The use of private constructor is to serve singleton classes. A singleton class is one which limits the number of objects creation to one. Using private constructor we can ensure that no more than one object can be created at a time. By providing a private constructor you prevent class instances from being created in any place other than this very class. We will see in the below example how to use private constructor for limiting the number of objects for a singleton class.
Example of Private Constructor
public class SingleTonClass {
//Static Class Reference
private static SingleTonClass obj=null;
private SingleTonClass(){
/*Private Constructor will prevent
* the instantiation of this class directly*/
}
public static SingleTonClass objectCreationMethod(){
/*This logic will ensure that no more than
* one object can be created at a time */
if(obj==null){
obj= new SingleTonClass();
}
return obj;
}
public void display(){
System.out.println("Singleton class Example");
}
public static void main(String args[]){
//Object cannot be created directly due to private constructor
//This way it is forced to create object via our method where
//we have logic for only one object creation
SingleTonClass myobject= SingleTonClass.objectCreationMethod();
myobject.display();
}
}
Output:
Singleton class Example
Paru says
April 6, 2015 at 3:25 AMHi,
when I tried the above example, I was able to instantiate the class using new keyword,
i.e, SingleTonClass obj3=new SingleTonClass();
I presumed this wud not be possible for a private Constructor.
Pls advice if my assumption is wrong and why if that is the case.
Thanks,
Paru
Daryll David says
April 12, 2015 at 7:59 AM“SingleTonClass obj3=new SingleTonClass();” is inside the main method which is in the class SingleTonClass. Compilation succeeds because although it’s a private constructor, private members can be accessed within the class only.
remove the main method from the SingleTonClass and create a separate .java file with main method and test “SingleTonClass obj3=new SingleTonClass();”
omar says
January 11, 2016 at 9:45 AMI was going to say that , thanks !
Sandeep Saproo says
August 17, 2017 at 1:46 PMprivate Constructor is used to prevent object creation outside the class.
We can create more than one object in Singleton class.
Trinanjana says
May 23, 2015 at 4:23 AMCan you please explain the difference between a static class and a singleton class?
Ashir says
August 18, 2015 at 1:27 PMThe static class means that call the method without creating it instance of object. In static class you freely called the method without making refrernce of object but in same class you to do….
The singleTOn class in which you have to follow the private constructor rule ..
Gautham says
October 4, 2015 at 4:29 PMWhat is the use of not creating more than one object at a time? Does certain programs require them?
karan says
December 10, 2015 at 5:17 AMYes, operations like print spooling etc would require the use of singleton pattern.
RAJ says
October 8, 2015 at 7:08 PMI have created a child class “trial” and instead of creating objects from parent class, i am creating it from child class and i have created 3 objects.
I think it should have given error by doing this?
Er.chandramohan says
March 25, 2016 at 6:22 AMthank you sir but i need another best example for private constructor…i am using a private constructor in another class i can access a private constructor in main class
Hoan Nguyen says
May 17, 2016 at 5:54 PMDear guys,
could you please explain to me the following code:
” private static SingleTonClass obj=null; ”
Thank you in advance,
Hoan
Hau Nguyen says
August 5, 2016 at 10:35 AMprivate static SingleTonClass obj=null;
↓
１．private -> only this SingleTonClass can use it.
２．static -> Because objectCreationMethod() use this variable “obj”. But, objectCreationMethod() is static method, so “obj” must be static.
３．null ->
– At the first time we call objectCreationMethod(), “obj” is null, then we create new object by this command:
obj= new SingleTonClass(); (*)
– The second time we call objectCreationMethod(), “obj” is NOT null because of above command (*).
So, do not create new object, reuse the object which is created at the first time we called objectCreationMethod().
– The same logic for n time calling objectCreationMethod(). There is one and only one object of SingleTonClass is created at the first time.
This is why it is called SingleTon.
Hope this help.
Thanks,
Hau Nguyen
swetha says
May 30, 2016 at 2:23 PMwhat if we declare more than one constructor as private.I mean parameterised constructor..still we can create more than one instance..please provide some more clarity on private constructor
VallaDanger says
August 15, 2016 at 9:59 PMNot thread-safe, violates singleton pattern. You either use Enum or double-check.
Pranav says
July 20, 2017 at 1:33 PMi tried to make two objects at the same time, and i didn’t face any issue, objects got created and methods could be called easily … why so ??