Java 8: performance of Streams vs Collections

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  1. Stop using LinkedList for anything but heavy removing from the middle of the list using iterator.

  2. Stop writing benchmarking code by hand, use JMH.

Proper benchmarks:

@OutputTimeUnit(TimeUnit.NANOSECONDS)
@BenchmarkMode(Mode.AverageTime)
@OperationsPerInvocation(StreamVsVanilla.N)
public class StreamVsVanilla {
    public static final int N = 10000;

    static List<Integer> sourceList = new ArrayList<>();
    static {
        for (int i = 0; i < N; i++) {
            sourceList.add(i);
        }
    }

    @Benchmark
    public List<Double> vanilla() {
        List<Double> result = new ArrayList<>(sourceList.size() / 2 + 1);
        for (Integer i : sourceList) {
            if (i % 2 == 0){
                result.add(Math.sqrt(i));
            }
        }
        return result;
    }

    @Benchmark
    public List<Double> stream() {
        return sourceList.stream()
                .filter(i -> i % 2 == 0)
                .map(Math::sqrt)
                .collect(Collectors.toCollection(
                    () -> new ArrayList<>(sourceList.size() / 2 + 1)));
    }
}

Result:

Benchmark                   Mode   Samples         Mean   Mean error    Units
StreamVsVanilla.stream      avgt        10       17.588        0.230    ns/op
StreamVsVanilla.vanilla     avgt        10       10.796        0.063    ns/op

Just as I expected stream implementation is fairly slower. JIT is able to inline all lambda stuff but doesn’t produce as perfectly concise code as vanilla version.

Generally, Java 8 streams are not magic. They couldn’t speedup already well-implemented things (with, probably, plain iterations or Java 5’s for-each statements replaced with Iterable.forEach() and Collection.removeIf() calls). Streams are more about coding convenience and safety. Convenience — speed tradeoff is working here.

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