Create a Defensive Copy in the Constructor

Question

The following is just example code to explain the problem I have trouble understanding:

Lets say I have the following Professor class, note the public getters and setters:

    public class Professor
    {
        public string id {get; set; }
        public string firstName{get; set;}
        public string lastName {get; set;}

        public Professor(string ID, string firstName, string lastname)
        {
            this.id = ID;
            this.firstName = firstName;
            this.lastName = lastname;
        }


    }

and Course:

public class Course
{
    string courseCode {get; private set;}
    string courseTitle {get; private set;}
    Professor teacher {get; private set;}

    public Course(string courseCode, string courseTitle, Professor teacher)
    {
        this.courseCode = courseCode;
        this.courseTitle = courseTitle;

    }
}

How would I make a defensive copy of the Professor object in the Course class? The example provided here does it like this with the date object.

fDateOfDiscovery = new Date(aDateOfDiscovery.getTime());

Can the same be done with the professor object in the Course class?

Update:

Taking the answer that was provided this is what I've come up with, is it correct?

public class Professor
{
 public string id {get; set; }
 public string firstName{get; set;}
 public string lastName {get; set;}

 Professor(string ID, string firstName, string lastname)
  {
       this.id = ID;
       this.firstName = firstName;
       this.lastName = lastname;
  }

 //This method can be either static or not
 //Please note that i do not implement the ICloneable interface. There is discussion in the community it should be marked as obsolete because one can't tell if it's doing a shallow-copy or deep-copy
 public static Professor Clone(Professor original)
 {
   var clone = new Professor(original.id, original.firstName, original.lastName);
   return clone;
 }
}

//not a method, but a constructor for Course
public Course (string courseCode, string courseTitle, Professor teacher)
{
    this.courseCode = courseCode;
    this.courseTitle = courseTitle;
    this.teacher = Professor.Clone(teacher)

}

You need to Deep-Clone your professor instance. Any more help? — Luis Filipe, Feb 18 '16 at 01:14
@How would I deep-clone it? Could you modify my professor instance and show me? — , Feb 18 '16 at 01:15
@Aron the asker points us to http://www.javapractices.com/topic/TopicAction.do?Id=15 — Luis Filipe, Feb 18 '16 at 01:55
Yes, you can clone it inside the constructor of Course class — Luis Filipe, Feb 18 '16 at 15:34
@LuisFilipe I've read that page several times, it has explanations, example code, even well commented from which you draw the conclusion that it's about defensive copying but it fails in saying what is defensive copying. At least an informal definition. I've also googled it to no success. Not even on Wikipedia. — mireazma, Nov 28 '20 at 10:50

Luis Filipe · Accepted Answer · 2016-02-18T01:54:11.857

1

You can Clone your professor instance.

Clone logic should be within Professor class.

You can then receive an already cloned professor instance in the Course constructor

public class Professor
{
 public string id {get; set; }
 public string firstName{get; set;}
 public string lastName {get; set;}

 Professor(string ID, string firstName, string lastname)
  {
       this.id = ID;
       this.firstName = firstName;
       this.lastName = lastname;
  }

 //This method can be either static or not
 //Please note that i do not implement the ICloneable interface. There is discussion in the community it should be marked as obsolete because one can't tell if it's doing a shallow-copy or deep-copy
 public static Professor Clone(Professor original)
 {
   var clone = new Professor(original.id, original.firstName, original.lastName);
   return clone;
 }
}

Then, when you invoke a new Course you'll do this

public Course AddCourse(string courseCode, string courseTitle, Professor original)
{
  var clonedProfessor = Professor.Clone(original);
  var course = new Course(courseCode, courseTitle, clonedProfessor);
  return course;
}

edited Feb 18 '16 at 01:54

answered Feb 18 '16 at 01:20

Luis Filipe

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Now, any changes that are made to the teacher object outside of the class would not reflect on the course object correct? The Course object is truly immutable? – Feb 18 '16 at 01:24
You must think on these objects as pointers (references). When you clone a professor you have two different professors which happen to have the same property values. If you then change one professor the other will not change – Luis Filipe Feb 18 '16 at 01:30
@Sebas i agree and will remove it – Luis Filipe Feb 18 '16 at 01:52
The lazy in me says just serialize the `Professor` instance, save that as a memento inside the `Course`, and rehydrate it whenever a disaster occurs. Your choice of json or xml... – code4life Feb 18 '16 at 03:25
Instead of having an AddCourse method in course, could I call Professor.Clone from inside the Course Constructor? Why did you remove the other method of creating a defensive copy? – Feb 18 '16 at 13:05
@Spaceman Not sure i completely understood your doubt. AddCourse method should be located in a separate class, namely a business class – Luis Filipe Feb 18 '16 at 15:00
@Spaceman I removed the other method because it is the wrong way of doing it. The classes themselves should be the ones to know how to clone themselves – Luis Filipe Feb 18 '16 at 15:01
Many thanks for your time/help. Bare with, I'll update my code to show what I mean. – Feb 18 '16 at 15:15

Create a Defensive Copy in the Constructor

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