public class HashMap<K,V> extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable {
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
static final int MAXIMUM_CAPACITY = 1 << 30;
static final float DEFAULT_LOAD_FACTOR = 0.75f;
transient Node<K,V>[] table;
transient int size;
transient int modCount;
static class Node<K,V> implements Map.Entry<K,V> {
final int hash;
final K key;
V value;
Node<K,V> next;
Node(int hash, K key, V value, Node<K,V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
... ...
}
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1)
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
if (e != null) {
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
public V get(Object key) {
Node<K,V> e;
return (e = getNode(hash(key), key)) == null ? null : e.value;
}
final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
if (first.hash == hash &&
((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null) {
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
... ...
... ...
}
- Node《K,V》 table就是Node數組(Node是HashMap的內部類,實現自Map.Entry《K,V》;成員屬性有key,value分別存鍵和值;Node類型的next成員,在產生衝突時形成鏈表存入一個指定槽中),向HashMap裏添加的一對新的鍵值對,就是向這個table即添加一個新的Node元素。
- 構建以及插入的步驟:計算得到鍵值對中鍵的hash碼(HashMap中的hash()方法),再經過計算(
i = (n - 1) & hash
)得到這個新的鍵值對作爲table數組中的下標,如果這個位置bucket(一般稱數組中的每個位置爲槽)已經有了元素,說明鍵值對中鍵的hash碼和新插入鍵值對中鍵的hash碼,這時判斷這兩個鍵是否相同(具體取決於鍵的類型K的equals方法和 == ),相同則新鍵值對替換舊的鍵值對並返回舊的鍵值對中的value。不同則,使用鏈表的形式,將新的Node追加到原有Node後面。最後兩種情況都要modcount修改次數加1.
- get的步驟:首先是獲取你想要的Node,有了Node返會
Node.value
直接就是你想要的值了。
getNode的步驟是:由hashCode經過計算得到在數組中的位置index,取出這個index對應槽中存的Node節點,如果有衝突的話該Node節點後面還有接Node節點形成鏈表,如果沒有衝突的話,就只有這個Node節點。從第一個節點向後遍歷,判斷這一個個節點哪個的key和用戶傳進來的key相等(取決於key類型的equals方法和 ==),只有節點的hashCode和用戶傳來key的hashCode相等並且兩者key值相等,纔是真正要返回的節點。