Question

如何访问public Map<String, Object> getListInsideMapObject(Long id, Date from) { Map<String, Object> result = new HashMap<>(); List<Map<String, Object>> mapList = new ArrayList<>(); List<MappedList> conTime = new ArrayList<>(); conTime = xxxRepository.findByxxx(id, from); Map<String, Object> map = xxxService.xxx(id); List<String> times = (List<String>) map.get("xxx"); for (MappedList t : conTime) { int num = 0; Map<String, Object> res = new HashMap<>(); res.put("x", ""); res.put("status", null); for (Contraction c : con) { num++; res.put("status", "stat"); res.put("x", new Date()); } } res.put("y", num); mapList.add(res); result.put("mapList", mapList); result.put("mapListA", mapListA); result.put("mapListB", mapListB); //etc return result; }类型地图中的列表？

getListInsideMapObject

我正在尝试调用此服务（dangerouslySetInnerHTML）并从此映射访问每个列表并遍历每个列表。例如在类xxx中我想调用getListInsideMapObject（Long id，Date from）作为服务并从地图访问每个列表

Answer 1

我想你想要这样的东西：

public NewClass1() {
    // have an instance from the class that gives you the map
    ClassThatBuildsTheMap mapClass = new ClassThatBuildsTheMap();
    // get the map.  must provide id and date
    Map <String, Object> myMap = mapClass.getListInsideMapObject(id, date);
    // access the lists inside the map
    useListInsideAMap(myMap);
}

private void useListInsideAMap(Map<String, Object> map){
    // Prior to Java8:
    for (Map.Entry<String, Object> entry : map.entrySet()) {
        String key = entry.getKey(); // if you want to use the key
        Object value = entry.getValue();
        if(value instanceof List){
            // I supose it is a list of sytrings
            List l = (List) value;
            for (Object item : l) {
                // Do something with the item from the list
            }
        }
    }

    // Java8:
    // loop through the map
    map.forEach((String key, Object value)->{
        // if you want to use key, just call key

        // checks if the value (Object) from the map is a list
        if(value instanceof List){
            List l = (List)value;
            // loop through the list
            l.forEach((Object item)->{
                // Do something with the item from the list
            });
        }
    });
}

Answer 2

您可以将Object转换为List

Map<String, Object> result = new HashMap<>();
List<Map<String, Object>> mapList= new ArrayList<>();
result.put("1", mapList);
-------------------------------------------------------
List<Map<String, Object>> = (List<Map<String, Object>>)result.get("1")

然后，您可以使用for / foreach正常循环。

Answer 3

此外，您可以使用Optional对象按键获取地图值，并在转换为所需类型之前检查它（以避免ClassCastException问题）：

Map<String, Object> someMap = new HashMap<>();

List<Map<String, Object>> result = Optional.ofNullable(someMap.get("id"))
        .filter(obj -> obj instanceof List)
        .map(obj -> (List<Map<String, Object>>)obj)
        .orElse(Collections.emptyList());

在这种情况下，您将有空List（如果元素未命中或类型不是List）或预期List值。

如何访问地图中的列表并循环浏览该列表

3 个答案: