java.util.HashMap.containsValue() 是如何工作的？ 它的时间复杂度是多少？

Question

I am using java.util.HashMap . I'm curious to know it's time complexity? Is it O(n) or O(1) ? Does method containsValue() goes for all mapp and check the values or it get immediately? It's very important for time executing.

Answer 1

Your answers will probably be here for the general case. For your specific question,

3.1. ArrayList The ArrayList in Java is backed by an array...

contains() – implementation is based on indexOf(). So it will also run in O(n) time

Answer 2

The HashMap essentially is a key-value store, which can access it's keys with a complexity of O(1). Checking a value, however, there's nothing the HashMap can do but check all values and see if they're equal to the one you're searching.

Therefore,

The complexity is O(n) with n being the number of elements in the HashMap.

Answer 3

how does method containsValue work in HashMap?

The Javadoc doesn't specify how it is implemented and it might be better than O(n) in theory, however in practice the OpenJDK version uses brute force to find a matching value.

// from OpenJDK Java 8.
// the implementation could be different for another implementation, or subclass.
public boolean containsValue(Object value) {
    Node<K,V>[] tab; V v;
    if ((tab = table) != null && size > 0) {
        for (int i = 0; i < tab.length; ++i) {
            for (Node<K,V> e = tab[i]; e != null; e = e.next) {
                if ((v = e.value) == value ||
                    (value != null && value.equals(v)))
                    return true;
            }
        }
    }
    return false;
}