1. 定义一个字符串列表类型的Tuple类：

public class StringListTuple {
    public String key;
    public List value;

    public StringListTuple() {}

    public StringListTuple(String key, List value) {
        this.key = key;
        this.value = value;
    }

    public static StringListTuple of(String key, List value) {
        return new StringListTuple(key, value);
    }
}

2. 使用group by和reduce进行聚合：

DataStream> dataStream = ...;

DataStream resultStream = dataStream
  .keyBy(value -> value.f0)
  .reduce((value1, value2) -> {
      List list = new ArrayList<>();
      list.addAll(value1.f1);
      list.addAll(value2.f1);
      return Tuple2.of(value1.f0, list);
  })
  .map(value -> StringListTuple.of(value.f0, value.f1));

3. 将结果转换为字符串：

DataStream stringResult = resultStream
  .map(value -> value.key + ": " + String.join(", ", value.value));