SparseArray 的性能
對(duì)于 HashMap<Integer, Object> 青灼，Google 推薦使用 SparseArray<Object>徒役，因?yàn)樾阅芨镁澹劣跒槭裁矗匆幌略创a就知道了钠糊。
源碼解讀
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挟秤。和普通的數(shù)組不同，SparseArrays 沒有空位抄伍，每個(gè)位置上都有元素艘刚。
* SparseArrays 避免了鍵的自動(dòng)裝箱和它的數(shù)據(jù)結(jié)構(gòu)決定了每個(gè)映射不需要依賴于額外的對(duì)象，因此 SparseArrays
* 的內(nèi)存效率比 HashMap<Integer, Object> 要好截珍。
*
* <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 使用二分查找來查找鍵攀甚，因此 SparseArrays 不適合大數(shù)據(jù)量的情況。
* 因?yàn)椴檎沂鞘褂枚植檎腋诤恚黾雍蛣h除時(shí)涉及到數(shù)組的挪動(dòng)秋度，因此 SparseArrays 通常會(huì)比 HashMap 慢。
* 數(shù)據(jù)量為上百個(gè)的時(shí)候钱床，性能差異不是很明顯荚斯，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>
*
* 為了優(yōu)化性能，在移除元素時(shí)事期，SparseArrays 做了一些優(yōu)化：移除元素時(shí)滥壕，SparseArray 不是馬上壓縮數(shù)組，
* 而是標(biāo)記為 DELETE兽泣。這樣元素就可以重復(fù)利用绎橘，或者在多次刪除元素之后通過一次 gc 操作將數(shù)組壓縮。
* gc 操作會(huì)在下列幾種情況執(zhí)行：數(shù)組需要增長撞叨、要獲取 SparseArrays 鍵值對(duì)個(gè)數(shù)金踪、獲取 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)牵敷、valueAt(int) 遍歷 SparseArrays 中的元素胡岔。
*/
public class SparseArray<E> implements Cloneable {
// 要?jiǎng)h除的元素被標(biāo)記為 DELETE
// 有元素要被刪除時(shí)，將該元素標(biāo)記為 DELETE枷餐，并設(shè)置 mGarbage 為 true
private static final Object DELETED = new Object();
private boolean mGarbage = false;
// 存儲(chǔ)鍵
private int[] mKeys;
// 存儲(chǔ)值
private Object[] mValues;
// 元素的個(gè)數(shù)
private int mSize;
/**
* Creates a new SparseArray containing no mappings.
*/
public SparseArray() {
this(10); // 默認(rèn)容量是10個(gè)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或者標(biāo)記為 DELETE靶瘸，就是沒有找到元素
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 就是找到該元素了
if (mValues[i] != DELETED) {
// 標(biāo)記為 DELETE
mValues[i] = DELETED;
// 將 gc 標(biāo)志設(shè)為 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) {
// 刪除指定位置的值
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();
}
}
參考資料
http://blog.csdn.net/u013493809/article/details/21699121
https://github.com/cheyiliu/All-in-One/wiki/SparseArray
http://www.tuicool.com/articles/Ej6FJff
