如何在List<String>
期间合并groupingBy
?
给定一个自定义对象EligibilityRequest
的列表,其中有一些重复,我需要将那些相等的EligibilityRequest
实例分组,然后合并到String列表中一个特定的属性,这对Equals
方法没有贡献。
这是详细信息:
public class EligibilityRequest {
private String channel; // it's part of Equals
...
private List<String> productCodes // it's not part of Equals and need to be aggregated within instance of EligibilityRequest "equal"
}
我尝试了以下方法:
Map<EligibilityRequest, List<String>> uniqueEligibilityRequests = new HashMap<>();
uniqueEligibilityRequests = requests.stream()
.collect(groupingBy(request -> request,
Collectors.mapping(request -> request.getProductsCode(), toList())
));
只要productCodes
代表字符串而不是List<String>
,它就可以工作。我无法弄明白如何在List<String>
中合并不同的groupingBy
。
有人可以帮忙吗?
问候,温琴佐
您拥有的代码适用于创建值为列表列表的映射:
Map<EligibilityRequest, List<List<String>>> result =
requests.stream()
.collect(groupingBy(request -> request,
Collectors.mapping(request -> request.getProductsCode(),
toList())
));
如果您的目的是将所有内部列表收集到一个内部列表中,那么您需要这样来展平内部列表:
Map<EligibilityRequest, List<String>> res =
requests.stream()
.collect(Collectors.groupingBy(Function.identity(),
Collectors.mapping(
EligibilityRequest::getProductCodes,
Collectors.collectingAndThen(
Collectors.toList(),
list -> list.stream()
.flatMap(List::stream)
.collect(Collectors.toList())))
)
);
在这种情况下,我发现无流代码更舒适。
public static <K extends Comparable<K>> Map<K, Set<String>> groupBy(List<EligibilityRequest> requests, Function<EligibilityRequest, K> getKey) {
Map<K, Set<String>> map = new TreeMap<>();
for (EligibilityRequest request : requests) {
map.compute(getKey.apply(request), (key, productCodes) -> {
productCodes = productCodes == null ? new TreeSet<>() : productCodes;
productCodes.addAll(request.getProductCodes());
return productCodes;
});
}
return map;
}