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分析源码之前先来介绍一下ArrayMap的存储结构，ArrayMap数据的存储不同于HashMap和SparseArray，在上一篇《Android SparseArray源码详解》中我们讲到SparseArray是以纯数组的形式存储的，一个数组存储的是key值一个数组存储的是value值，今天我们分析的ArrayMap和SparseArray有点类似，他也是以纯数组的形式存储，不过不同的是他的一个数组存储的是Hash值另一个数组存储的是key和value，其中key和value是成对出现的，key存储在数组的偶数位上，value存储在数组的奇数位上，我们先来看其中的一个构造方法
public ArrayMap(int capacity) {
if (capacity == 0) {
mHashes = ContainerHelpers.EMPTY_INTS;
mArray = ContainerHelpers.EMPTY_OBJECTS;
} else {
allocArrays(capacity);
}
mSize = 0;
} private static void freeArrays(final int[] hashes, final Object[] array, final int size) {
if (hashes.length == (BASE_SIZE*2)) {
synchronized (ArrayMap.class) {
if (mTwiceBaseCacheSize < CACHE_SIZE) {
array[0] = mTwiceBaseCache;
array[1] = hashes;
for (int i=(size<<1)-1; i>=2; i--) {
array[i] = null;
}
mTwiceBaseCache = array;
mTwiceBaseCacheSize++;
if (DEBUG) Log.d(TAG, "Storing 2x cache " + array
+ " now have " + mTwiceBaseCacheSize + " entries");
}
}
} else if (hashes.length == BASE_SIZE) {
synchronized (ArrayMap.class) {
if (mBaseCacheSize < CACHE_SIZE) {
array[0] = mBaseCache;
array[1] = hashes;
for (int i=(size<<1)-1; i>=2; i--) {
array[i] = null;
}
mBaseCache = array;
mBaseCacheSize++;
if (DEBUG) Log.d(TAG, "Storing 1x cache " + array
+ " now have " + mBaseCacheSize + " entries");
}
}
}
} private void allocArrays(final int size) {
if (mHashes == EMPTY_IMMUTABLE_INTS) {
throw new UnsupportedOperationException("ArrayMap is immutable");
}
if (size == (BASE_SIZE*2)) {
…………………………
} else if (size == BASE_SIZE) {
synchronized (ArrayMap.class) {
if (mBaseCache != null) {
final Object[] array = mBaseCache;
mArray = array;
mBaseCache = (Object[])array[0];
mHashes = (int[])array[1];
array[0] = array[1] = null;
mBaseCacheSize--;
if (DEBUG) Log.d(TAG, "Retrieving 1x cache " + mHashes
+ " now have " + mBaseCacheSize + " entries");
return;
}
}
}
mHashes = new int[size];
mArray = new Object[size<<1];
} int indexOf(Object key, int hash) {
final int N = mSize;
// Important fast case: if nothing is in here, nothing to look for.
if (N == 0) {
return ~0;
}
int index = ContainerHelpers.binarySearch(mHashes, N, hash);
// If the hash code wasn't found, then we have no entry for this key.
if (index < 0) {
return index;
}
// If the key at the returned index matches, that's what we want.
if (key.equals(mArray[index<<1])) {
return index;
}
// Search for a matching key after the index.
int end;
for (end = index + 1; end < N && mHashes[end] == hash; end++) {
if (key.equals(mArray[end << 1])) return end;
}
// Search for a matching key before the index.
for (int i = index - 1; i >= 0 && mHashes[i] == hash; i--) {
if (key.equals(mArray[i << 1])) return i;
}
// Key not found -- return negative value indicating where a
// new entry for this key should go. We use the end of the
// hash chain to reduce the number of array entries that will
// need to be copied when inserting.
return ~end;
}这个方法很简单，就是根据二分法查找来确定hash值在数组中的位置，如果没找到就返回一个负数，注意下面还有两个循环，这是因为mHashes数组中的hash值不是唯一的，只有hash值相同并且key也相同才会返回所在的位置，否则就返回一个负数。下面就来看一下put(K key, V value)这个方法。
@Override
public V put(K key, V value) {
final int hash;
int index;
if (key == null) {
hash = 0;
index = indexOfNull();
} else {
hash = key.hashCode();
index = indexOf(key, hash);
}
//通过查找，如果找到就把原来的替换，
if (index >= 0) {
index = (index<<1) + 1;
final V old = (V)mArray[index];
mArray[index] = value;
return old;
}
//在上一篇《Android SparseArray源码详解》讲过，根据二分法查找，如果没有找到就会返回一个负数，这里进行取反
index = ~index;
//如果满了就扩容
if (mSize >= mHashes.length) {
//扩容的尺寸，三目运算符
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;
//扩容
allocArrays(n);
//如果原来有数据就把原来的数据拷贝到扩容后的数组中
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);
}
//根据上面的二分法查找，如果index小于mSize，说明新的数据是插入到数组之间index位置，插入之前需要把后面的移位
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);
}
//数据保存，mHashes只有hash值，mArray即保存key值又保存value值，
mHashes[index] = hash;
mArray[index<<1] = key;
mArray[(index<<1)+1] = value;
mSize++;
return null;
} /**
* Special fast path for appending items to the end of the array without validation.
