How to make class to allow creation of 3 objects only

Question

i was asked this question in my interview that how to make a class that can only allows 3 objects creation.

i suggested to take a static variable and a static function for creation of object and while returning reference of new object just check value in static variable to check count of no. of object already created.

second approach i told him to take 3 static object of same class in that class only and let the user use those object only.

please tell me the best approach to perform above operation.

Thanks

I would not want to work for a company that thinks this is a good interview question in any way. I would run away screaming, really. — Karl Knechtel, Dec 01 '10 at 07:38
@Karl - An interview question is not necessarily indicative of programming practices. I think your comment is a massive overreaction. — Quick Joe Smith, Dec 01 '10 at 07:48

Denis Tulskiy · Accepted Answer · 2010-12-01T07:50:25.357

3

I think your first approach is closer to the question: make private constructor and a static newInstance() method that checks how many instances were created before and returns null if it's more than 3.

Second approach is also ok, though.

EDIT @Saurabh: even though the question says nothing about what to do in case the object is gc-ed, let's develop this:

A dirty hack to the first solution: override finalize() method that decrements the static counter of objects.
A pool of objects with some sort of a locking mechanism that only allows three, let's say, users, use the objects ate the same time.

edited Dec 01 '10 at 07:50

answered Dec 01 '10 at 07:36

Denis Tulskiy

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What about if one of the object garbage collected – TalentTuner Dec 01 '10 at 07:39
Does it matter if any of the objects were GCed? The question is how to only all 3 to be created, not ensure that only 3 exist at any point in time. – Gabe Dec 01 '10 at 07:48
@Saurabh - Seems very unlikely so long as there is a static reference to them. – Quick Joe Smith Dec 01 '10 at 07:49

score 3 · Answer 2 · answered Dec 01 '10 at 07:42

3

In Java, the simplest solution would be to use an enum.

answered Dec 01 '10 at 07:42

Daniel Pryden

59,486
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score 2 · Answer 3 · answered Dec 01 '10 at 08:13

2

Step one would be to write your own VM. Reflection could be used to circumvent most of the answers here, a customised classloader would defeat them all. Moral of the story: sufficiently privileged code can do whatever it wants to your classes.

answered Dec 01 '10 at 08:13

CurtainDog

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+1 : for Reflection bit , i were aware of what you are talking about , simpley whole Singleton Pattern can be broken with reflection and i know that but sometime you just need to answer what is asked. – TalentTuner Dec 01 '10 at 08:54

score 1 · Answer 4 · answered Dec 01 '10 at 08:07

my implementation is would be as below , it is not perfect but my thoughts at initial

 public class ThreeInstances
{

    private static int TOTALINSTANCECOUNT = 0;

    private ThreeInstances()
    {
    }

     private object objLock = new object();

     private static List<ThreeInstances> objThreeInstances = new List<ThreeInstances>();

     public static ThreeInstances GetInstance()
     {

         if (TOTALINSTANCECOUNT < 3)
         {
             lock (objLock)
             {
                 objThreeInstances.Add(new ThreeInstances());
                 Interlocked.Increment(ref TOTALINSTANCECOUNT);
                 return objThreeInstances[TOTALINSTANCECOUNT];

             }

         }
         else
         {
             Random r = new Random(0);
          int value =    r.Next(2);
          return objThreeInstances[value];
         }


     }

     ~ThreeInstances()
     {
         Interlocked.Decrement(ref TOTALINSTANCECOUNT);

         if (TOTALINSTANCECOUNT < 3)
         {
             lock (objLock)
             {
                 objThreeInstances.Add(new ThreeInstances());
                 Interlocked.Increment(ref TOTALINSTANCECOUNT);
                 return objThreeInstances[TOTALINSTANCECOUNT];

             }

         }
     }
}

score 0 · Answer 5 · answered Apr 22 '15 at 12:40

public class Threeobject
{
    public static int objectcount { get; set; }

    public Threeobject()
    {
        if (Threeobject.objectcount >= 3)
        {
            throw new Exception("You cannot create more then three object");
        }
        Threeobject.objectcount++;
    }
}

score 0 · Answer 6 · answered Dec 01 '10 at 07:40

0

Singleton design pattern with some modifications two support three instances instead of one.

answered Dec 01 '10 at 07:40

sherbeny

11
3

score 0 · Answer 7 · answered Dec 01 '10 at 07:58

Something like this... simple Singleton Pattern... You can add direct access for the three instances via Getters.

import java.util.ArrayList;
import java.util.Collections;

public class ThreeInstanceClazz {

    private static ArrayList<ThreeInstanceClazz> instances= null;

    private ThreeInstanceClazz(){
        // Only a private constructor!!
    }

    public static ThreeInstanceClazz getInstance(){
        if(instances == null){
            instances = new ArrayList<ThreeInstanceClazz>(3);
        }

        if(instances.size() < 3){
            ThreeInstanceClazz newOne = new ThreeInstanceClazz();
            instances.add(new ThreeInstanceClazz());

            // return newly created
            return newOne;
        }else{
            // return random instance ... or anything else
            Collections.shuffle(instances);
            return instances.get(0);
        }
    }
}

score 0 · Answer 8 · edited May 23 '17 at 11:51

0

Check out Noldorin's answer in this thread. May this would be helpful.

edited May 23 '17 at 11:51

Community

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answered Dec 01 '10 at 10:10

VJOY

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How to make class to allow creation of 3 objects only

8 Answers8