为什么随机不起作用？

Question

I have to write a method, which is able to find random Vacation in the database. But when I use this part in the test

Random id = new Random();
vacationId = id.nextInt(2500);

I get NullPoiterExeption . But when i just print

vacationId = 2500;

or some other number, everything is okay.

This is the whole method

public Vacation findRandomVacation() {
    Integer vacationId = null;
    Query query = entityManager.createQuery("from Vacation v where v.id = :id")
                .setParameter("id", vacationId);
    if  ( query.getResultList().isEmpty()) {
        Random id = new Random();
        vacationId = id.nextInt(2500);
        query = entityManager.createQuery("from Vacation v where v.id = :id")
               .setParameter("id", vacationId);
    }
    return (Vacation) query.getResultList().get(0);
}

And this is the main body of the test:

@Test
public void testVacationApprovalResultDao () {
    Vacation testVacation = findRandomVacation();
    VacationApproval testVacationApproval = findVacationApprovalDAO(testVacation);
    List<VacationApprovalResult> testVacationApprovalResult = getVacationApprovalResultByManager(testVacationApproval);
    Set<VacationApprovalResult> setVacationApprovalResult = testVacationApproval.getVacationApprovalResults();   //mistake is here
    List<VacationApprovalResult> testVacationApprovalResult2 = new ArrayList<VacationApprovalResult> ();
    testVacationApprovalResult2.addAll(setVacationApprovalResult);

    assertEquals(testVacationApprovalResult, testVacationApprovalResult2);
}

Answer 1

I suspect this is where you get the NullPointerException :

Integer vacationId = null;
Query query = entityManager.createQuery("from Vacation v where v.id = :id")
            .setParameter("id", vacationId);

This is before you've set it from Random - you're setting the parameter to a null value.

To be honest though, this isn't a good way of finding a random record from the database unless you know that there are vacations with every ID from 0 to 2499.

Basically you want the random choice to be performed at the database as that's where the data is. An alternative would be to fetch all the vacation IDs, pick a random element of that set, and then load the data for it - but that's not pleasant either.

Answer 2

change

 Integer vacationId = null;

to

Integer vacationId = 0;

Answer 3

Why would you do this ?

Integer vacationId = null;
Query query = entityManager.createQuery("from Vacation v where v.id = :id")
            .setParameter("id", vacationId);

Is it possible for vacationId to hold a null value ? if not then try with the random at once.