys.length) {            int n = ArrayUtils.idealIntArraySize(mSize + 1);            int[] nkeys = new int[n];            Object[] nvalues = new Object[n];            // Log.e("SparseArray", "grow " + mKeys.length + " to " + n);            System.arraycopy(mKeys, 0, nkeys, 0, mKeys.length);            System.arraycopy(mValues, 0, nvalues, 0, mValues.length);            mKeys = nkeys;            mValues = nvalues;        }        if (mSize - i != 0) {            // Log.e("SparseArray", "move " + (mSize - i));            System.arraycopy(mKeys, i, mKeys, i + 1, mSize - i);            System.arraycopy(mValues, i, mValues, i + 1, mSize - i);        }        mKeys[i] = key;        mValues[i] = value;        }}

所以，修改数据实际也有两种方法：

public void put(int key, E value)public void setValueAt(int index, E value)

最后再来看看如何查找数据。有两个方法可以查询取值：

public E get(int key)public E get(int key, E valueIfKeyNotFound)

其中get(int key)也只是调用了 get(int key,E valueIfKeyNotFound),最后一个从传参的变量名就能看出，传入的是找不到的时候返回的值.get(int key)当找不到的时候，默认返回null。

查看第几个位置的键：

public int keyAt(int index)

有一点需要注意的是，查看键所在位置，由于是采用二分法查找键的位置，所以找不到时返回小于0的数值，而不是返回-1。返回的负值是表示它在找不到时所在的位置。

查看第几个位置的值：

public E valueAt(int index)

查看值所在位置，没有的话返回-1：

public int indexOfValue(E value)

最后，发现其核心就是折半查找函数（binarySearch），算法设计的很不错。

private static int binarySearch(int[] a, int start, int len, int key) {    int high = start + len, low = start - 1, guess;    while (high - low &gt; 1) {        guess = (high + low) / 2;        if (a[guess] &lt; key)            low = guess;        else            high = guess;    }    if (high == start + len)        return ~(start + len);    else if (a[high] == key)        return high;    else        return ~high;}相应的也有SparseBooleanArray，用来取代HashMap<Integer, Boolean>，SparseIntArray用来取代HashMap<Integer, Integer>，大家有兴趣的可以研究。

总结

SparseArray是android里为<Interger,Object>这样的Hashmap而专门写的类,目的是提高效率，其核心是折半查找函数（binarySearch）。在Android中，当我们需要定义    HashMap<Integer, E> hashMap = new HashMap<Integer, E>();时，我们可以使用如下的方式来取得更好的性能.    SparseArray<E> sparseArray = new SparseArray<E>();

最后，看看重构后的ActivityGroupManager：

public class ActivityGroupManager {    static final String TAG = ActivityGroupManager.class.getName();    private SparseArray<View> array;    private ViewGroup container;    public ActivityGroupManager() {    array = new SparseArray<View>();    }    public void setContainer(ViewGroup container) {    this.container = container;    }    public void showContainer(int num, View view) {    if (array.get(num) == null) {    array.put(num, view);    container.addView(view);    }    for (int i = 0; i < array.size(); i++) {    View v = array.valueAt(i);    v.setVisibility(View.GONE);    }    view.setVisibility(View.VISIBLE);    }}




• 2013 年 08 月 01 日
• android

最近在重构one的项目，其中用HashMap来缓存ActivityGroup加载过的View，Eclipse给出了一个警告，之前考虑项目进度没怎么在意，这次仔细看了下提示，如下：

Use new SparseArray<View> (...) instead for better performance

意思就是说用SparseArray来替代，以获取更好的性能。对SparseArray根本不熟悉，甚至都没听过，第一感觉应该是Android提供的类，于是F3进入SparseArray的源码，果不其然，确实是Android提供的一个工具类,部分源码如下：

* Copyright (C) 2006 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * *      http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */package android.util;import com.android.internal.util.ArrayUtils;/** * SparseArrays map integers to Objects.  Unlike a normal array of Objects, * there can be gaps in the indices.  It is intended to be more efficient * than using a HashMap to map Integers to Objects. */public class SparseArray<E> implements Cloneable {    private static final Object DELETED = new Object();    private boolean mGarbage = false;    private int[] mKeys;    private Object[] mValues;    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.     */    public SparseArray(int initialCapacity) {        initialCapacity = ArrayUtils.idealIntArraySize(initialCapacity);        mKeys = new int[initialCapacity];        mValues = new Object[initialCapacity];        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) {        int i = binarySearch(mKeys, 0, mSize, key);        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 = binarySearch(mKeys, 0, mSize, key);        if (i >= 0) {            if (mValues[i] != DELETED) {                mValues[i] = DELETED;                mGarbage = true;            }        }    }    /**     * Alias for {@link #delete(int)}.     */    public void remove(int key) {        delete(key);    }    /**     * Removes the mapping at the specified index.     */    public void removeAt(int index) {        if (mValues[index] != DELETED) {            mValues[index] = DELETED;            mGarbage = true;        }    }    ...

