Java中的HashMap和HashSet size（）

Question

I tried to find information about it but failed.

How is size() in HashMap or HashSet implemented? How does it work? Is it an O(1) or O(n) operation?

Answer 1

It takes O(1) time. HashMap has a size instance variable that gets modified when entries are added or removed:

This is the HashMap implementation:

/**
 * Returns the number of key-value mappings in this map.
 *
 * @return the number of key-value mappings in this map
 */
public int size() {
    return size;
}

HashSet calls size() of the backing HashMap .