I recently came to know that in Java 8 hash maps uses binary tree instead of linked list and hash code is used as the branching factor.I understand that in case of high collision the lookup is reduced to O(log n) from O(n) by using binary trees.My question is what good does it really do as the amortized time complexity is still O(1) and maybe if you force to store all the entries in the same bucket by providing the same hash code for all keys we can see a significant time difference but no one in their right minds would do that.
Binary tree also uses more space than singly linked list as it stores both left and right nodes.Why increase the space complexity when there is absolutely no improvement in time complexity except for some spurious test cases.
HashMap class uses Node as a subclass for holding key-value entries. Members of this class are: final int hash; final K key; V value; Node<K,V> next; The binary tree implementation is a Red-Black tree. A red-black tree is a sorted tree, in which search takes max log(n) operations.
Binary Search Trees are generally memory-efficient since they do not reserve more memory than they need to. On the other hand, Hash tables can be a bit more demanding if we don't know the exact number of elements we want to store.
That's because it has an array and linked list implemented in it (Duh!). The array is used to store the hash of the key and the linked list is used to store the data and the key and other stuff.
It actually uses a singly linked list implemented by chaining the hash table entries. (By contrast, a LinkedList is doubly linked, and it requires a separate Node object for each element in the list.)
This is mostly security-related change. While in normal situation it's rarely possible to have many collisions, if hash keys arrive from untrusted source (e.g. HTTP header names received from the client), then it's possible and not very hard to specially craft the input, so the resulting keys will have the same hashcode. Now if you perform many look-ups, you may experience denial-of-service. It appears that there's quite a lot of code in the wild which is vulnerable to this kind of attacks, thus it was decided to fix this on the Java side.
For more information refer to JEP-180.
Your question contains some wrong premises.
Map
’s size, you can’t raise the array size arbitrarily to avoid bucket collisions. There’s even the theoretical limitation that an array size can be at max 2³¹ while there are 2³² possible hash codes.String
s are an obvious example, but even a Point
bearing two int
values or a plain Long
key have unavoidable hash collisions. So they might be more common than you think and it heavily depends on the use case.Comparable
, its natural order will be used. The examples you might have found on the web deliberately use the same hash code for objects to demonstrate the use of the Comparable
implementation which otherwise would not show up. What they trigger is only the last resort of the implementation.If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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