entity can not be null?

Question

I wanted to retrieve found object or a null from a database.

In my ProjectFollowerImpl, for a method useFollowingProject I return a single instance that matches the query, or null if the query returns no results.

@Override
public ProjectFollower userFollowingProject(Integer userId, Integer projectId) {
    return (ProjectFollower) session.createCriteria(classType)
            .add(Restrictions.eq("user.id", userId))
            .add(Restrictions.eq("project.id", projectId))
            .uniqueResult();
}

Then in a tapestry page, I have following methods:

public ProjectFollower getUserFollowingProject() {
    return projectFollowerDao.userFollowingProject(loggedInUser.getId(), project.getId());
}

@CommitAfter
void onActionFromFollowProject() {
    ProjectFollower pf = getUserFollowingProject();
    if (pf != null) {
        projectFollowerDao.delete(pf.getId());
    } else {
        pf = new ProjectFollower();
        pf.setProjectId(project);
        pf.setUserId(loggedInUser);
        projectFollowerDao.merge(pf);
    }
}

However, tapestry throws null pointer exception, stack trace:

com.rile.issuetracker.pages.Tracker getUserFollowingProject()   Tracker.java    75
com.rile.issuetracker.pages.Tracker advised$onActionFromFollowProject_6d8b9673baf() Tracker.java    80

So why is it a problem to return a null value for a object? What am I doing wrong ?

UPDATE:

public class Tracker {

@Property
@SessionState
private User loggedInUser;

@Property
@Inject
private ProjectDao projectDao;
@Property
private Project projectP1, project;
@Property
private List<Project> projectList;
@Property
@Inject
private ProjectFollowerDao projectFollowerDao;

@Property
@Inject
private TicketDao ticketDao;
@Property
private List<Ticket> ticketList;
@Property
private Ticket ticketP1;
@Property
@Inject
private TicketFollowerDao ticketFollowerDao;

@Property
private Util util = new Util();

public boolean getLoggedIn() {
    return loggedInUser.getEmail() != null;
}    

@PageLoaded
void onPageLoad() {
    projectList = projectDao.loadAll();
    ticketList = ticketDao.loadAll();
}

void onActivate(Integer contextValue) {
    if (contextValue != null) {  
        project = projectDao.getByID(contextValue);
    }
    if (project != null) {
        List ticketListByProjectID = ticketDao.getTicketsByProjectID(project.getId());
        if (!ticketListByProjectID.isEmpty()) {
            ticketList = ticketListByProjectID;
        } else {
            ticketList = null;
        }
    }
}

public ProjectFollower getUserFollowingProject() {
        return projectFollowerDao.userFollowingProject(loggedInUser.getId(), project.getId());        

}

@CommitAfter
void onActionFromFollowProject() {
    ProjectFollower pf = getUserFollowingProject();
    if (pf != null) {
        projectFollowerDao.delete(pf.getId());
    } else {
        pf = new ProjectFollower();
        pf.setProjectId(project);
        pf.setUserId(loggedInUser);
        projectFollowerDao.merge(pf);
    }
}

public boolean getIsUserFollowingTicket() {
    return ticketFollowerDao.isUserFollowingTicket(loggedInUser.getId(), ticketP1.getId());
}

@CommitAfter
void onActionFromFollowTicket() {}

public String getActiveFor(String parameter) {
    if (parameter == null || parameter.isEmpty()) {
        return null;
    }
    switch (parameter) {
        case "userFollowingProject":
            return getUserFollowingProject() != null ? "active" : "null";
        case "userFollowingTicket":
            return getIsUserFollowingTicket() ? "anchor-active" : "anchor-inactive";
        default: 
            return null;
    }
}

}

Answer 1

From the complete code, it looks like the best bet is that project is null, after checking complete class, we see that its not injected, and its value is only set in onActivate function, if contextValue is not null and projectDao.getByID(contextValue) returns a non-null value.

Could you please confirm that is happenning correctly before the call to onActionFromFollowProject