Java-8流：将list转换为map

Question

我有一个标签类，它包含一个项目列表

class Tag {

private String tagName
private List<String> items

}

我有一个标签列表，其中每个标签都有一个项目列表

List<Tag> tagList =   [
                       {"tagName": "popular", "items": ["Item1","Item2","Item3","Item4"]},
                       {"tagName": "expensive" , "items":  ["Item2","Item4","Item5"]},
                       {"tagName": "mostwanted", "items":  ["Item1","Item2","Item5"]}
                      ]

我想把它转换成一个映射，其中的项目是key，而tagName是值。

预期产出：

{
    "Item1" : ["popular","mostwanted"],
    "Item2" : ["popular","expensive","mostwanted"],
    "Item3" : ["popular","mostwanted"],
    "Item4" : ["popular","expensive"],
    "Item5" : ["expensive","mostwanted"]
}

我尝试了Stream/FaltMap/GroupingBy的各种组合，但没有得到预期的输出。你能帮帮我吗。谢谢

Answer 1

下面是使用java-8中的新特性（如Map::ComputeIfPresent和Map::ComputeIfEstent)的过程方法，但不使用Java流：

Map<String, List<String>> map = new HashMap<>();
    tagList.forEach(tag -> {
        String tagName = tag.getTagName();
        tag.getItems().forEach(item -> {
            map.computeIfPresent(item, (k, v) -> { v.add(tagName); return v; });
            map.computeIfAbsent(item, k -> new ArrayList<>(Arrays.asList(tagName)));
   });
});

map.forEach((k, v) -> System.out.println(k + " " + v));

如果要将这些项从项1排序为项5，请使用不同的实现：

Map<String, List<String>> map = new TreeMap<>();

此外，只要item3只出现一次，您的预期输出就不匹配：

Item1 [popular, mostwanted]
Item2 [popular, expensive, mostwanted]
Item3 [popular]
Item4 [popular, expensive]
Item5 [expensive, mostwanted]