I have a list of String. I want to generate one single hash code from all the strings of the list. How can I do that?
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2add them to a list and get list.hashCode()khachik– khachik2018-10-27 14:17:12 +00:00Commented Oct 27, 2018 at 14:17
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3 Answers
If you have a List of objects, you can do
List<String> list = ...
int hashCode = list.hashCode();
The hashCode uses the contents. There are lots of options for improving the hash code if you need but this is the simplest.
2 Comments
João Matos
Does this apply to every Collection children, or just List?
Peter Lawrey
@JoãoMatos All collections generate a hashCode based on the contents of the elements, keys/values. Note: if you change any element, key or value and this is in a HashSet or a key of a Map this can corrupt that collection.
Try this:
int hashCode = Objects.hash(list.toArray());
6 Comments
Peter Lawrey
I would make sure this doesn't hash the array instead of the contents.
Bohemian
@peter it doesn’t, because
toArray() returns an Object[], which is passed as the whole varargs.Bohemian
@peter but I like your answer better :)
Bohemian
@peter yeah, re DV, although clunky, this is the general answer to hashing a bunch o’ stuff
Peter Lawrey
Or
Arrays.asList( ... ).hashCode() for older versions of Java. |
You could just take the hashCode of the list, but that may be dodgy if you intend to have different implementations of List hold the same strings. A more robust solution, that relies only on the strings themselves could be to use Arrays#hashCode:
int hash = Arrays.hashCode(list.toArray());
Note, however, that this hash code depends on the order of the elements of the array, so if you don't care about the order of the strings in the list, you may want to sort this array so that the same strings produce the same hash code.
1 Comment
Peter Lawrey
This is better if you don't trust how the List has been implemented, or I would avoid using a List implementation I don't trust.