Java Map - 如何将自定义对象列表检索为单个列表

Question

I have the following Java Map, where the values are lists of a custom type (EmployeeInfo):

Map<String, List<EmployeeInfo>> myMap;

My goal is to retrieve all the values as one single List from this map. So far I have tried the following, but haven't made it work yet:

// ERROR: The constructor ArrayList<EmployeeInfo>(Collection<List<EmployeeInfo>>) is undefined
List<EmployeeInfo> info = new ArrayList<EmployeeInfo>(myMap.values());

// ERROR: java.lang.ClassCastException: java.util.HashMap$Values cannot be cast to java.util.List
List<EmployeeInfo> info = (List)myMap.values();

Could anyone provide any help? Thanks in advance!

Answer 1

You need to go through every key in myMap , and append every element in the current List to your result List .

List<EmployeeInfo> res = new LinkedList<>(); // Can be any list, not just linkedlist, but linkedlist works best for this.
for(List<EmployeeInfo> l : myMap.values()) {
    for(EmployeeInfo e : l) {
        res.add(e);
    }
}

Note: I made this on the stop in StackOverflow itself, so please fix any small syntax errors.

Answer 2

You need to convert the Map<String, List<EmployeeInfo>> or {[e1,e2], [e3,e4]} to List<EmployeeInfo> or [e1,e2,e3,e4] . You can do this by flat mapping the values to list. Here is a approach using streams:

List<EmployeeInfo> list = myMap.values() // gives you [[e1,e2],[e3,e4]]
                               .stream() // stream over them
                               .flatMap(List::stream) // convert to [e1,e2,e3,e4]
                               .collect(Collectors::toList); // collect back

Answer 3

Like @Nishant Chatterjee KMS said, you need to flatten the values.

Here is a java 8 =< way of doing so:

Map<String, List<String>> foo = new HashMap<>();
List<String> bar = new ArrayList<>();
foo.values().forEach(bar::addAll);