如何使用来自另一个 List<Object2> 的字段 String 和 Map<String, Set<String>> 创建 List<Object>

Question

Object2 类具有标准 getter，并具有 String 字段 folder、file 和 version。 它被命名为SourceInfo

List<SourceInfo> source 包含上述三个字段。

我的目标是从List<SourceInfo> 创建一个List<Info>。

新List的类是Info，如下图。

    public class Info {
      private final String folder;
      private final Map<String, Set<String>> file;
    
      public static Builder builder() {
        return new Builder();
      }
    
      public static Builder builder(Info info) {
        return new Builder(info);
      }
    
      private Info(Builder builder) {
        this.folder = builder.folder;
        this.file = builder.file;
      }
    
      public String getFolder() {
        return folder;
      }
    
      public Map<String, Set<String>> getFile() {
        return file;
      }
    
      // autogenerated toString, hashCode, and equals
    
      public static class Builder {
        private String folder;
        private Map<String, Set<String>> file;
    
        private Builder() {}
    
        private Builder(Info info) {
          this.folder = info.folder;
          this.file = info.file;
        }
    
        public Builder with(Consumer<Builder> consumer) {
          consumer.accept(this);
          return this;
        }
    
        public Builder withFolder(String folder) {
          this.folder = folder;
          return this;
        }
    
        public Builder withFile(Map<String, Set<String>> file) {
          this.file = file;
          return this;
        }
    
        public Info build() {
          return new Info(this);
        }
      }

到目前为止，我尝试的是在构建器模式中创建一个集合。

    List<SourceInfo> source;
    
    // error: gc overhead limit exceeded 

    List<Info> infoList = source.stream()
            .map(e -> Info.builder()
                .withFolder(e.getFolder())
                .withFile(source.stream()
                    .collect(Collectors.groupingBy(SourceInfo::getKey,
                        Collectors.mapping(SourceInfo::getVersion, Collectors.toSet()))))
                .build())
            .collect(Collectors.toList());
    
    
    Map<String, Set<String>> map = source.stream()
        .collect(Collectors
            .groupingBy(SourceInfo::getKey,
                Collectors.mapping(SourceInfo::getVersion, Collectors.toSet())));
    
    List<Info> info = source.stream()
        .map(e -> Info.builder()
            .withFolder(e.getFolder())
            .withFile(map.entrySet()
                .stream()
                .collect(Collectors.toMap(Map.Entry::getKey,
                    Map.Entry::getValue)))
            .build())
        .collect(Collectors.toList());

所需的输出。以下语法可能已关闭。

// [String, Map<String, Set<String>>]

Info [folder, [key=file [value=version]]]
...

我是 Java 新手，不胜感激。

我想了解如何使用 java8 和 for 循环来做到这一点。

谢谢。

Answer 1

首先收集到Map<String, Map<String, Set<String>>，然后使用Collectors#collectingAndThen 和Collectors#groupingBy 将其映射到List<Info>

List<Info> result = list.stream()
        .collect(Collectors.collectingAndThen(
                Collectors.groupingBy(
                        Object2::getFolder,
                        Collectors.groupingBy(
                                Object2::getFile,
                                Collectors.mapping(
                                        Object2::getVersion,
                                        Collectors.toSet()
                                )
                        )
                ),
                map -> map.entrySet()
                        .stream()
                        .map(entry -> new Info.Builder()
                                              .withFolder(entry.getKey())
                                              .withFile(entry.getValue())
                                              .build()
                        )
                        .collect(Collectors.toList())
        ));