Unable to have correct value with SELECT query using MAX() in JPA

Question

I am new in JPA and I have a problem when I try to query to the database using MAX() function. Code of my function is following. Can anyone help me? Thank you.

public int getMaxId(){

    entityManager = this.entityManagerFactory.createEntityManager();

    Query query = entityManager.createQuery("SELECT * FROM user WHERE id = (SELECT MAX(u.id) FROM user u)");
    User user = (User) query.getSingleResult();

    int id = user.getId();
    return id;
}

I am using JPA, TopLink and Apache Derby. My method should return the maximum id of table users.

Edit: I call that function from a service:

try {
        int id = userDAO.getMaxId();
        logger.info("Max id: " + id);
        user.setId(id+1);

    }
    catch (Exception ex){
        logger.error("Unable to get the max id.");
    }

Value of user.setId() is always '0'.

Edit(2): Log

    Caused by: Exception [EclipseLink-8034] (Eclipse Persistence Services - 2.3.0.v20110604-r9504): org.eclipse.persistence.exceptions.JPQLException
Exception Description: Error compiling the query [SELECT u FROM user u WHERE u.id = (SELECT MAX(uu.id) FROM user uu)]. Unknown entity type [user].
    at org.eclipse.persistence.exceptions.JPQLException.entityTypeNotFound(JPQLException.java:483)
    at org.eclipse.persistence.internal.jpa.parsing.ParseTreeContext.classForSchemaName(ParseTreeContext.java:138)
    at org.eclipse.persistence.internal.jpa.parsing.SelectNode.getClassOfFirstVariable(SelectNode.java:327)
    at org.eclipse.persistence.internal.jpa.parsing.SelectNode.getReferenceClass(SelectNode.java:316)
    at org.eclipse.persistence.internal.jpa.parsing.ParseTree.getReferenceClass(ParseTree.java:436)
    at org.eclipse.persistence.internal.jpa.parsing.ParseTree.adjustReferenceClassForQuery(ParseTree.java:75)
    at org.eclipse.persistence.internal.jpa.parsing.JPQLParseTree.populateReadQueryInternal(JPQLParseTree.java:103)
    at org.eclipse.persistence.internal.jpa.parsing.JPQLParseTree.populateQuery(JPQLParseTree.java:84)
    at org.eclipse.persistence.internal.jpa.EJBQueryImpl.buildEJBQLDatabaseQuery(EJBQueryImpl.java:219)
    at org.eclipse.persistence.internal.jpa.EJBQueryImpl.buildEJBQLDatabaseQuery(EJBQueryImpl.java:190)
    at org.eclipse.persistence.internal.jpa.EJBQueryImpl.<init>(EJBQueryImpl.java:142)
    at org.eclipse.persistence.internal.jpa.EJBQueryImpl.<init>(EJBQueryImpl.java:126)
    at org.eclipse.persistence.internal.jpa.EntityManagerImpl.createQuery(EntityManagerImpl.java:1475)
    ... 35 more

My entity User is declared as follows:

@Entity
@Table(name = "user")
public class User {

@Id
private int id;
private String name;
private String lastName;
private String city;
private String password;

Answer 1

You can directly use more simple JPQL

return (Integer)entityManager.createQuery("select max(u.id) from User u").getSingleResult();

Or use TypedQuery

return entityManager.createQuery("select max(u.id) from User u", Integer.class).getSingleResult();

EDIT:

Unknown entity type [user]

You should use User instead of user.

Answer 2

Well it is hard to say from your comments and you haven't posted any logging.

How about this:

Query query = entityManager.createQuery("SELECT u FROM users u WHERE u.id = (SELECT MAX(u.id) FROM users u)");

Answer 3

Im Using that code:

    Query qry = em.createQuery("SELECT MAX(t.column) FROM Table t");
    Object obj = qry.getSingleResult();
    if(obj==null) return 0;
    return (Integer)obj;

Cause if there is no elements on Table "t", NullpointerException is throwing.