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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; }

当capacity不为0的时候调用allocArrays方法分配数组大小,在分析allocArrays源码之前,我们先来看一下freeArrays方法,

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"); } } } }

BASE_SIZE的值为4,ArrayMap对于hashes.length为4和8的两种情况会进行缓存,上面的两种情况下原理都是一样的,我们就用下面的一种情况进行分析,缓存的数量也不是无线大的,当大于等于10(CACHE_SIZE)的时候也就不再进行缓存了,缓存的原理就是让array数组的第一个位置保存之前缓存的mBaseCache,第二个位置保存当前的hashes数组,其他的全部置为空,下面我们再来看一下之前的allocArrays方法,

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]; }

如果分配的尺寸不为4或者8,就初始化,我们看到最下面两行mArray的大小是mHashes的两倍,这是因为mArray存储的是key和value两个值。如果分配的尺寸为4或者8,就判断之前对这两种情况是否进行了缓存,如果缓存过就从缓存中取,取出来的时候会把array的值置空,在上面的freeArrays方法中我们知道array的第一个位置和第二个位置保存的有值,其他的都置为空,在这里把array[0]和array[1]也置为了空,但是有一点奇葩的地方就是mHashes的值确保留了下来,无论是在freeArrays方法中还是在allocArrays方法中,都没有把他置为默认值。通过ArrayMap的源码发现,这里mHashes的值无论改不改变基本上都没有什么太大影响,因为put的时候如果存在就被替换了,但在indexOf的方法中如果存在还要在继续比较key的值,只有hash和key都一样才会返回。我们下面来看一下indexOf(Object key, int hash)这个方法,

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; }

还有clear()方法和erase()方法,这两个区别就是clear()把所有的数据清空,并释放空间,erase()清空数据但没有释放空间,并且erase()只清mArray数据,mHashes数据并没有清空,这就是上面讲到的mHashes即使没清空也不会有影响,代码比较少就不在看了。在看一下和put类似的一个方法append(K key, V value)

/** 
 * 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; }

我们看注释这个方法是隐藏的,没有开放,因为这个方法不稳定,如果调用可能就会出现问题,看上面的注释,意思是说这个方法存储数据的时候没有验证,因为在最后存储的时候,是直接存进去的,这就会有一个问题,如果之前存过相同的key和value,再调用这个方法,很可能会再次存入,就可能会有两个key和value完全一样的,我个人认为如果把上面的if (index > 0 && mHashes[index-1] > hash)改为if (index > 0 && mHashes[index-1] >= hash)应该就没问题了,因为如果有相同的就调用put方法把原来的替换,不明白他为什么要这样写,下面再看一个方法validate()

/** 
 * 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); } } } }

看上面的注释也是隐藏的,存储的时候可能会存在多个相同的key,这个方法就是用来验证的,这个方法很好理解,因为我们存储数据的时候是按照二分法查找然后存储的,如果hash值相同,那么存储的时候肯定是挨着的,在这里进行验证,对挨着相同hash值的数据进行key比较,如果key相同,则说明已经存在了,就会报异常。我们再来看最后一个方法

 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; 

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转载自www.cnblogs.com/dujiaCC/p/10940782.html