Java hashmap对字符串哈希的复杂性

Question

I was looking at java source code in HashMap class.

final int hash(Object k) {
    int h = 0;
    if (useAltHashing) {
        if (k instanceof String) {
            return sun.misc.Hashing.stringHash32((String) k);
        }
        h = hashSeed;
    }

    h ^= k.hashCode();

So, what is the time complexity of hashmapObject.put("somestring") ? Is it O(1) or O(n) where n is number of characters in a string.

Answer 1

In worst case time(In practise it happens rarely , only when we have a bad hashing function) complexity for put method in hashmap is O(N) , because although we add the element at the beginning of the linked list( O(1) ) but we still need to loop through the bucket(linked list) to determine if that new element already exists or not.
Updated: As per Peter Lawery comment in java 8 its O(log n) . This optimization is described here but in a nutshell an ad-hoc implementation of tree map is used as a bucket when the size of the bucket crosses the threshold value. The threshold value is setted by the variable static final int TREEIFY_THRESHOLD = 8; in HashMap.java

Answer 2

It is O(1) wrt the size of the map, which is what is usually of interest. It is O(N) wrt the length of the string.