我正在阅读 JDK 11 中的 LinkedHashMap 源代码,我发现了一段死代码(我不确定)
众所周知,LinkedHashMap 使用双向链表来保存所有元素的顺序。它有一个成员叫accessOrder
final boolean accessOrder;
默认情况下它是假的,但如果它被设置为真,每次运行
get
,它将把它得到的元素移动到链表的末尾。这就是函数afterNodeAccess
做。//if accessOrder were set as true, after you visit node e, if e is not the end node of the linked list,
//it will move the node to the end of the linkedlist.
void afterNodeAccess(Node<K, V> e) {
LinkedHashMap.Entry<K, V> last;
if(accessOrder && (last = tail) != e) {
//if enter `if` ,it indicates that e is not the end of the linked list, because (last=tail!=e)
//then `a` as the after node of p(p is e after casting to LinkedHashMap.Entry) is never gonna be null. Only if p is last node of the linked list then a will be null.
LinkedHashMap.Entry<K, V> p = (LinkedHashMap.Entry<K, V>) e, b = p.before, a = p.after;
p.after = null;
if(b == null) {
head = a;
} else {
b.after = a;
}
// Is the if else clasue redundant? `a` must not be null.. the else clase will never be excuted.
if(a != null) {
a.before = b;
} else {
last = b;
}
if(last == null) {
head = p;
} else {
p.before = last;
last.after = p;
}
tail = p;
++modCount;
}
}
所以我的问题来了:
(accessOrder && (last = tail) != e
意味着 e 不是链表的最后一个节点。如果 e 已经是最后一个节点,我们就不用做任何事情了吧?然后
a
作为 p 的后节点,(p 转换为 LinkedHashMap.Entry 后为 e),它不能为空。仅当 p
是最后一个节点然后 a
可以为空。那么下面这段代码有什么意义呢?
// Is the if else clasue redundant? `a` must not be null.. the else clase will never be excuted.
if(a != null) {
a.before = b;
} else {
last = b;
}
a
总是 != null
, else 子句 last = b
永远不会被执行......所以它是死代码吗?我也用
accessorder
做了一个实验设为 true
,然后我 get
Debug模式下的最后一个节点,似乎我永远无法进入上述 else caluse last = b
有什么建议?
最佳答案
OP中提供的代码是单链表的节点移除算法,将移除的节点设置为链表的尾部(重新定位到尾部):
LinkedHashMap.Entry<K, V> current = (LinkedHashMap.Entry<K, V>) e
LinkedHashMap.Entry<K, V> pred = current.before, succ = current.after;
current.after = null;
// position the successor of the removed node correctly
// (either as the head of the list or as the successor of the node BEFORE the removed node)
if(pred == null) {
head = succ;
} else {
pred.after = succ ;
}
// position the predecessor of the removed node correctly
// (either as the tail of the list or as the predecessor of the node AFTER the removed node)
if(succ != null) {
succ.before = pred;
} else { // unreachable for non tail nodes
last = pred;
}
// final step - if the predecessor of the removed node was null then the head
// of the list is the removed node (the list contains a single node).
// otherwise update the removed node as the tail of the list -
// its predecessor will be the previous tail of the list
if(last == null) { // unreachable for non tail nodes
head = current;
} else {
current.before = last;
last.after = current;
}
tail = current;
该算法处理给定节点的所有可能情况,该节点应重新定位为链接案例的尾部。
在
afterNodeAccess
的上下文中方法,由于(last = tail) != e
,重新定位的节点永远不会在列表的尾部,因此在一般情况下算法中会有一些冗余。 .因此,更有效的算法是: current.after = null;
// position the successor of the removed node correctly
// (either as the head of the list or as the successor of the node BEFORE the removed node)
if(pred == null) {
head = succ;
} else {
pred.after = succ ;
}
// position the predecessor of the removed node correctly
// (as the predecessor of the node AFTER the removed node)
// update the removed node as the tail of the list -
// its predecessor will be the previous tail of the list
succ.before = pred;
current.before = last;
last.after = current;
tail = current;
如 holger在评论中提到 - 这是一个经典的“复制粘贴”解决方案,恕我直言,它表明在某些情况下重用代码似乎效率低下且不清楚。
正如 Johannes Kuhn 所建议的,您可以考虑向 OpenJDK 社区提交无法访问代码的修复程序。请参阅有关如何完成此操作的引用资料。
引用:
关于java - JDK11中LinkedHashMap中的死代码吗?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/61634520/