Java11：使用收集器将List<String>转换为TreeMap<String,List<String>>
我有一个这样的清单
List<String> customList = Arrays.asList(
"5000 Buruli ulcer is an infectious disease",
"6000 characterized by the development",
"7000 of painless open wounds.",
"8000 The disease largely occurs in",
"10000 sub-Saharan Africa and Australia."
);
我想把它转换List成TreeMap<String, List<String>>这样：
"5000", ["Buruli", "ulcer", "is", "an", "infectious", "disease"]
"6000", ["characterized", "by", "the", "development"]
// etc
到目前为止我的代码：
TreeMap<String, List<String[]>> collect = customList.stream()
.map(s -> s.split(" ", 2))
.collect(Collectors
.groupingBy(a -> a[0], TreeMap::new, Collectors.mapping(a -> a[1].split(" "), Collectors.toList())));
我有两个问题。
- 首先，这
TreeMap::new可能不起作用，因为顺序与原始List. - 其次，我似乎没有找到一种方法将其
List<String[]>变成List<String>.
有任何想法吗？
回答
您想使用 aLinkedHashMap来保留原始顺序。所以你的代码应该是这样的：
Map<String, List<String>> collect = customList.stream()
.map(s -> s.split(" +"))
.collect(Collectors.toMap(a -> a[0], a -> Arrays.asList(a)
.subList(1, a.length), (a, b) -> a, LinkedHashMap::new));
如果您的键不是唯一的，您可以使用具有以下内容的分组收集器（Collectors.flatMapping需要 Java 9+）：
collect = customList.stream()
.map(s -> Arrays.asList(s.split(" +")))
.collect(Collectors.groupingBy(l -> l.get(0),
LinkedHashMap::new,
Collectors.flatMapping(l -> l.stream().skip(1), Collectors.toList())));
- @Thomas Yes, I'm assuming the keys are unique. Otherwise, groupBy, as Arvind suggests, is the way to do it (or of course a proper merge function in toMap).
回答
还有一个更新：
此更新是为了满足 OP 在答案下方作为评论提到的以下要求：
我希望每个单词都作为列表中的一个单独元素。对于您的解决方案，所有元素都在同一个 List 条目中。例如，我想要 10000=[撒哈拉以南、非洲和澳大利亚。]
为了实现这一点，您不应拆分单词串。
演示：
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.TreeMap;
import java.util.stream.Collectors;
public class Main {
public static void main(String[] args) {
List<String> customList = Arrays.asList(
"5000 Buruli ulcer is an infectious disease",
"6000 characterized by the development",
"7000 of painless open wounds.",
"8000 The disease largely occurs in",
"10000 sub-Saharan Africa and Australia."
);
TreeMap<String, List<String>> collect = customList.stream().map(s -> s.split(" ", 2))
.collect(Collectors.groupingBy(a -> a[0],
() -> new TreeMap<String, List<String>>(Comparator.comparingInt(Integer::parseInt)),
Collectors.mapping(a -> a[1], Collectors.toList())));
System.out.println(collect);
}
}
输出：
{5000=[Buruli ulcer is an infectious disease], 6000=[characterized by the development], 7000=[of painless open wounds.], 8000=[The disease largely occurs in], 10000=[sub-Saharan Africa and Australia.]}
或者基于我原来的答案：
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.TreeMap;
import java.util.stream.Collectors;
public class Main {
public static void main(String[] args) {
List<String> customList = Arrays.asList(
"5000 Buruli ulcer is an infectious disease",
"6000 characterized by the development",
"7000 of painless open wounds.",
"8000 The disease largely occurs in",
"10000 sub-Saharan Africa and Australia."
);
Map<String, List<String>> collect = customList.stream().map(s -> s.split("\s+", 2))
.collect(Collectors.groupingBy(a -> a[0], TreeMap::new,
Collectors.mapping(a -> a[1], Collectors.toList())));
System.out.println(collect);
}
}
输出：
{10000=[sub-Saharan Africa and Australia.], 5000=[Buruli ulcer is an infectious disease], 6000=[characterized by the development], 7000=[of painless open wounds.], 8000=[The disease largely occurs in]}
Aniket建议的解决方案：
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.TreeMap;
import java.util.stream.Collectors;
public class Main {
public static void main(String[] args) {
List<String> customList = Arrays.asList(
"5000 Buruli ulcer is an infectious disease",
"6000 characterized by the development",
"7000 of painless open wounds.",
"8000 The disease largely occurs in",
"10000 sub-Saharan Africa and Australia."
);
TreeMap<String, List<String>> collect = customList.stream().map(s -> s.split(" ", 2))
.collect(Collectors.groupingBy(a -> a[0],
() -> new TreeMap<String, List<String>>(Comparator.comparingInt(Integer::parseInt)),
Collectors.mapping(a -> Arrays.toString(a[1].split(" ")), Collectors.toList())));
System.out.println(collect);
}
}
输出：
{5000=[[Buruli, ulcer, is, an, infectious, disease]], 6000=[[characterized, by, the, development]], 7000=[[of, painless, open, wounds.]], 8000=[[The, disease, largely, occurs, in]], 10000=[[sub-Saharan, Africa, and, Australia.]]}
原答案：
你快到了。你可以这样做：
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.TreeMap;
import java.util.stream.Collectors;
public class Main {
public static void main(String[] args) {
List<String> customList = Arrays.asList(
"5000 Buruli ulcer is an infectious disease",
"6000 characterized by the development",
"7000 of painless open wounds.",
"8000 The disease largely occurs in",
"10000 sub-Saharan Africa and Australia."
);
Map<Object, List<Object>> collect = customList.stream().map(s -> s.split("\s+", 2))
.collect(Collectors.groupingBy(a -> a[0], TreeMap::new,
Collectors.mapping(a -> Arrays.asList(a[1].split("\s+")), Collectors.toList())));
System.out.println(collect);
}
}
输出：
{10000=[[sub-Saharan, Africa, and, Australia.]], 5000=[[Buruli, ulcer, is, an, infectious, disease]], 6000=[[characterized, by, the, development]], 7000=[[of, painless, open, wounds.]], 8000=[[The, disease, largely, occurs, in]]}
THE END
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