如何在Java中将对象的ArrayList转换为Integer列表

Question

I used this native query to fetch data. select c.id,c.name from customer c,information i where i.status=1 and c.id=i.customerId and i used the below code to convert the object list into integer list as

public List<Integer> getAllIdsByGroupId(Integer groupId) throws Exception {
    List<Object[]> list = repository.getAllIdsByGroupId(groupId);
    List<Integer> ids = repository.mapIds(list);
    return ids;
}


public List<Object[]> getAllIdsByGroupId(Integer groupId) {  
    Query query = entityManager.createNativeQuery("select c.id,c.name from customer c,information i where i.status=1 and c.id=i.customerId and c.groupId=:groupId");
    query.setParameter("groupId", groupId);
    List<Object[]> list = query.getResultList();
    if (list == null || list.isEmpty()) {
        return null;
    }
    return list;
}


public List<Integer> mapIds(List<Object[]> list) {
    List<Integer> ids = new ArrayList<Integer>();
    if (list != null && !list.isEmpty()) {
        Integer id;
        for (Object[] object : list) {
            id = (object[0] != null ? (Integer) object[0] : null);
            ids.add(id);
        }
    }
    return ids;
}

The query retrieves a list with id and name.and the above code gives output without exception.But i require List with only ids. when i remove c.name from query as follows select c.id from customer c,information i where i.status=1 and c.id=i.customerId the above code fails and produce the exception

java.lang.Integer cannot be cast to [Ljava.lang.Object;

Any helps? thanks in advance.

Answer 1

If you're using JPA / Hibernate for fetching single column data then it returns List<Integer> implicitly (based on type of that column), so no need to convert it to Integer list.

General rule:

• If you are selecting an entity, then result is list of that entity typed items
• If you are selecting primitive, then result is list of that primitive wrapper typed items
• If you are selecting multiple expressions (id, name), then result is Object[] containing the corresponding primitives/entities

Answer 2

The issue is in the typecasting Change List<Object[]> list = query.getResultList();

to List<Object> list = query.getResultList()

It is list of object where as your code is trying to convert and integer into List of Array.

List<Integer> list = query.getResultList() will also suffice if you are just trying to get ids

Answer 3

try this one may not try in my editor but may i thing one of this solution help you.

public List<Integer> mapIds(List<Object[]>  list) {
    List<Integer> ids = new ArrayList<Integer>();
    if(list != null && !list.isEmpty()) {
        int id;
        for(Object[] object : list) {
            id = (int) (object[0] != null ?  object[0] : null);
            ids.add(id);
     }
 }
 return ids;
}

Answer 4

Although the question is lacking a lot of information, the most likely reason for the ClassCastException is that you're trying to cast objects that are not of type Integer to type Integer . I'm not sure what's in the list that you're passing into the method, but if the list does indeed contain some objects of type Integer , then what you could try is to check for that before converting element object[0] into Integer ,

public List<Integer> mapIds(List<Object[]> list) {
    List<Integer> ids = new ArrayList<Integer>();
    if (list != null && !list.isEmpty()) {
        Integer id;
        for (Object[] object : list) {
            id = ((object[0] != null && object[0] instanceof Integer) ? (Integer) object[0] : null);
            ids.add(id);
            }
        }
    }
    return ids;
}

However, if the objects in list don't contain any Integer objects, then the only way to achieve what you're doing is to initialize an Integer object for each object[0] by somehow retrieving the int value in each. Only you know how to do this.