Why is assertEquals false if it is the same date? Hibernate

Question

I'm generating one date and saving in a database through hibernate, and when I get the value and I compare with the value before it was inserted. The result is not equal!

I created the date as following

Date rightnow = Calendar.getInstance().getTime();

Task t1 = new Task("My task", rightnow);
taskDao.saveOrUpdate(t1);

Task taskR1 = taskDao.get(t1.getIdTask());
assertEquals("They should have to be equal dates",taskR1.getDate(),t1.getDate());

I'm getting this error

<2014-04-11 23:13:13.0> is different to <Fri Apr 11 23:13:13 CEST 2014>

java.lang.AssertionError:  
They should have to be equal dates  
expected:<2014-04-11 23:13:13.0>  
but was:<Fri Apr 11 23:13:13 CEST 2014>

---

Extra info related with the problem

Class Task

@Entity
@Table(name = "t_task")
public class Task {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "idTask")
    private long idTask;
    ...
    @Column(name = "date")
    private Date date;
    ...

Mysql table t_task

CREATE TABLE IF NOT EXISTS `mytask`.`t_task` (
  `idTask` INT(11) UNSIGNED NOT NULL AUTO_INCREMENT,
  `date` DATETIME NOT NULL
  ...

I created a new hashCode() and equals() functions in Task, with only date field and even so it is different.

@Override
public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + ((date == null) ? 0 : date.hashCode());
    return result;
}

@Override
public boolean equals(Object obj) {
    if (this == obj)
        return true;
    if (obj == null)
        return false;
    if (!(obj instanceof Task))
        return false;
    Task other = (Task) obj;
    if (date == null) {
        if (other.date != null)
            return false;
    } else if (!date.equals(other.date))
        return false;
    return true;
}

Any idea?

Answer 1

This is a complete mess caused by the java.sql.Timestamp messed up design, and by Hibernate returning instances of this class. Indeed, you're storing a java.util.Date instance into your entity. Hibernate transforms that to a java.sql.Timestamp to insert it in the database. But when it reads the data from the database, it doesn't trasform back the Timestamp into a java.util.Date. That works fine, because Timestamp extends Date.

But Timestamp should never have extended Date. Indeed, Date is precise up to the millisecond, whereas Timestamp is precise up to the nanosecond. To be able to compare the nanoseconds parts of two Timestamp, Timestamp overrides the equals() method, but breaks its general contract by doing so. The end result is that you can have date.equals(timestamp) being true, but timestamp.equals(date) being false.

My advice: never compare Date instances with equals(). Use compareTo() instead.