java - 使用流api java计算嵌套元素的数量

Question

使用流计算内容数量

class Subject {
  private String id;
  private String name;
  private List<Unit> units;
}

class Unit {
  private String id;
  private String name;
  private List<Topic> topics;
}

class Topic {
  private String id;
  private String name;
  private List<Content> contents;
}

class Content {
  private String id;
  private String contentType;
  private SubTopic subtopic;
}

使用 Java 8 和 Streams，我想要 contentType 等于视频的 Content 元素的计数。

为了计算主题，我试过这个:

int topicCount = subject.getUnits().stream()
    .map(Unit::getTopics)
    .filter(topics -> topics != null)
    .mapToInt(List::size)
    .sum();

Answer 1

您可以平面映射嵌套元素并计算它们:

long videoContentCount = 
    subject.getUnits()
           .stream()
           .flatMap(u -> u.getTopics().stream())
           .flatMap(t -> t.getContents().stream())
           .filter(c -> c.getCountetType().equals("video"))
           .count();