CODEWITHSUNDEEP
c#javaoop
Dr. Rajesh Rolen
Dr. Rajesh Rolen

Reputation: 14285

How to make class to allow creation of 3 objects only

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

Upvotes: 2

Views: 5480

Answers (8)

Uzair Ahmed
Uzair Ahmed

Reputation: 1

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++;
    }
}

Upvotes: 0

VJOY
VJOY

Reputation: 3792

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

Upvotes: 0

CurtainDog
CurtainDog

Reputation: 3205

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.

Upvotes: 2

TalentTuner
TalentTuner

Reputation: 17556

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];

             }

         }
     }
}

Upvotes: 1

Gruber
Gruber

Reputation: 551

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);
        }
    }
}

Upvotes: 0

Denis Tulskiy
Denis Tulskiy

Reputation: 19177

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:

  1. A dirty hack to the first solution: override finalize() method that decrements the static counter of objects.
  2. 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.

Upvotes: 3

Daniel Pryden
Daniel Pryden

Reputation: 60957

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

Upvotes: 3

sherbeny
sherbeny

Reputation: 11

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

Upvotes: 0

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