Question

按编号分组，查找大小超过1的组 2.检查组是否包含条件（字符串！= null）

2a. if yes ---> remove all rows which do not have a condition (string == null)
2b. if no ---> return the group as it is (no filtering at all)

我正在尝试下面的代码，但我无法在此处过滤条件。有没有简单的方法呢？

groupByList = list.stream().collect(Collectors.groupingBy(student::no));
groupByList.values().stream().map(group -> group.size() > 1)
                 .collect(Collectors.toList());

Answer 1

您需要同时使用Stream.filter（..）和Stream.map（..）

// Groups students by group number
Map<String, List<Student>> allGroups = list.stream().collect(Collectors.groupingBy(student::no));
// Filters groups that have more than 1 member
Map<String, List<Student>> filteredGroups = allGroups.entrySet().stream().filter(entry -> entry.getValue().size() > 1).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
// Removes all non-null members
Map<String, List<Student>> cleanGroups = filteredGroups.entrySet().stream()
    .map(entry -> {
        entry.setValue(entry.getValue().stream().filter(student -> student.name != null).collect(Collectors.toList()));
        return entry;
    }).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

您可以将执行管道化为一个大链：

Map<String, List<Student>> cleanGroups = list.stream()
      .collect(Collectors.groupingBy(student::no))
      .entrySet().stream()
      .filter(entry -> entry.getValue().size() > 1)
      .map(entry -> {
         entry.setValue(entry.getValue().stream()
                             .filter(student -> student.name != null)
                             .collect(Collectors.toList()));
        return entry;
      })
      .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

请注意，您有一个隐藏的对象转换，上面有.filter（..）。collect（..）。如果你需要跳过它（以双重比较为代价），你可以将lambda方法扩展为：

 .map(entry -> {
     if (entry.getValue().stream.anyMatch(student.name == null)) {
        entry.setValue(entry.getValue().stream().filter(student -> student.name != null).collect(Collectors.toList()));
     }
     return entry;
 })

使用java 8 groupBy条件

1 个答案: