HashMap要关注的点:
- 内部数据结构
- Hash算法
- 冲突解决方法
- 扩容策略
/**
* The table, resized as necessary. Length MUST Always be a power of two.
*/
transient HashMapEntry<K,V>[] table = (HashMapEntry<K,V>[]) EMPTY_TABLE;
static class HashMapEntry<K,V> implements Map.Entry<K,V> {
final K key;
V value;
HashMapEntry<K,V> next;
int hash;
/**
* Creates new entry.
*/
HashMapEntry(int h, K k, V v, HashMapEntry<K,V> n) {
value = v;
next = n;
key = k;
hash = h;
}
public final K getKey() {
return key;
}
public final V getValue() {
return value;
}
public final V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}
public final boolean equals(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry e = (Map.Entry)o;
Object k1 = getKey();
Object k2 = e.getKey();
if (k1 == k2 || (k1 != null && k1.equals(k2))) {
Object v1 = getValue();
Object v2 = e.getValue();
if (v1 == v2 || (v1 != null && v1.equals(v2)))
return true;
}
return false;
}
public final int hashCode() {
return Objects.hashCode(getKey()) ^ Objects.hashCode(getValue());
}
public final String toString() {
return getKey() + "=" + getValue();
}
/**
* This method is invoked whenever the value in an entry is
* overwritten by an invocation of put(k,v) for a key k that's already
* in the HashMap.
*/
void recordAccess(HashMap<K,V> m) {
}
/**
* This method is invoked whenever the entry is
* removed from the table.
*/
void recordRemoval(HashMap<K,V> m) {
}
}
table是一个HashMapEntry的数组,HashMapEntry实际为一个链表,每个节点包括key,value和hash。
从这里可以看出Android里面HashMap的实现实际是使用了数组+链表法。
/**
* Returns the entry associated with the specified key in the
* HashMap. Returns null if the HashMap contains no mapping
* for the key.
*/
final Entry<K,V> getEntry(Object key) {
if (size == 0) {
return null;
}
int hash = (key == null) ? 0 : sun.misc.Hashing.singleWordWangJenkinsHash(key);
for (HashMapEntry<K,V> e = table[indexFor(hash, table.length)];
e != null;
e = e.next) {
Object k;
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
}
return null;
}
以上是getEntry的代码,从代码可以看出Android HashMap中使用的hash算法是sun.misc.Hashing.singleWordWangJenkinsHash算法。 Wang/JenkinsHash算法具体内容这里我们不深究。具体可以百度。
public V put(K key, V value) {
if (table == EMPTY_TABLE) {
inflateTable(threshold);
}
if (key == null)
return putForNullKey(value);
int hash = sun.misc.Hashing.singleWordWangJenkinsHash(key);
int i = indexFor(hash, table.length);
for (HashMapEntry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
modCount++;
addEntry(hash, key, value, i);
return null;
}
void addEntry(int hash, K key, V value, int bucketIndex) {
if ((size >= threshold) && (null != table[bucketIndex])) {
resize(2 * table.length);
hash = (null != key) ? sun.misc.Hashing.singleWordWangJenkinsHash(key) : 0;
bucketIndex = indexFor(hash, table.length);
}
createEntry(hash, key, value, bucketIndex);
}
void createEntry(int hash, K key, V value, int bucketIndex) {
HashMapEntry<K,V> e = table[bucketIndex];
table[bucketIndex] = new HashMapEntry<>(hash, key, value, e);
size++;
}
冲突解决也是利用数组+链表方法解决的;
上面的put函数中,首先计算key的hash,根据hash算对应的索引, 在索引对应的链表里面进行遍历,发现有相同key的数据,进行value的替换; 如果没有,在该链表的最头部加上新节点。
void resize(int newCapacity) {
HashMapEntry[] oldTable = table;
int oldCapacity = oldTable.length;
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return;
}
HashMapEntry[] newTable = new HashMapEntry[newCapacity];
transfer(newTable);
table = newTable;
threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
}
void transfer(HashMapEntry[] newTable) {
int newCapacity = newTable.length;
for (HashMapEntry<K,V> e : table) {
while(null != e) {
HashMapEntry<K,V> next = e.next;
int i = indexFor(e.hash, newCapacity);
e.next = newTable[i];
newTable[i] = e;
e = next;
}
}
}
HashMap的默认加载因子是0.75,即达到当前容量的0.75倍时,开始扩容操作,按照double size的容量进行扩容。
扩容后,将老table的数据重新计算每个项的index然后移到新table里面,即重新rehash操作。