Accessing private variable of Model class without getters

Question

I have a Model class defined in my project. and as usual it has some private variables and public getters and setters

public class Person{

 private ArrayList<String> mark;

 public void setMark(ArrayList<String> mark){
  this.mark = mark;
 }

 public void getMark(){
  return this.mark;
 }
}

Suppose in some other class I am using this Model like

Person person = new Person();
ArrayList<String> mark = new ArrayList();
mark.add("10");
mark.add("15");
mark.add("18");
person.setMark();

then the private variable of person holds the value "my name", the I am accessing the variable using public getter of the class like

ArrayList<String> localMark = person.getMark()

so as per my knowledge person.getMark() returns the reference of private variable name, so if I modify the local variable 'localMark', then it will effect the private variable of Person class, so there it breaks the private property of the variable

ex:

 ArrayList<String> localMark = person.getMark();
 System.out.println(localMark.get(0)); // will be "10"
 localMark.set(0,"25") // person.mark will be changed
 System.out.println(person.getMark().get(0)); //will be printing "25"

most of the developers following the same design pattern I guess, but what is the correct way to create Models

EDIT

As per the comment of vinod I checked, and Strings it passes value but not reference but for ArrayList... it returns reference.

*System.out.println(person.getName()); //will be printing "new Name"* ==> Did you check?. I think you should check what is printed :) — TheLostMind, Nov 27 '15 at 09:53
I din't but if it returns the reference it should be the value isn't it ? — droidev, Nov 27 '15 at 09:54
@Raf locaName is a local variable of another class, I can modify it with any value there — droidev, Nov 27 '15 at 09:56
I down-voted your question because you posted something which won't happen to Strings. *re-assigning* the reference won't change a thing because java is *pass-by-value*. Next, the changes you do on the instance in a different class will affect the actual instance if you have *mutable* instances returned by your getter (which you shouldn't, you should return defensive copies of mutable instances) — TheLostMind, Nov 27 '15 at 09:56
@VinodMadyalkar I don't think, it requires a down vote. Adding an answer expalining what's wring will be the nice approach afaik :) — Suresh Atta, Nov 27 '15 at 09:57
@sᴜʀᴇsʜᴀᴛᴛᴀ - But he should try his code before posting. :). Don't you think my comment gives him enough info / hints? :) — TheLostMind, Nov 27 '15 at 09:59
@VinodMadyalkar That is what my point. You can guide him with an answer. — Suresh Atta, Nov 27 '15 at 10:00
@Raf - Well, *Reflection* could be used to break *immutability*. But the OP is merely re-assigning references — TheLostMind, Nov 27 '15 at 10:10
@VinodMadyalkar interesting. I am yet to learn Reflections. Thanks. — Raf, Nov 27 '15 at 10:15
@Raf - No problemo. Check [this](http://stackoverflow.com/questions/20945049/is-a-java-string-really-immutable) if you are interested :) — TheLostMind, Nov 27 '15 at 10:17
please reload the question, I have tried and checked,for strings there is no issues, it returns the value, not reference. — droidev, Nov 27 '15 at 10:21
@VinodMadyalkar I think its the time to remove the down vote :) — droidev, Nov 27 '15 at 10:32
@VividVervet - Done. Check my answer as to why the value is reflected in case `List` is used — TheLostMind, Nov 27 '15 at 10:36

score 3 · Answer 1 · answered Nov 27 '15 at 10:02

You have a reference (name) to an object instance (the value of name). As the reference is private, you're in full control of it.

When you return a reference, you in fact return it 'by value', meaning that a copy of the reference is returned. Both references point to the same value (the String instance)).

An outside caller obtaining the reference can assign a new value, but your model's own reference is unaffected by that and still points to the value.

It's like a dog (object) on a leash (reference).

When you return a reference you're returning a new leash onto the same dog.
The owner of the new reference can modify your dog (pet it, shave it, whatever) when the dog is mutable (which Strings are not, so it cannot be modified)
...or he can attach a new dog to his leash
...but he can never (reflection aside) attach YOUR leach to another dog.

Liked the Dog and leash analogy. +1 – TheLostMind Nov 27 '15 at 10:10 — TheLostMind, Nov 27 '15 at 10:10

score 1 · Accepted Answer · answered Nov 27 '15 at 10:05

If the instance being exposed by call to get() is mutable, then whatever changes you make in some other place will be reflected in the instance everywhere it is used.

Example :

methodX classA - 
 List<String> locaNamesList = person.getNamesList();
 locaNamesList.clear();

Somewhere else
methodY classB -
List<String> locaNamesList = person.getNamesList(); // note the same person instance should be used.
//locaNamesList will be empty here

Just re-assigning the reference won' change anything.

List<String> locaNamesList = person.getNamesList();
locaNamesList = null; // won't change the actual list. You are setting local field locaNamesList to null and not the actual instance.

You have to use defensive-copies of mutable instances and pass them around if you don't want the original instance to be changed by external players (provided you can't make the instance itself immutable)

then how do I redesign my model class ? what you mean by defencive copies ? — droidev, Nov 27 '15 at 11:23
@VividVervet - Defensive copy means - *creating a new Object which is exactly same as the original. This is usually done via a copy constructor . You can use a factory or other method*. Thus you will return a copy of your instance and not the actual instance itself — TheLostMind, Nov 27 '15 at 11:59

Accessing private variable of Model class without getters

2 Answers2