java - 重用部分 Stream mapping 和 filtering 组成两个不同的结果

Question

我想知道是否有一种好的方法可以重用最终针对不同输出而变化的通用流操作。下面的示例正是我试图压缩成一步操作的内容:

public static DepartmentInfo extractDepartmentInfo(BaselinePolicy resource) throws ResourceProcessorError {
    Function<Exception, Exception> rpe = e -> new ResourceProcessorError(e.getMessage());
    List<String> parents = 
        Objects.requireNonNull(
            Exceptions.trying(
                () -> Arrays.asList(Exceptions.dangerous(resource::getParentIds).expecting(CMException.class).throwing(rpe))
                            .stream()
                            .map(cId -> Exceptions.dangerous(cId, resource.getCMServer()::getPolicy).expecting(CMException.class).throwing(rpe))
                            .filter(policy -> PagePolicy.class.isAssignableFrom(policy.getClass()))
                            .map(PagePolicy.class::cast)
                            .filter(page -> Exceptions.dangerous(page,
                                                                 p -> Boolean.valueOf(p.getComponentNotNull(ComponentConstants.POLOPOLY_CLIENT, 
                                                                                                            ComponentConstants.IS_HOME_DEPARTMENT,
                                                                                                            Boolean.FALSE.toString())).booleanValue())
                                                      .expecting(CMException.class).throwing(rpe))
                            .map(page -> Exceptions.dangerous(page, p -> p.getExternalId().getExternalId()).expecting(CMException.class).throwing(rpe)), ResourceProcessorError.class)
                            .collect(Collectors.toList()));
    String externalId = parents.get(parents.size()-1).toString();
    List<String> list = 
        Objects.requireNonNull(
            Exceptions.trying(
                () -> Arrays.asList(Exceptions.dangerous(resource::getParentIds).expecting(CMException.class).throwing(rpe))
                            .stream()
                            .map(cId -> Exceptions.dangerous(cId, resource.getCMServer()::getPolicy).expecting(CMException.class).throwing(rpe))
                            .filter(policy -> PagePolicy.class.isAssignableFrom(policy.getClass()))
                            .map(PagePolicy.class::cast)
                            .map(page -> 
                                Exceptions.dangerous(page, 
                                        p -> p.getChildPolicy(PATH_SEGMENT) != null && 
                                             StringUtils.hasLength(SingleValued.class.cast(p.getChildPolicy(PATH_SEGMENT)).getValue())? 
                                             SingleValued.class.cast(p.getChildPolicy(PATH_SEGMENT)).getValue(): p.getName()).expecting(CMException.class).throwing(rpe))
                            .filter(val -> val != null && !val.isEmpty()), ResourceProcessorError.class)
                            .collect(Collectors.toList()));
    if(list.size() > 3) {
        list = list.subList(list.size() - 3, list.size()-1);
    }
    switch(list.size()) {
        case 0: {
            throw new ResourceProcessorError("br.com.oesp.XMLRender.error.noProduct");
        }
        case 1: {
            return DepartmentInfo.withProduct(list.get(0), externalId);
        }
        case 2: {
            return DepartmentInfo.withProduct(list.get(0), externalId).withDepartment(list.get(1));
        }
        default: {
            return DepartmentInfo.withProduct(list.get(0), externalId).withDepartment(list.get(1)).withSubDepartment(list.get(2));
        }
    }
}

请注意，对两者都重复了第一步:

List<String> parents = 
    Objects.requireNonNull(
        Exceptions.trying(
            () -> Arrays.asList(Exceptions.dangerous(resource::getParentIds).expecting(CMException.class).throwing(rpe))
                        .stream()
                        .map(cId -> Exceptions.dangerous(cId, resource.getCMServer()::getPolicy).expecting(CMException.class).throwing(rpe))
                        .filter(policy -> PagePolicy.class.isAssignableFrom(policy.getClass()))
                        .map(PagePolicy.class::cast)

这不仅是阅读的问题，而且特别是因为我要重做一个繁重的操作两次，同时以一种更命令的方式我会做一次。

Answer 1

您要尝试做两件事:

• 避免创建输入数组的冗余工作
• 避免map/filter/map的冗余代码

第一个很简单:

    List<Id> list = Arrays.asList(Exceptions.dangerous(resource::getParentIds)
                    .expecting(CMException.class)
                    .throwing(rpe));

现在您可以从这个源中拉取流两次而无需重新实现它。

下一位只是一个从 List 到 Stream 的函数:

Function<List<Id>, Stream<Something>> asStream = 
    list -> list.stream().map(...).filter(...).map(...);

现在，用这个开始你的流:

asStream.apply(list).moreStuff().moreStuff()