一、SparseIntArray API
SparseIntArrays map integers to integers. Unlike a normal array of integers, there can be gaps in the indices. It is intended to be more memory efficient than using a HashMap to map Integers to Integers, both because it avoids auto-boxing keys and values and its data structure doesn't rely on an extra entry object for each mapping.
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%.
It is possible to iterate over the items in this container using keyAt(int) andvalueAt(int). Iterating over the keys usingkeyAt(int) 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 ofvalueAt(int).
SparseArray是android里为<Interger,Object>这样的Hashmap而专门写的class,目的是提高效率，其核心是折半查找函数（binarySearch）
http://developer.android.com/reference/android/util/SparseIntArray.html
二、源码
http://www.oschina.net/code/explore/android-2.2-froyo/android/util/SparseIntArray.java
/**
* 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;
/***
* SparseIntArrays map integers to integers. Unlike a normal array of integers,
* there can be gaps in the indices. It is intended to be more efficient
* than using a HashMap to map Integers to Integers.
*/
public class SparseIntArray {
/***
* Creates a new SparseIntArray containing no mappings.
*/
public SparseIntArray() {
this(10);
}
/***
* Creates a new SparseIntArray containing no mappings that will not
* require any additional memory allocation to store the specified
* number of mappings.
*/
public SparseIntArray(int initialCapacity) {
initialCapacity = ArrayUtils.idealIntArraySize(initialCapacity);
mKeys = new int[initialCapacity];
mValues = new int[initialCapacity];
mSize = 0;
}
/***
* Gets the int mapped from the specified key, or <code>0</code>
* if no such mapping has been made.
*/
public int get(int key) {
return get(key, 0);
}
/***
* Gets the int mapped from the specified key, or the specified value
* if no such mapping has been made.
*/
public int get(int key, int valueIfKeyNotFound) {
int i = binarySearch(mKeys, 0, mSize, key);
if (i < 0) {
return valueIfKeyNotFound;
} else {
return 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) {
removeAt(i);
}
}
/***
* Removes the mapping at the given index.
*/
public void removeAt(int index) {
System.arraycopy(mKeys, index + 1, mKeys, index, mSize - (index + 1));
System.arraycopy(mValues, index + 1, mValues, index, mSize - (index + 1));
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, int value) {
int i = binarySearch(mKeys, 0, mSize, key);
if (i >= 0) {
mValues[i] = value;
} else {
i = ~i;
if (mSize >= mKeys.length) {
int n = ArrayUtils.idealIntArraySize(mSize + 1);
int[] nkeys = new int[n];
int[] nvalues = new int[n];
// Log.e("SparseIntArray", "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("SparseIntArray", "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;
mSize++;
}
}
/***
* Returns the number of key-value mappings that this SparseIntArray
* currently stores.
*/
public int size() {
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
* SparseIntArray stores.
*/
public int keyAt(int index) {
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
* SparseIntArray stores.
*/
public int valueAt(int index) {
return mValues[index];
}
/***
* 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) {
return binarySearch(mKeys, 0, 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.
* 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.
*/
public int indexOfValue(int value) {
for (int i = 0; i < mSize; i++)
if (mValues[i] == value)
return i;
return -1;
}
/***
* Removes all key-value mappings from this SparseIntArray.
*/
public void clear() {
mSize = 0;
}
/***
* 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, int value) {
if (mSize != 0 && key <= mKeys[mSize - 1]) {
put(key, value);
return;
}
int pos = mSize;
if (pos >= mKeys.length) {
int n = ArrayUtils.idealIntArraySize(pos + 1);
int[] nkeys = new int[n];
int[] nvalues = new int[n];
// Log.e("SparseIntArray", "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;
}
mKeys[pos] = key;
mValues[pos] = value;
mSize = pos + 1;
}
折半查找
private static int binarySearch(int[] a, int start, int len, int key) {
int high = start + len, low = start - 1, guess;
while (high - low > 1) {
guess = (high + low) / 2;
if (a[guess] < key)
low = guess;
else
high = guess;
}
if (high == start + len)
return ~(start + len);
else if (a[high] == key)
return high;
else
return ~high;
}
private void checkIntegrity() {
for (int i = 1; i < mSize; i++) {
if (mKeys[i] <= mKeys[i - 1]) {
for (int j = 0; j < mSize; j++) {
Log.e("FAIL", j + ": " + mKeys[j] + " -> " + mValues[j]);
}
throw new RuntimeException();
}
}
}
private int[] mKeys;
private int[] mValues;
private int mSize;
}
