Rule 4:Prefer collection over stream as return type
when writing a method that returns a sequence of elements, remember that some of your users may want to process them as a stream while others may want to iterate over them. Try to accommodate both groups. If it’s feasible to return a collection, do so. If you already have the elements in a collection or the number of elements in the sequence is small enough to justify creating a new one, return a standard collection such as ArrayList.
Otherwise, consider implementing a custom collection as we did for the power set. If it isn’t feasible to return a collection, return a stream or iterable, whichever seems more natural.
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