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Showing posts from April, 2014

HashMap performance improvements in Java 8

HashMap<K, V> is fast, versatile and ubiquitous data structure in every Java program. First some basics. As you probably know, it uses hashCode() and equals() method of keys to split values between buckets. The number of buckets (bins) should be slightly higher than the number of entries in a map, so that each bucket holds only few (preferably one) value. When looking up by key, we very quickly determine bucket (using hashCode() modulo number_of_buckets) and our item is available at constant time.

This should have already been known to you. You probably also know that hash collisions have disastrous impact on HashMap performance. When multiple hashCode() values end up in the same bucket, values are placed in an ad-hoc linked list. In worst case, when all keys are mapped to the same bucket, thus degenerating hash map to linked list - from O(1) to O(n) lookup time. Let's first benchmark how HashMap behaves under normal circumstances in Java 7 (1.7.0_40) and Java 8 (1.8.0-b132)…