java 8流-如何在映射后获取原始对象引用

Question

Is at all possible to simply the below code using Java 8 stream?

Code:-

private double calculateBCMMatrixCostForAnalyte(Long startingMaterialId) {

    double bcmMatrixCost = 0;

    StartingMaterialMatrix smm = startingMaterialMatrixRepository.findByStartingMaterialId(startingMaterialId);

    if (smm.getBlending1MatrixConcentration() != null) {
        bcmMatrixCost += (priceService.findNeatMaterialCostByStartingMaterialId(smm.getBlending1Matrix().getId()) * smm.getBlending1MatrixConcentration()) / 1000000f;
    }

    if (smm.getBlending2MatrixConcentration() != null) {
        bcmMatrixCost += (priceService.findNeatMaterialCostByStartingMaterialId(smm.getBlending2Matrix().getId() * smm.getBlending2MatrixConcentration())) / 1000000f;
    }

    return Precision.round(bcmMatrixCost, 4);
}

I tried something like below

    Stream.of(startingMaterialMatrixRepository.findByStartingMaterialId(startingMaterialId))
            .filter(smm -> smm.getBlending1MatrixConcentration() != null)
            .mapToDouble(smm ->  priceService.findNeatMaterialCostByStartingMaterialId(smm.getStartingMaterial().getId()) * smm.getBlending1MatrixConcentration() / 1000000f);

But after this, I won't have any reference to the original 'smm' object to do further checks.

Answer 1

I honestly would not be hell-bent on turning this into a stream, it doesn't really need to be.

If it's overly complex to turn something into a stream, it may not be worth it, is there a specific reason?

that being said, you could make two streams, filter one, one way, then filter the other way, then use Stream.concat to combine the two streams, and call .sum but I do not think it'd be worth it.

edit:

You may need DoubleStream.concat , since you are mapping to double.

https://docs.oracle.com/javase/8/docs/api/java/util/stream/DoubleStream.html#concat-java.util.stream.DoubleStream-java.util.stream.DoubleStream-

Answer 2

Usage of Stream would be good if you change your StartingMaterialMatrix class. Instead of have Blending1Matrix and Blending2Matrix you should have collection List<BlendingMatrix> or Set and BlendingMatrix should have concentration and id And then it will looks like this:

smm.getBlendingMatrixes()
.stream()
.filter(blendingMatrix -> blendingMatrix.getConcentration() != null)
.mapToDouble(blendingMatrix -> (priceService.findNeatMaterialCostByStartingMaterialId(blendingMatrix .getId()) * blendingMatrix.getConcentration())) / 1000000f)
.sum();

Answer 3

This is really not readable, but here it is

public double streamWay(Long startingMaterialId) {

    return Stream.of(startingMaterialMatrixRepository.findByStartingMaterialId(startingMaterialId))
            .map(Stream::of)
            .flatMapToDouble(smmStream ->
                    DoubleStream.concat(
                            smmStream.filter(smm -> smm.getBlending1MatrixConcentration() != null)
                                    .mapToDouble(smm -> priceService.findNeatMaterialCostByStartingMaterialId(smm.getStartingMaterial().getId()) * smm.getBlending1MatrixConcentration() / 1000000f),
                            smmStream.filter(smm -> smm.getBlending1MatrixConcentration() != null)
                                    .mapToDouble(smm -> priceService.findNeatMaterialCostByStartingMaterialId(smm.getBlending1Matrix().getId() * smm.getBlending2MatrixConcentration()) / 1000000f)
                    )).sum();


}