根据唯一值将列表拆分为子列表

Question

我有一份清单清单: List<ArrayList<String>> D = new ArrayList<>();

当它被填充时,它可能看起来像:
["A","B","Y"]
["C","D","Y"]
["A","D","N"]

我想根据唯一属性值将列表列表拆分为分区(比如索引1).

所以索引1的属性有两个唯一值,"B"和"D",所以我想分成:
["A","B","Y"]

["C","D","Y"]
["A","D","N"]

把它们放进去 List<ArrayList<ArrayList<String>>> sublists;

有没有一种聪明的方法可以做到这一点,或者我只是做这样的事情:

List<ArrayList<ArrayList<String>>> sublists = new ArrayList<>();
int featIdx = 1;

// generate the subsets
for (ArrayList<String> record : D) {
    String val = record.get(featIdx);

    // check if the value exists in sublists
    boolean found = false;
    for (ArrayList<ArrayList<String>> entry : sublists) {
        if (entry.get(0).get(featIdx).equals(val)) {
            entry.add(record);
            found = true;
            break;
        }
    }

    if (!found) {
        sublists.add(new ArrayList<>());
        sublists.get(sublists.size()-1).add(record);
    }
}

这是C4.5决策树算法的一个步骤,所以如果有人有这方面的经验,如果你能告诉我这是否是生成子列表的正确方法,我将不胜感激.

谢谢.

Answer 1

使用Java 8,您可以使用groupingBy收集器:

Map<String, List<List<String>>> grouped = D.stream()
                .collect(Collectors.groupingBy(list -> list.get(1)));
Collection<List<List<String>>> sublists = grouped.values();

或者@AlexisC的建议:

import static java.util.stream.Collectors.collectingAndThen;
import static java.util.stream.Collectors.groupingBy;

Collection<List<List<String>>> sublists = D.stream()
             .collect(collectingAndThen(groupingBy(list -> list.get(1)), Map::values));