java - 如何将 list 转换为保留所有重复键和值的映射

Question

我有一个列表如下:

List<Address> address;

哪里

Address:
city
country
state

我想将其转换为如下所示

Map <String,String> convertedMap=list.stream().collect
(Collectors.toMap(Address:getCity+Address:getCountry ,Address:getState)); 

我想在生成的映射中保留所有重复的键和值，如下所示。

(key=City1country1, value= state1) ,(key=City1country1, value=state2),(key=City1country1,value= state1) ;

Answer 1

正如评论中提到的， map 不存储重复的键，因此您必须使用 Map<String, List<String>>相反 Map<String, String> 。

总而言之，您只需使用 Collectors.toMap方法 mergeFunction您可以在其中处理重复键的参数。每次出现相同的按键时都会调用该操作。在本例中，我们只需将两个列表合并为一个。看一下下面的代码(使用 JDK 11 编译)，我相信它完全满足您的需要并打印预期结果(当然使用 List<String>)。

import java.util.*;
import java.util.stream.Collectors;

public class ListToMapWithDuplicatedKeysSample {

    public static void main(String[] args) {
        List<Address> addresses = List.of(
                new Address("City1", "country1", "state1"),
                new Address("City1", "country1", "state2"),
                new Address("City1", "country1", "state1"),
                new Address("City3", "country3", "state3")
        );
        Map<String, List<String>> result = addresses.stream()
                .collect(
                        Collectors.toMap(
                                address -> address.getCity() + address.getCountry(),
                                address -> Collections.singletonList(address.getState()),
                                ListToMapWithDuplicatedKeysSample::mergeEntriesWithDuplicatedKeys
                        )
                );
        System.out.println(result);
    }

    private static List<String> mergeEntriesWithDuplicatedKeys(List<String> existingResults, List<String> newResults) {
        List<String> mergedResults = new ArrayList<>();
        mergedResults.addAll(existingResults);
        mergedResults.addAll(newResults);
        return mergedResults;
    }

    private static class Address {

        private final String city;
        private final String country;
        private final String state;

        public Address(String city, String country, String state) {
            this.city = city;
            this.country = country;
            this.state = state;
        }

        String getState() {
            return state;
        }

        String getCountry() {
            return country;
        }

        String getCity() {
            return city;
        }
    }
}