Why is 31 used in hashcode

Please log in or register to add a comment. Please log in or register to answer this question. This is what described in Effective Java a book : The value 31 was chosen because it is an odd prime. Related questions. Why does String. Is the idea after the first resolution it'll rely on OS caching? Still this seems inefficient and in cases of multiple domains resolving to the same IP, incorrect.

What am I missing? Why do I need to override the equals and hashCode methods in Java? Recently I read through this Developer Works Document. The document is all about defining hashCode and equals effectively and correctly, however I I am trying to understand how one can utilize Auto Layout to position items relative to other views percentage-wise.

For example, I recently learned that I'm losing my wits here with this 'simple' problem: In the colorbar illustrated in picture in matplotlib I need to move offsetText base multiplier Most other processors would require a separate shift and subtract instruction. However, if your multiplier is slow this is still a win. Modern processors tend to have fast multipliers so it doesn't make much difference, so long as 32 goes on the correct side. It's not a great hash algorithm, but it's good enough and better than the 1.

Python Javascript Linux Cheat sheet Contact. A common source of bugs is the failure to override the hashCode method. You must override hashCode in every class that overrides equals.

Failure to do so will result in a violation of the general contract for Object. The key provision that is violated when you fail to override hashCode is the second one: equal objects must have equal hash codes. For example, consider the following simplistic PhoneNumber class, whose equals method is constructed according to the recipe in Item At this point, you might expect m.

Notice that two PhoneNumber instances are involved: one is used for insertion into the HashMap, and a second, equal, instance is used for attempted retrieval.

Therefore the get method is likely to look for the phone number in a different hash bucket from the one in which it was stored by the put method. Fixing this problem is as simple as providing a proper hashCode method for the PhoneNumber class.

So what should a hashCode method look like? This one, for example, is always legal but should never be used:. Therefore, every object hashes to the same bucket, and hash tables degenerate to linked lists. Programs that should run in linear time instead run in quadratic time.