For loop in java 8 streams

Question

I would like to convert below code with Java's 8 Stream. I'm new to streams. Please help to improve this code.

List<ValidationErrorResult> errors = new ArrayList<>();
    for(String stageKey : stage.getParentStages()){
        
        // get stage from db
        Stage dbStage = stageService.getStageByKey(stageKey);

        //comparing enum
        if(!StageType.DEV.name().equals(dbStage.getType().name())){
            errors.add(ValidationErrorResult.builder().validationError(StageValidationError.DEV_STAGE_VALIDATION).build());
        } else if(!StageType.TEST.name().equals(dbStage.getType().name())){
            errors.add(ValidationErrorResult.builder().validationError(StageValidationError.TEST_STAGE_PARENT_ENV_VALIDATION).build());
        }
    }
return errors;

Thanks

Answer 1

Firstly, convert your list to stream with stream() . Then, with map() method, you can create new stream with others type.

At the end, I used Collectors.toList() to convert it as basic list, and "end" the stream.

return stage.getParentStages().stream().map(stageService::getStageByKey).map((dbStage) -> {
    if(!StageType.DEV.name().equals(dbStage.getType().name())){
        return ValidationErrorResult.builder().validationError(StageValidationError.DEV_STAGE_VALIDATION).build());
    } else if(!StageType.TEST.name().equals(dbStage.getType().name())){
        return ValidationErrorResult.builder().validationError(StageValidationError.TEST_STAGE_PARENT_ENV_VALIDATION).build());
    }
    return null;
}).collect(Collectors.toList());

Just, such as I didn't know what you would like to do when you don't add something in errors list, it return null.