Java 8 Stream vs Collection Storage

Question

I have been reading up on Java 8 Streams and the way data is streamed from a data source, rather than have the entire collection to extract data from.

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Answer 1

1. Collection is a data structure. Based on the problem you decide which collection to be used like ArrayList, LinekedList (Considering time and space complexity) . Where as Stream is just a processing kind of tool, which makes your life easy.

2. Other difference is, you can consider Collection as in-memory data structure, where you can add , remove element. Where as in Stream you can perform two kind of operation:

   a. Intermediate operation : Filter, map ,sort,limit on the result set
   b. Terminal operation : forEach ,collect the result set to a collection.

   But if you notice, with stream you can't add or remove elements.

3. Stream is kind of iterator, you can traverse collection through stream. Note, you can traverse stream only once, let me give you an example to have better understanding:

Example1:

List employeeNameList = Arrays.asList("John","Peter","Sachin");
    Stream s = employeeNameList.stream();

    // iterate through list
    s.forEach(System.out :: println);  // this work's perfectly fine
    s.forEach(System.out :: println);  // you will get IllegalStateException, stating stream already operated upon

So, what you can infer is, collection you can iterate as many times as you want. But for the stream, once you iterate , it won't remember what it is supposed to do. So, you need to instruct it again.

I hope, it is clear.