the good way of returning a mutable object

Question

Let's say I have a class Comment and I have a private field named commentDate which is a java.util.Date and with a getter named getCommentDate.

Why it's better to return a copy of that date ( return new Date(commentDate.getTime()) ) than simply returning that date...

How can a user change the object state of that Date since it's a getter, not a setter?

Answer 1

Since java.util.Date implements Cloneable you can easily clone the date, as:

public class DateTest {
    private Date date;

    public DateTest() {

    }

    public Date getDate() {
        return (Date) date.clone();
    }

    public void setDate(Date date) {
        this.date = (Date) date.clone();
    }       
}