Java Example program to restrict a class from creating not more than three objects

Question: How to restrict a class from creating multiple objects?

• Restring a class from creating multiple objects or restricting class from creating not more than three objects this type of interview questions will come for experience interview or written tests.


• Yes we can restrict a class from creating multiple objects.
• As we know using singleton we can restrict a class from creating multiple objects it will create single object and share it.
• Same design pattern we can apply here with counter 
• In this we will take a static variable counter to check how many objects created
• As we can access static variables in constructor and static variables are class level we can stake help of static variable to count number object created.
• First three time we will create new objects and forth time we need to return 3rd object reference. if we don't want same object 4th time we can return null
• By printing the hashcode() of the object we can check how many objects created. 
•  

Basic java example program to restrict a class from not to create more than three instances

1. package com.instanceofjava;
2.  
3. public class RestrictObjectCreation{
4. 
   
5. private static RestrictObjectCreationobject;
6. public static int objCount = 0;
7.  
8. private RestrictObjectCreation()
9. {
10.      System.out.println("Singleton(): Private constructor invoked");
11.  
12. objCount  ++;
14. }
15.  
16. public static RestrictObjectCreation getInstance()
17. {
18.  
19. if (objCount < 3)
20. {
21.  
22. object = new RestrictObjectCreation();
23.  
24.  }
25.  
26. return object;
27.  
28. }
29.  
30. }
     

1. package instanceofjava;
2.  
3. public class Test{
4. 
   
5. public static void main(String args[]) {
6.  
7. RestrictObjectCreation obj1= RestrictObjectCreation.getInstance();
8. RestrictObjectCreation obj2= RestrictObjectCreation.getInstance();
9. RestrictObjectCreation obj3= RestrictObjectCreation.getInstance();
10. RestrictObjectCreation obj4= RestrictObjectCreation.getInstance();
11. RestrictObjectCreation obj5= RestrictObjectCreation.getInstance();
12.  
13. System.out.println(obj1.hashCode());
14. System.out.println(obj2.hashCode());
15. System.out.println(obj3.hashCode());
16. System.out.println(obj4.hashCode());
17. System.out.println(obj5.hashCode());
18.  
19. }
20. }

Output:

1. RestrictObjectCreation(): Private constructor invoked
2. RestrictObjectCreation(): Private constructor invoked
3. RestrictObjectCreation(): Private constructor invoked
4. 705927765
5. 366712642
6. 1829164700
7. 1829164700
8. 1829164700

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