Android ArrayMap源码解析

数据结构：

    int[] mHashes;Object[] mArray;

为两个数组实现，mHashes 负责记录Key的hash,key的hash所在的位置index，在mArray对应的位置 index2 为Key index2 + 1 为value
mHashes 里面的元素是经过排序的支持二分查找

增删改查

Put操作：

    public V put(K key, V value) {final int osize = mSize;final int hash;int index;if (key == null) {hash = 0;index = indexOfNull();} else {hash = mIdentityHashCode ? System.identityHashCode(key) : key.hashCode();index = indexOf(key, hash);}//已经存在相同的keyif (index >= 0) {index = (index<<1) + 1;final V old = (V)mArray[index];mArray[index] = value;return old;}//不存在key ~index 就会返回key应该所在的位置index = ~index;if (osize >= mHashes.length) {final int n = osize >= (BASE_SIZE*2) ? (osize+(osize>>1)): (osize >= BASE_SIZE ? (BASE_SIZE*2) : BASE_SIZE);final int[] ohashes = mHashes;final Object[] oarray = mArray;allocArrays(n);if (mHashes.length > 0) {if (DEBUG) Log.d(TAG, "put: copy 0-" + osize + " to 0");System.arraycopy(ohashes, 0, mHashes, 0, ohashes.length);System.arraycopy(oarray, 0, mArray, 0, oarray.length);}freeArrays(ohashes, oarray, osize);}if (index < osize) {if (DEBUG) Log.d(TAG, "put: move " + index + "-" + (osize-index)+ " to " + (index+1));System.arraycopy(mHashes, index, mHashes, index + 1, osize - index);System.arraycopy(mArray, index << 1, mArray, (index + 1) << 1, (mSize - index) << 1);}mHashes[index] = hash;mArray[index<<1] = key;mArray[(index<<1)+1] = value;mSize++;return null;}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;}//二分查找Key的位置int index = binarySearchHashes(mHashes, N, hash);//<0  表示不存在  同时~index 会表示Key应该所在的位置// If the hash code wasn't found, then we have no entry for this key.if (index < 0) {return index;}//hasCode相同的情况呢？ 会对key进行比较equals 直到找到对应的Key// 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;}

Key对应的Hash 放入数组的时候，会进行排序
这样对于hasCode 就可以进行二分查找

get比较简单直接返回mArray的value值即可

    public V get(Object key) {final int index = indexOfKey(key);return index >= 0 ? (V)mArray[(index<<1)+1] : null;}

删除
正常删除，当数组满足回收条件的时候回收数组下一次使用

    public V remove(Object key) {final int index = indexOfKey(key);if (index >= 0) {return removeAt(index);}return null;}public V removeAt(int index) {final Object old = mArray[(index << 1) + 1];final int osize = mSize;final int nsize;if (osize <= 1) {// Now empty.if (DEBUG) Log.d(TAG, "remove: shrink from " + mHashes.length + " to 0");final int[] ohashes = mHashes;final Object[] oarray = mArray;mHashes = EmptyArray.INT;mArray = EmptyArray.OBJECT;freeArrays(ohashes, oarray, osize);nsize = 0;} else {nsize = osize - 1;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.final int n = osize > (BASE_SIZE*2) ? (osize + (osize>>1)) : (BASE_SIZE*2);if (DEBUG) Log.d(TAG, "remove: shrink from " + mHashes.length + " to " + n);final int[] ohashes = mHashes;final Object[] oarray = mArray;allocArrays(n);if (index > 0) {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 < nsize) {if (DEBUG) Log.d(TAG, "remove: copy from " + (index+1) + "-" + nsize+ " to " + index);System.arraycopy(ohashes, index + 1, mHashes, index, nsize - index);System.arraycopy(oarray, (index + 1) << 1, mArray, index << 1,(nsize - index) << 1);}} else {if (index < nsize) {if (DEBUG) Log.d(TAG, "remove: move " + (index+1) + "-" + nsize+ " to " + index);System.arraycopy(mHashes, index + 1, mHashes, index, nsize - index);System.arraycopy(mArray, (index + 1) << 1, mArray, index << 1,(nsize - index) << 1);}mArray[nsize << 1] = null;mArray[(nsize << 1) + 1] = null;}}if (CONCURRENT_MODIFICATION_EXCEPTIONS && osize != mSize) {throw new ConcurrentModificationException();}mSize = nsize;return (V)old;}

为什么省内存？

1.初始化内存较小
ArrayMap 默认为4 BASE_SIZE = 4
HashMap默认为16 static final int DEFAULT_INITIAL_CAPACITY = 1 << 4
2.增长策略不同
HashMap每次都是翻倍
newCap = oldCap << 1
ArrayMap 为：4 -> 8 -> 1.5倍

            final int n = osize >= (BASE_SIZE*2) ? (osize+(osize>>1)): (osize >= BASE_SIZE ? (BASE_SIZE*2) : BASE_SIZE);

3.ArrayMap 不需要创建Entry对象因为全是数组保存的Key Value
4.ArrayMap使用了缓存池
当创建的数组不用的时候会进行回收

 //长度为4的数组静态缓存池static Object[] mBaseCache;static int mBaseCacheSize;//长度为8的数组静态缓存池static Object[] mTwiceBaseCache;static int mTwiceBaseCacheSize;

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Android ArrayMap源码解析

数据结构：

增删改查

为什么省内存？

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