如何用单语句将List<Definition>转为Map<String, Map<String, Definition>>？

需求：已将对象数组转为List<Definition>，需要转换为Map<String, Map<String, Definition>>，外层Map的键为defName，内层Map的键为defDesc，且同一defName下的defDesc唯一。

原始对象数组：

[
    {
        "defName": "Project",
        "defDesc": "Project 1",
        "defID": "abcd123",
        "defClass": "class1"
    },
    {
        "defName": "Project",
        "defDesc": "Project 2",
        "defID": "asdoihw1345",
        "defClass": "class2"
    },
    {
        "defName": "Project",
        "defDesc": "Project 3",
        "defID": "ewrwu123",
        "defClass": "class3"
    },
    {
        "defName": "Product",
        "defDesc": "Product 1",
        "defID": "pgkorejgoi1234",
        "defClass": "class1"
    },
    {
        "defName": "Product",
        "defDesc": "Product 2",
        "defID": "asadsad2343",
        "defClass": "class2"
    },
    {
        "defName": "Product",
        "defDesc": "Product 3",
        "defID": "4dgaksld",
        "defClass": "class3"
    }
]

原有两步实现：

List<Definition> definitions = // 从JSON获取的列表
Map<String, List<Definition>> definitionByName = definitions.stream().collect(Collectors.groupingBy(Definition::getDefName));
Map<String, Map<String, Definition>> definitionMap = new HashMap<>();
definitionByName.forEach((name, definitionList) -> {
    Map<String, Definition> definitionDescMap = definitionList.stream()
            .collect(Collectors.toMap(def -> def.getDefDesc(), Function.identity()));
    definitionMap.put(name, definitionDescMap);
});

Java 8 单语句解决方案 #

可以直接使用Collectors.groupingBy的重载方法，指定下游收集器为Collectors.toMap，一步完成嵌套Map的转换：

List<Definition> definitions = // 从JSON获取的列表
Map<String, Map<String, Definition>> definitionMap = definitions.stream()
        .collect(Collectors.groupingBy(
                Definition::getDefName,
                Collectors.toMap(Definition::getDefDesc, Function.identity())
        ));

说明 #

• 第一个参数Definition::getDefName是外层Map的分组键提取函数；
• 第二个参数是下游收集器，将每个分组内的List<Definition>直接转换为以defDesc为键、Definition对象为值的内层Map；
• 由于你明确同一defName下的defDesc唯一，无需处理键冲突，所以不用额外指定冲突合并函数。

Java 8+ 优化参考 #

如果后续使用Java 16及以上版本，写法逻辑和Java 8一致，但可以根据场景补充冲突处理逻辑（比如出现defDesc重复时）：

Map<String, Map<String, Definition>> definitionMap = definitions.stream()
        .collect(Collectors.groupingBy(
                Definition::getDefName,
                Collectors.toMap(
                        Definition::getDefDesc,
                        Function.identity(),
                        (existing, replacement) -> replacement // 冲突时保留新对象
                )
        ));

内容的提问来源于stack exchange，提问作者iCode