首先從構(gòu)造方法說起旁蔼，
初始化有兩個(gè)參數(shù)
initialCapacity 代表初始容量，實(shí)際容量為大于initialCapacity的2的冪次數(shù)
之后會初始化一個(gè)長度為initialCapacity的數(shù)組彼乌，姑且把數(shù)組中每個(gè)元素定義為一個(gè)桶铐殃。默認(rèn)桶內(nèi)個(gè)數(shù)為0 。隨著桶中個(gè)數(shù)的增加佳励，當(dāng)平均每個(gè)桶的元素個(gè)數(shù)大于loadFactor時(shí)休里，則觸發(fā)重構(gòu)蛆挫。
public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY) {
initialCapacity = MAXIMUM_CAPACITY;
} else if (initialCapacity < DEFAULT_INITIAL_CAPACITY) {
initialCapacity = DEFAULT_INITIAL_CAPACITY;
}
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
// Android-Note: We always use the default load factor of 0.75f.
// This might appear wrong but it's just awkward design. We always call
// inflateTable() when table == EMPTY_TABLE. That method will take "threshold"
// to mean "capacity" and then replace it with the real threshold (i.e, multiplied with
// the load factor).
threshold = initialCapacity;
init();
}
HashMap里存放的是一個(gè)特殊的entity，格式如下
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) {
}
}
既然是HashMap那肯定跟Hash有關(guān)份帐。每次添加元素時(shí)璃吧，根據(jù)key的hash值計(jì)算出一個(gè)hashint，之后根據(jù)hashing%桶的個(gè)數(shù)獲取一個(gè)index废境，再根據(jù)這個(gè)index吧元素放到對應(yīng)的桶里畜挨。如果桶里有舊對象則讓新對象指向舊對象。如果key為null則放在第0個(gè)桶里
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;
}
get方法也是類似根據(jù)key確定具體哪個(gè)桶里再遍歷鏈表比對key值獲取元素
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;
}
當(dāng)總對象個(gè)數(shù)超過預(yù)定閾值觸發(fā)resize 桶的個(gè)數(shù)翻倍
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);
}
