如何将此表达式转换为 lambda 表达式？

Question

我想把我正在做的事情变成 lambda，在这种情况下，我会在另一个列表中滚动 (listRegistrationTypeWork)，检查子列表 (getRegistrationTypeWorkAuthors) 是否为 != null，如果是也就是说，滚动它以查找 authorCoautor = 类型，并增加一个计数，以找出列表中有多少记录具有相同的类型。

public int qtyMaximumWorksByAuthorCoauthor(AuthorCoauthor type) {
    int count = 0;
    for (RegistrationTypeWork tab : listRegistrationTypeWork) {
        if (CollectionUtils.isNotEmpty(tab.getRegistrationTypeWorkAuthors())) {
            for (RegistrationTypeWorkAuthors author : tab.getRegistrationTypeWorkAuthors()) {
                if (author.getAuthorCoauthor().equals(type))
                    count++;
            }
        }
    }
    return count;
}

Answer 1

虽然您的陈述对转换为 lambda 表达式 的含义不够清楚，但我假设您希望将命令式循环步骤转换为函数式 stream 和 lambda em> 基于一个。

这应该很简单：

• filter 从您的两个集合中过滤掉不需要的值
• flatMap 将所有内部集合扁平化为单个流，以便您可以将 count 作为单个源对其进行操作

public int qtyMaximumWorksByAuthorCoauthor(AuthorCoauthor type) {
    return listRegistrationTypeWork.stream()
            .filter(tab -> tab.getRegistrationTypeWorkAuthors() != null)
            .flatMap(tab -> tab.getRegistrationTypeWorkAuthors().stream())
            .filter(author -> type.equals(author.getAuthorCoauthor()))
            .count();
}

Answer 2

除了 Thomas 的精美评论之外，我认为您还想编写类似这样的流。

long count = listRegistrationTypeWork.stream()
  // to make sure no lists that are actual null are mapped.
  // map all RegistrationTypeWork into optionals of lists of RegistrationTypeWorkAuthors
  .map(registrationTypeWork -> Optional.ofNullable(registrationTypeWork.getRegistrationTypeWorkAuthors()))
  // this removes all empty Optionals from the stream
  .flatMap(Optional::stream)
  // this turns the stream of lists of RegistrationTypeWorkAuthors into a stream of plain RegistrationTypeWorkAuthors
  .flatMap(Collection::stream)
  // this filters out RegistrationTypeWorkAuthors which are of a different type
  .filter(registrationTypeWorkAuthors -> type.equals(registrationTypeWorkAuthors.getAuthorCoauthor()))
  .count();

// count returns a long so you either need to return a long in your method signature or cast the long to an integer.
 return (int) count;