![]() ![]() ![]() We know that the length is a power of two (the leftmost bit is set) and it is easy to create a mask to get the index value 256 – 1 = 255 or 11111111. To perform a search, we use the hash function to determine which linked list (or BST) to traverse using and operation:įirst we are trying to get the index of the linked list in the array using the &(and) operation: first = tab.įor example we may have this hash code: 10010011_00111110_01011001_01001110 and tab.length is 00000001_00000000(256). To keep the same traversal order, it uses the doubly linked list that’s why it extends LinkedHashMap.Entry: This is performance optimization in case we have a lot of collisions. TreeNode is implementation of a binary search tree ( red black tree).Last node has a null pointer (See picture above, John Smith has a pointer to next node which is Sandra Dee) It has a pointer to the next Node in a chain. Basic node Node is implementation of a linked list.There are two implementations of the node: HashMap.java keeps all key/value pairs in the nodes:Įach node contains a key and a value. You have an array of buckets and each bucket contains a linked list of objects. HashMap.java uses separate chaining strategy. be consistent-equal keys must produce the same hash value.In general a good hash function for a given data type should: The common pattern to create a hash code for custom object is getting hash codes for all fields and multiplying them by small prime number, for example: The multiplier usually is a prime number (for better distribution) and precise value is not important. If hash is 0 it calculates it using loop through all characters and multiplying the value by 31. Let’s consider how Java implements hash code for different data types: If x.equals(y), then x.hashCode() = y.hashCode(). Hash function must return the same values for equal objects. If we use the power of two, we can replace modulo (%) operation with and operation(&) to calculate the chain index. ![]() The reason is performance because the modulo operation is expensive. Java HashMap uses the power of two for the array length. Many hash functions implement a function that finds the next prime number. They all have the same index 0! It’s a good idea to make sure the size of the array is a prime number. Now consider what happens if the size of the array is 10 and a lot of keys end with 0 (100, 110, 120, 130, 140, etc). ![]() Returning key % arraySize seems a reasonable choice. In the example above all keys are integers. Using hashCode mod arraySize we can find the index in the array for the value. Hash function transforms the object to number. The main problem with this solution is to deal with situation when to different keys map to the same index. Ideally, hash function should be simple to compute and ensure that any two distinct keys get different indexes of the array. But what if we have keys in the range from 0 to 1.000.000.000? It is not space efficient to create an array containing so many elements to keep 4 values.Ī hash function helps to map arbitrary keys to array numbers from 0 to N, where N is a table size. We have the cost O(1) for search, insert and delete operation. This is the most efficient way to keep these key/values. If our keys are small integers, we could use an array. Hash map is a data structure that maps keys to values. Oleksii Shyshkov Follow Java HashMap implementation ![]()
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