一步一步解析java集合框架HashMap源码（3）

public static void main(String[] args) {
Map<String,String> map=new HashMap<String,String>();
map.put("firstKey","firstValue");
map.put("secondKey","secondValue");
map.put("fir","thirdValue");
map.put("first","fourthValue");

System.out.println("firstKey:"+map.get("firstKey"));
}

/**
* 通过键获取值
*/
public V get(Object key) {
Node<K,V> e;
return (e = getNode(hash(key), key)) == null ? null : e.value;
}

/**
* 获取键对应节点
* @param hash hash for key
* @param key 键
* @return the node, or null if none
*/
final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
//(n - 1) & hash直接定位索引，再散列之后依然可以定位
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
if (first.hash == hash && // 首先检查第一个节点是否是查询值
((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);
//循环遍历查询
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}

public static void main(String[] args) {
Map<String,String> map=new HashMap<String,String>();
map.put("firstKey","firstValue");
map.put("secondKey","secondValue");
map.put("fir","thirdValue");
map.put("first","fourthValue");
map.remove("firstKey");
System.out.println("firstKey:"+map.get("firstKey"));
}

/**
* 返回删除节点值
* 不存在返回null
*/
public V remove(Object key) {
Node<K,V> e;
return (e = removeNode(hash(key), key, null, false, true)) == null ?
null : e.value;
}

/**
*返回删除的节点
*
* @param hash hash for key
* @param key 键
* @param value 值
* @param matchValue if true only remove if value is equal
* @param movable if false do not move other nodes while removing
* @return the node, or null if none
*/
final Node<K,V> removeNode(int hash, Object key, Object value,
boolean matchValue, boolean movable) {
Node<K,V>[] tab; Node<K,V> p; int n, index;
if ((tab = table) != null && (n = tab.length) > 0 &&
(p = tab[index = (n - 1) & hash]) != null) {
Node<K,V> node = null, e; K k; V v;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
node = p;
else if ((e = p.next) != null) {
if (p instanceof TreeNode)
node = ((TreeNode<K,V>)p).getTreeNode(hash, key);
else {
do {
if (e.hash == hash &&
((k = e.key) == key ||
(key != null && key.equals(k)))) {
node = e;//查找到节点
break;
}
p = e;//提取到上一节点，以便后续指定链接
} while ((e = e.next) != null);
}
}
//又重新判断了一遍
if (node != null && (!matchValue || (v = node.value) == value ||
(value != null && value.equals(v)))) {
if (node instanceof TreeNode)
((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
else if (node == p)
tab[index] = node.next;
else
p.next = node.next;//重新指定链接
++modCount;
--size;
afterNodeRemoval(node);
return node;
}
}
return null;
}