Object and Index Streams

Ashley FriezeAugust 26th, 2019Last Updated: August 21st, 2019

1 42 2 minutes read

I was going to write a post about how to blend a stream with the indices of each element, but the folks over on Baeldung have covered this subject enormously well! Given I’m part of their editorial team, I’m proud of them/us.

It’s interesting how functional programming in Java, Streams especially, have trained us out of some common patterns. Example:

List<Foo> foos = new ArrayList<>();
for (int i=0; i < bars.size(); i++) {
    foos.add(new Foo(i, bars.get(i));
}

The above code creates a new Foo from the index and the bar from a source list.

We don’t like using add in the midst of a for loop, when Stream.collect is a bigger signal that we’re pulling data from an iteration. Similarly, we don’t like hiding the effective Stream.map inside the loop, when we could express it more explicitly in a streaming declaration. We all know that the above ought to read:

List<Foo> foos = bars.stream()
   .map( ... something ...)
   .collect(toList());

The awkward challenge is that the something can’t easily be done.

How to choose what to do

On the whole, cooking a complex brew of functions around a streaming operation is a way of hiding the very thing that the stream was meant to show us – namely how the data from one place goes to another.

So here are some thoughts:

Maybe just have a for loop – it might be easier
If you find that the main player within the body of the operation is the index, then make a stream of indices, and mix in the object with a List.get at the last minute, assuming your list is efficiently indexed
If the main player is an object, then maybe store a counter (AtomicInteger is a good bet) and increment it at the last minute, watching out for ordering and parallel streams
If the code gets complex, try to divorce the streaming logic from your business logic by extracting a function
Maybe delegate this to a library – the Baeldung article shows ProtonPack and others that might just be better and which come ready tested and documented.

Whatever you do, don’t make things difficult for yourself on a point of principle.

Published on Java Code Geeks with permission by Ashley Frieze, partner at our JCG program. See the original article here: Object and Index Streams

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