Junit mockito when(..).thenReturn() throws NullPointerException

Question

Can anyone explain me the below scenario
Code to be tested
UserTransaction.java

@Override
public ServiceResponse<User> get(String name) {
    ServiceResponse<User> response = new ServiceResponse<User>();
    List<Map<String, Object>> exp = new ArrayList<Map<String, Object>>();
    Map<String, Object> map = new HashMap<String, Object>();
    map.put("expression", "eq");
    map.put("property", "name");
    map.put("value", name);
    exp.add(map);
    List<User> users = userDao.getByCriteria(exp);
    if (!users.isEmpty()) {
        response.setResponse(users.get(0));
    } else {
        response.setResponse(null);
    }
    return response;
}   

UserDao.java

public List<User> getByCriteria(List<Map<String, Object>> exp) {
  DetachedCriteria criteria = DetachedCriteria.forClass(User.class);
  for (Integer i=0;i<exp.size();i++){
    String expression = (String) exp.get(i).get("expression");
    String property = (String) exp.get(i).get("property");
    if(expression.equals("eq"){
       criteria.add(Restrictions.eq(property,exp.get(i).get("value")));
    }
  }
  return hibernateTemplate.findByCriteria(criteria);
 }

UserTransactionTest.java

private UserTransaction userTransactions = new UserTransaction();
private UserDao userDao = mock(UserDao.class);

@Test
public void testGet() {
   User user = new User();
   user.setName("Raman");
    try {
        when(userDao.getByCriteria(anyList())).thenReturn(user);
    } catch (Exception e) {
        e.printStackTrace();
    }
    ServiceResponse<User> response = userTransactions.get("raman");
    User result = response.getResponse();
    assertEquals("Raman", result.getName());
    assertEquals(0, response.getErrors().size());
}

works fine.

But instead of "anyList()" I passed a user-defined list "myList"

List<Map<String,Object>> myList = new ArrayList<Map<String,Object>>();
Map<String,Object> map = new HashMap<String,Object>();
map.put("expression","eq");
map.put("property","name");
map.put("value","raman");
myList.add(map);
when(userTransactions.getByCriteria(myList)).thenReturn(user);

Throws NullPointerException at the line assertEquals(). Why? What actually happens if anyList() is given?

Answer 1

I'm sure you've already solved your problem by now, but in case anyone stumbles upon the same issue, here's the answer:

In the code you've provided, you are not using the mocked myList you've created. The get() method always calls userDao.getByCriteria(exp), a local variable.

This is why anyList() works, while myList doesn't.

If you do want to test the expression, List<Map<String,Object>> exp should be a member of your class, not a local variable:

public class UserTransaction {
    private List<Map<String,Object>> exp;

    public UserTransaction() {
        // creating a default exp value
        Map<String, Object> map = new HashMap<String, Object>();
        map.put("expression", "eq");
        map.put("property", "name");
        map.put("value", name);
        exp.add(map);
    }

    // getters and setters for exp

    public ServiceResponse<User> get(String name) {
        ServiceResponse<User> response = new ServiceResponse<User>();
        List<User> users = userDao.getByCriteria(exp);
        if (!users.isEmpty()) {
            response.setResponse(users.get(0));
        } else {
            response.setResponse(null);
        }
        return response;
    }
}

And in your Test:

private UserTransaction userTransactions = new UserTransaction();
private UserDao userDao = mock(UserDao.class);

@Test
public void testGet() {
    User user = new User();
    user.setName("Raman");

    // creating a custom expression
    List<Map<String,Object>> myList = new ArrayList<Map<String,Object>>();
    Map<String,Object> map = new HashMap<String,Object>();
    map.put("expression","eq");
    map.put("property","name");
    map.put("value","raman");
    myList.add(map);

    // replacing exp with the list created
    userTransactions.setExp(myList);
    // return user when calling getByCriteria(myList)
    when(userDao.getByCriteria(myList)).thenReturn(user);

    ServiceResponse<User> response = userTransactions.get("raman");
    User result = response.getResponse();
    assertEquals("Raman", result.getName());
    assertEquals(0, response.getErrors().size());
}

Answer 2

If your code is complete (I suspect it may not be) then you haven't specified the mock object that contains the get() method. This should be present in the call to when(...)

I am expecting code like this...

UserDao mockDao = mock(UserDao.class);

when(mockDao.get(list)).thenReturn(users);