Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified.This way, very few, ideally no equals comparisons are required to implement contains.Īs equals, hashCode is defined on Object. Only elements therein are compared to the instance.
OVERRIDING EQUALS METHOD MAP CODE
When an instance is given to contains, its hash code is used to compute the bucket.If other, non-equal elements have the same hash code, they end up in the same bucket and must be bundled together, e.g.When an element is added, its hash code is used to compute the index in an internal array (called a bucket).
(Or should have, we will discuss this shortly.) Such data structures are often named after this technique, recognizable by the Hash in their name, with HashMap the most notable representative.
Instances with the same hash code are not necessarily equal but equal instances have the same hash code. This shortcut is the hash code, which can be seen as an object’s equality boiled down to an integer value. Instead of comparing the requested instance with each element they contain, they use a shortcut that reduces the number of potentially equal instances and then only compare those. The variable contains is true because, while instances of "b" are not identical (again, ignoring String interning), they are equal.Ĭomparing every element with the instance given to contains is wasteful, though, and a whole class of data structures uses a more performant approach. asList ( "a", "b", "c" ) boolean contains = list. Most data structures use equals to check whether they contain an element. If we were being a little hard on them, we could say that they are just an implementation detail to improve performance. While equality makes sense from a general perspective, hash codes are much more technical.
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