Java generate a unique id from an unordered set/list of objects / values

Question

Following is the scanerio.

How can we generate a unique id (string/number), from a given unordered collection/set of objects (having their own uids).
How to make sure the generated id will always be the same even if the order of the objects changes in the collection/set.

I am working with groups of objects and each group needs a uid, to represent certain objects are present in it regardless of the order of the object, hope it makes sence, thanks in advance for your help.

Lets say you have an object A and it is composed of different objects (collection). To check if the object A contains a certain object you will have to itterate the entire collection but with uid you can just check the uid to be the same as required. — John Smith
– John Smith, Commented Jan 23, 2018 at 5:49
Is there an (almost) unlimited number of possible objects that can be in the collection (for example, each object represents a random String)? Or is it limited (for example, each object represents a country, and only 200 or so of those exist in the world)? — D M
– D M, Commented Jan 23, 2018 at 5:53
No the number of objects in the collection are limited ~20 and each object is unique with its own uid. — John Smith
– John Smith, Commented Jan 23, 2018 at 6:02
Lets say you have 5 total objects as ob1, ob2, ob3, ob4, ob5 and you create objects as O1{ ob1,ob2} and O2{ob2,ob4,ob1} and O3{ob2,ob1}. Is there a way to generate a uid for the collections of objects ob1to ob5, keeping in mind the id generated has to be same for O1 and O3 as both have same collection? — John Smith
– John Smith, Commented Jan 23, 2018 at 6:12

D M · Accepted Answer · 2018-01-23 06:14:36Z

1

Given that the number of possible objects are so limited, you can easily do this. Give each object a value which is a power of 2 (1,2,4,8,16, etc.) Then, for the collection, just sum the values of the objects in the collection. The result should fit in a 32 bit integer.

answered Jan 23, 2018 at 6:14

D M

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