Java 8 got a new implement to hashmap (using a tree). I have understand that in the worst case, it may be O(n) for lookup.
Will changing this implement to an avl tree change this O(n) case to something better (without changing the other complexities)?
here are some notes on it:
This map usually acts as a binned (bucketed) hash table, but when bins get too large, they are transformed into bins of TreeNodes, each structured similarly to those in java.util.TreeMap. Most methods try to use normal bins, but relay to TreeNode methods when applicable (simply by checking instanceof a node). Bins of TreeNodes may be traversed and used like any others, but additionally support faster lookup when overpopulated. However, since the vast majority of bins in normal use are not overpopulated, checking for existence of tree bins may be delayed in the course of table methods.
Tree bins (i.e., bins whose elements are all TreeNodes) are ordered primarily by hashCode, but in the case of ties, if two elements are of the same "class C implements Comparable", type then their compareTo method is used for ordering. (We conservatively check generic types via reflection to validate this -- see method comparableClassFor). The added complexity of tree bins is worthwhile in providing worst-case O(log n) operations when keys either have distinct hashes or are orderable, Thus, performance degrades gracefully under accidental or malicious usages in which hashCode() methods return values that are poorly distributed, as well as those in which many keys share a hashCode, so long as they are also Comparable. (If neither of these apply, we may waste about a factor of two in time and space compared to taking no precautions. But the only known cases stem from poor user programming practices that are already so slow that this makes little difference.)