* The array must already be large enough to contain the item.
* @hide
*/
public void append(K key, V value) {
int index = mSize;
final int hash = key == null ? 0 : key.hashCode();
if (index >= mHashes.length) {
throw new IllegalStateException("Array is full");
}
if (index > 0 && mHashes[index-1] > hash) {
RuntimeException e = new RuntimeException("here");
e.fillInStackTrace();
Log.w(TAG, "New hash " + hash
+ " is before end of array hash " + mHashes[index-1]
+ " at index " + index + " key " + key, e);
put(key, value);
return;
}
mSize = index+1;
mHashes[index] = hash;
index <<= 1;
mArray[index] = key;
mArray[index+1] = value;
} /**
* The use of the {@link #append} function can result in invalid array maps, in particular
* an array map where the same key appears multiple times. This function verifies that
* the array map is valid, throwing IllegalArgumentException if a problem is found. The
* main use for this method is validating an array map after unpacking from an IPC, to
* protect against malicious callers.
* @hide
*/
public void validate() {
final int N = mSize;
if (N <= 1) {
// There can't be dups.
return;
}
int basehash = mHashes[0];
int basei = 0;
for (int i=1; i<N; i++) {
int hash = mHashes[i];
if (hash != basehash) {
basehash = hash;
basei = i;
continue;
}
// We are in a run of entries with the same hash code. Go backwards through
// the array to see if any keys are the same.
final Object cur = mArray[i<<1];
for (int j=i-1; j>=basei; j--) {
final Object prev = mArray[j<<1];
if (cur == prev) {
throw new IllegalArgumentException("Duplicate key in ArrayMap: " + cur);
}
if (cur != null && prev != null && cur.equals(prev)) {
throw new IllegalArgumentException("Duplicate key in ArrayMap: " + cur);
}
}
}
} public V removeAt(int index) {
final Object old = mArray[(index << 1) + 1];
//如果小于等于1就全部清空
if (mSize <= 1) {
// Now empty.
if (DEBUG) Log.d(TAG, "remove: shrink from " + mHashes.length + " to 0");
freeArrays(mHashes, mArray, mSize);
mHashes = EmptyArray.INT;
mArray = EmptyArray.OBJECT;
mSize = 0;
} else {
// 如果数组比较大，但使用的比较少，就会重新分配空间
if (mHashes.length > (BASE_SIZE*2) && mSize < mHashes.length/3) {
// Shrunk enough to reduce size of arrays. We don't allow it to
// shrink smaller than (BASE_SIZE*2) to avoid flapping between
// that and BASE_SIZE.
//重新计算空间，当大于8的时候会1.5倍增长
final int n = mSize > (BASE_SIZE*2) ? (mSize + (mSize>>1)) : (BASE_SIZE*2);
if (DEBUG) Log.d(TAG, "remove: shrink from " + mHashes.length + " to " + n);
final int[] ohashes = mHashes;
final Object[] oarray = mArray;
// 重新分配空间
allocArrays(n);
mSize--;
if (index > 0) {
//如果删除的位置大于0，拷贝前半部分到新数组中
if (DEBUG) Log.d(TAG, "remove: copy from 0-" + index + " to 0");
System.arraycopy(ohashes, 0, mHashes, 0, index);
System.arraycopy(oarray, 0, mArray, 0, index << 1);
}
if (index < mSize) {
// 如果删除的位置小于mSize，把index位置以后的数据拷贝到新数组中
if (DEBUG) Log.d(TAG, "remove: copy from " + (index+1) + "-" + mSize
+ " to " + index);
System.arraycopy(ohashes, index + 1, mHashes, index, mSize - index);
System.arraycopy(oarray, (index + 1) << 1, mArray, index << 1,
(mSize - index) << 1);
}
} else {
mSize--;
if (index < mSize) {
//同上
if (DEBUG) Log.d(TAG, "remove: move " + (index+1) + "-" + mSize
+ " to " + index);
System.arraycopy(mHashes, index + 1, mHashes, index, mSize - index);
System.arraycopy(mArray, (index + 1) << 1, mArray, index << 1,
(mSize - index) << 1);
}
// 把移除的位置置空，上面的为什么没有置空，是因为上面的数据拷贝到一个新的数组中，而删除的就没有
//拷贝，这里要置空是因为这里数组没有扩容，还是在原来的数组操作，所以必须置空
mArray[mSize << 1] = null;
mArray[(mSize << 1) + 1] = null;
}
}
return (V)old;
}剩下的方法都比较简单，这里就不在一一分析。
