Type cast not working using map function in Java 8

问题:

I am comparing two lists.

List allUserGroups = UserBC.getAllGroupsForUser(userId, deptID);
List<String> confUserGroups= Arrays.asList(configuredSet);

List one returns Object which I need to typecast to GroupData entity. GroupData has multiple fields and want to compare for one of the fields 'id'. So I used map function to typecast as below,

isValuePresent = allUserGroups.stream().map(p -> (GroupData) p).anyMatch(p -> confUserGroups.contains(p.getId()));

Issue is, for p.getId() its again asking for typecast. Compiler asks to add cast again. Can anyone please suggest if I missed anything.

EDIT1: id is of long type otherwise I could have used ((GroupData)p).getId()

EDIT2: Modified the code as answered by Joop, but getting same error

回答1:

You might try to use something like this:

allUserGroups.stream()
    .map(GroupData.class::cast)
    .map(GroupData::getID)
    .anyMatch(confUserGroups.contains)

for example with String class:

List<Object> list = Arrays.asList("a","ab","abc");
list.stream()
        .map(String.class::cast) // cast to String
        .map(String::getBytes) // call getBytes on every element
        .forEach(System.out::println); 

回答2:

Both p's are different variables.

isValuePresent = allUserGroups.stream()
    .map(GroupData.class::cast)
    .anyMatch(p -> confUserGroups.contains(p.getId()));

isValuePresent = allUserGroups.stream()
    .map(GroupData.class::cast)
    .map(GroupData::getId)
    .anyMatch(confUserGroups::contains);

回答3:

If you already know the type of the values on the list, you could cast the list and then use the stream.

List<GroupData> list = (List<GroupData>)UserBC.getAllGroupsForUser(userId, deptID);
confUserGroups= Arrays.asList(configuredSet);

isValuePresent = allUserGroups.stream().anyMatch(p -> confUserGroups.contains(p.getId()));

回答4:

Adding String.valueOf and directly typecasting id solved my issue.

isValuePresent = allUserGroups.stream().anyMatch(p -> configuredVipUserGroups.contains(String.valueOf(((GroupData) p).getId())));

But I still wonder why .map(p -> (GroupData) p) was not able to typecast.

回答5:

Use casting in contains function no need to cast before it.

isValuePresent = allUserGroups.stream()
                            .map(p -> p)
                            .anyMatch(p -> confUserGroups.contains( (GroupData)p.getId()));