Java – get random elements from a sequence collection

I talked about an API and gave me a Java util. A collection of iterators This means I can iterate over it, but I can't access these elements directly / randomly

Now my question: I want to get a random element from this set What do I do? I think I can create a new collection that allows direct access, but isn't it a little memory consumption? I can also traverse the entire collection and "roll a die" for each element to see if I should take the element and exit the iteration or continue However, I can't get the size of the set I need from the iterator

Thank you in advance

Solution

There is a method that can be executed through the collection at one time without using a lot of extra memory (just the size of an element in the collection plus a float) Pseudo code:

By collecting iterations. > Generate random floating point numbers for each project. > If the float is the lowest (or highest, it doesn't matter) seen so far, store the current item in the collection in a temporary variable (the new lowest random value is also stored.) > Once you reach the end of the set, you have a random item in the temp variable

Obviously, every time you call it, you will traverse the whole collection, but you don't have much choice

Update: the name of this type of problem finally came back to me This is called reservoir sampling