mSize++;单纯从字面上来理解，SparseArray指的是稀疏数组(Sparse array)，所谓稀疏数组就是数组中大部分的内容值都未被使用（或都为零），在数组中仅有少部分的空间使用。因此造成内存空间的浪费，为了节省内存空间，并且不影响数组中原有的内容值，我们可以采用一种压缩的方式来表示稀疏数组的内容。

继续阅读SparseArray的源码，从构造方法我们可以看出，它和一般的List一样，可以预先设置容器大小，默认的大小是10：

public SparseArray() {    this(10);}public SparseArray(int initialCapacity) {    initialCapacity = ArrayUtils.idealIntArraySize(initialCapacity);    mKeys = new int[initialCapacity];    mValues = new Object[initialCapacity];    mSize = 0;}

再来看看它对数据的“增删改查”。

public void put(int key, E value) {}public void append(int key, E value){}

修改数据起初以为只有setValueAt(int index, E value)可以修改数据，但后来发现put(int key, E value)也可以修改数据，我们查看put(int key, E value)的源码可知，在put数据之前，会先查找要put的数据是否已经存在，如果存在就是修改，不存在就添加。

public void put(int key, E value) {    int i = binarySearch(mKeys, 0, 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 = ~binarySearch(mKeys, 0, mSize, key);        }        if (mSize >= mKeys.length) {            int n = ArrayUtils.idealIntArraySize(mSize + 1);            int[] nkeys = new int[n];            Object[] nvalues = new Object[n];            // Log.e("SparseArray", "grow " + mKeys.length + " to " + n);            System.arraycopy(mKeys, 0, nkeys, 0, mKeys.length);            System.arraycopy(mValues, 0, nvalues, 0, mValues.length);            mKeys = nkeys;            mValues = nvalues;        }        if (mSize - i != 0) {            // Log.e("SparseArray", "move " + (mSize - i));            System.arraycopy(mKeys, i, mKeys, i + 1, mSize - i);            System.arraycopy(mValues, i, mValues, i + 1, mSize - i);        }        mKeys[i] = key;        mValues[i] = value;        }}

所以，修改数据实际也有两种方法：

public void put(int key, E value)public void setValueAt(int index, E value)

最后再来看看如何查找数据。有两个方法可以查询取值：

public E get(int key)public E get(int key, E valueIfKeyNotFound)

其中get(int key)也只是调用了 get(int key,E valueIfKeyNotFound),最后一个从传参的变量名就能看出，传入的是找不到的时候返回的值.get(int key)当找不到的时候，默认返回null。

查看第几个位置的键：

public int keyAt(int index)

有一点需要注意的是，查看键所在位置，由于是采用二分法查找键的位置，所以找不到时返回小于0的数值，而不是返回-1。返回的负值是表示它在找不到时所在的位置。

查看第几个位置的值：

public E valueAt(int index)

查看值所在位置，没有的话返回-1：

public int indexOfValue(E value)

最后，发现其核心就是折半查找函数（binarySearch），算法设计的很不错。

private static int binarySearch(int[] a, int start, int len, int key) {    int high = start + len, low = start - 1, guess;    while (high - low &gt; 1) {        guess = (high + low) / 2;        if (a[guess] &lt; key)            low = guess;        else            high = guess;    }    if (high == start + len)        return ~(start + len);    else if (a[high] == key)        return high;    else        return ~high;}相应的也有SparseBooleanArray，用来取代HashMap<Integer, Boolean>，SparseIntArray用来取代HashMap<Integer, Integer>，大家有兴趣的可以研究。

总结

SparseArray是android里为<Interger,Object>这样的Hashmap而专门写的类,目的是提高效率，其核心是折半查找函数（binarySearch）。在Android中，当我们需要定义    HashMap<Integer, E> hashMap = new HashMap<Integer, E>();时，我们可以使用如下的方式来取得更好的性能.    SparseArray<E> sparseArray = new SparseArray<E>();

最后，看看重构后的ActivityGroupManager：

public class ActivityGroupManager {    static final String TAG = ActivityGroupManager.class.getName();    private SparseArray<View> array;    private ViewGroup container;    public ActivityGroupManager() {    array = new SparseArray<View>();    }    public void setContainer(ViewGroup container) {    this.container = container;    }    public void showContainer(int num, View view) {    if (array.get(num) == null) {    array.put(num, view);    container.addView(view);    }    for (int i = 0; i < array.size(); i++) {    View v = array.valueAt(i);    v.setVisibility(View.GONE);    }    view.setVisibility(View.VISIBLE);    }}