How to avoid multiple Streams with Java 8

Issue

I am having the below code

trainResponse.getIds().stream()
        .filter(id -> id.getType().equalsIgnoreCase("Company"))
        .findFirst()
        .ifPresent(id -> {
            domainResp.setId(id.getId());
        });

trainResponse.getIds().stream()
        .filter(id -> id.getType().equalsIgnoreCase("Private"))
        .findFirst()
        .ifPresent(id ->
            domainResp.setPrivateId(id.getId())
        );

Here I'm iterating/streaming the list of Id objects 2 times.

The only difference between the two streams is in the filter() operation.

How to achieve it in single iteration, and what is the best approach (in terms of time and space complexity) to do this?

Solution

IMO, the two streams solution is the most readable. And it may even be the most efficient solution using streams.

IMO, the best way to avoid multiple streams is to use a classical loop. For example:

// There may be bugs ...

boolean seenCompany = false;
boolean seenPrivate = false;
for (Id id: getIds()) {
   if (!seenCompany && id.getType().equalsIgnoreCase("Company")) {
      domainResp.setId(id.getId());
      seenCompany = true;
   } else if (!seenPrivate && id.getType().equalsIgnoreCase("Private")) {
      domainResp.setPrivateId(id.getId());
      seenPrivate = true;
   }
   if (seenCompany && seenPrivate) {
      break;
   }
}

It is unclear whether that is more efficient to performing one iteration or two iterations. It will depend on the class returned by getIds() and the code of iteration.

The complicated stuff with two flags is how you replicate the short circuiting behavior of findFirst() in your 2 stream solution. I don't know if it is possible to do that at all using one stream. If you can, it will involve something pretty cunning code.

But as you can see your original solution with 2 stream is clearly easier to understand than the above.

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The main point of using streams is to make your code simpler. It is not about efficiency. When you try to do complicated things to make the streams more efficient, you are probably defeating the (true) purpose of using streams in the first place.

Answered By - Stephen C