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();
    }       
}

Answer 2

First off, please, please, please avoid using getters and setters as much as possible. If you have both of them for the same field you are almost certainly doing something wrong. I don't care what the Java gurus are telling you. They don't know what they're talking about. This is not how OO works. OO is not a make-work project to turn field accesses into method calls. That doesn't actually encapsulate anything.

That said: if you return the date itself, then the calling code has a reference to your date object, and can use its full interface. Since dates are mutable objects, the interface includes things that can change the object state. Since the reference is to your date, your date's state will get changed. It doesn't matter how the calling code got the date (i.e. "with a getter").

Answer 3

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

Easily:

Comment comment = new Comment();
comment.getCommentDate().setTime(0); // now it's January 1, 1970 00:00:00 GMT.

Answer 4

Follow Tapas Bose example, we can do the following using JAVA 8 to handle NULL cases:

public class DateTest {
private Date date;

public DateTest() {

}

public Date getDate() {
    return Optional.ofNullable(date).map(Date::getTime).map(Date::new).orElse(null);
}

public void setDate(Date inputDate) {
    this.date= Optional.ofNullable(inputDate).map(Date::getTime).map(Date::new).orElse(null);
}}

Reference: Is there a way to copy Date object into another date Object without using a reference? (Nicolas Henneaux's answer)

Answer 5

The user can't "replace" the instance provided by getCommentDate(). However, the user can invoke getCommentDate().setMonth(10) and thereby modifying the date. Thus, if this is a concern, I'd advise you to return a copy of the "original" instance.

Answer 6

Since java.util.Date is mutable, it could be changed via the getter like this:

getCommentDate().setYear(2011)

This will cause the commentDate on the comment to be changed to the year 2011. All other set methods on Date can be called as well off course, just an example.

Answer 7

Note: Do not return mutable objects via getters eg. date (before Java 8). It can always be reset by a rogue programmer. Lets say you write a program where social security benefits of an employee is calculated based on the years of work.

public class Employee {
// instance fields
private String name;
private String nickName;
private double salary;
private Date hireDay;

// constructor
Employee(String name, String aNickName, double aSalary, int aYear,
        int aMonth, int aDay) {
    this.name = name;
    nickName = aNickName;
    salary = aSalary;
    GregorianCalendar cal = new GregorianCalendar(aYear, aMonth - 1, aDay);
    hireDay = cal.getTime();
}
//needs to be corrected or improved  because date is a mutable object
public Date getHireDay() {
    return hireDay;
}

A hacker/bad programmer can reset the date using a setter

Employee john = new Employee("John", "Grant", 50000, 1989, 10, 1);
    Date d = john.getHireDay();

    // Original hire date is Oct 1, 1989
    System.out.println("Original hire date "+ d.getTime()));

    long tenYearsInMilliseconds = 10 * 365 * 24 * 60 * 60 * 1000L;
    long time = d.getTime();

    // Hire date after hacker modifies the code
    d.setTime(time - tenYearsInMilliseconds);
    System.out.println("Hacked hire date "+john.getHireDay().getTime()));
}

Instead..return a clone of the date method for Java 7 or use LocalDate Class for Java 8

 // for Java 7
public Date getHireDay() {
    return (Date)hireDay.clone();
}

//for Java 8
public LocalDate getHireDay() {
    return hireDay;
}

Answer 8

In Java you are dealing with references. When you've a getter and returning your commentDate then you're in fact returning a reference to the object. That means that it is the same object like in your private field the caller can operate on due to reference returned by getter.