HashMap源码分析

hashmap 中存储的基础元素
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;
}

public final K getKey() { return key;}
public final V getValue() { return value;}
public final String toString() { return key + “=” + value;}

public final int hashCode() {
return Objects.hashCode(key) ^ Objects.hashCode(value);
}

public final V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}

public final boolean equals(Object o) {
if (o == this)
return true;
if (o instanceof Map.Entry) {
Map.Entry<?,?> e = (Map.Entry<?,?>)o;
if (Objects.equals(key, e.getKey()) &&
Objects.equals(value, e.getValue()))
return true;
}
return false;
}
}
hashmap 有一个 table 和链表或子节点组成，每个 table 子元素或链接小于 TREEIFY_THRESHOLD（8）个元素的时候，为链表，超过之后转化为树
transient Node<K,V>[] table;
transient Set<Map.Entry<K,V>> entrySet;
根据 key 值查找 value
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;
// 如果 table 不为空，根据 hash 找到元素所在的 entrySet
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n – 1) & hash]) != null) {
// 对比要查找的值是否为 entrySet，的首元素
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
// 如果不是循环向下查找
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;
}
新增节点
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;
// 如果 table 不存在，新生成 table
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
// 如果 table 不存在该 hash 子项，新建一个 table 子项，并存入
if ((p = tab[i = (n – 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
// 如果存在改 table 子项如下，如果改 enteset 中存在该 key 值如果 value 相同，指向改值，否则替换改 value，并返回旧值，如果不存在该 key 值，则直接存入新元素
else {
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
// 当该 table 后面连接的是树的时候
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
// 如果链接数不超过 TREEIFY_THRESHOLD（8）是增加节点作为链表，否则转化为红黑树
else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD – 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
if (e != null) {// existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}