java - What is the role of the afterNodeInsertion method in HashMap?

Question

Environment: jdk1.8
Problem: When learning HashMap, I found that the afterNodeInsertion method was called at the end of the putVal method

    ...
    ++modCount;
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;

I searched for the afterNodeInsertion method again and found that it is called in many places, but its implementation is

    void afterNodeInsertion(boolean evict) { }

An empty method? ? Want to know what this method does?

淡淡烟草味 · Answer

// Callbacks to allow LinkedHashMap post-actions
void afterNodeAccess(Node p) { }
void afterNodeInsertion(boolean evict) { }
void afterNodeRemoval(Node p) { }

In fact, it has been mentioned in the source code that these three methods are all for inheriting HashMap的LinkedHashMap class services.

LinkedHashMap 是 HashMap 的一个子类，它保留插入的顺序，如果需要输出的顺序和输入时的相同，那么就选用 LinkedHashMap.

LinkedHashMap中被覆盖的afterNodeInsertionMethod, used to callback to remove the earliest object placed in the Map

void afterNodeInsertion(boolean evict) { // possibly remove eldest
    LinkedHashMap.Entry first;
    if (evict && (first = head) != null && removeEldestEntry(first)) {
        K key = first.key;
        removeNode(hash(key), key, null, false, true);
    }
}