如何将包含字符串列表（内部列表）的对象列表转换为字符串列表

Question

I have class like

public class RoleAccess {

    private String roleId;

    private List<String> apiIdList;

    public String getRoleId() {
        return roleId;
    }

    public void setRoleId(String roleId) {
        this.roleId = roleId;
    }

    public List<String> getApiIdList() {
        return apiIdList;
    }

    public void setApiIdList(List<String> apiIdList) {
        this.apiIdList = apiIdList;
    }

}

I want to create a new list which will add all apiIdlist from roleaccess

List<String> apiIdList = new ArrayList<>();
for (RoleAccess roleAccess : roleAccessList) {
            if (roleAccess.getApiIdList() != null) {
                apiIdList.addAll(roleAccess.getApiIdList());
            }
        }

How can we do with stream api or which is best solution to do this?

I checked normal object list to list but I want to list of object and inner list to list I tried this

  List<String> apiIdList = roleAccessList.stream()
                          .map(RoleAccess::getApiIdList)
                          .collect(ArrayList::new, List::addAll, List::addAll);

Answer 1

Use flatMap instead of map :

List<String> apiIdList = roleAccessList.stream()
    .flatMap(e -> e.getApiIdList().stream())
    .collect(Collectors.toList());

Please take a look on the link for more info about flatMap .

Answer 2

List<String> apiIdList = roleAccessList.stream()
                       .map(RoleAccess::getApiIdList)
                       .filter(Objects::nonNull)
                       .flatMap(Collection::stream)
                       .collect(Collectors.toList());

Answer 3

An alternate solution would be:

List<String> apiIdList = roleAccessList.stream()
        .flatMap(a -> a.getApiIdList().stream()) // flat map the apiIds from list
        .filter(Objects::nonNull) // use only not null
        .collect(Collectors.toList());