Thread: how to call single instance of class? singlet.

Hey question about creating a single instance of a class.
I have two classes and the first class I would use
CourseOfferingController cOffer= new CourseOfferingController(); but later found out its wrong since it keeps on creating a new instance. I added this to my other class
// singleton object
static StudentRecordController controller;
public static StudentRecordController getController() {
return controller;
}


so, not sure what I do in the first class to call it, I tried
StudentRecordController sAdd = StudentRecordController.getController();
but now program crashes on this line Student s = sAdd.addStudent(studentid,studentname, studentnamel);
It worked before when I had CourseOfferingController cOffer= new CourseOfferingController(); but not now so Im guessing
StudentRecordController sAdd = StudentRecordController.getController(); is the problem.

thanks

--- Update ---

nvm solved it, had to add if(controller == null)
{
controller = new StudentRecordController();
}

Singleton concept is basically implemented when we require to use a single instance of a class throughout the application. For this we follow some rules.

1. We don't allow any class to create the object of the singleton class except the class itself for which we need to make the constructor the class private.

private StudentRecordController(){}
2. Then again we create the object of the singleton class in the same class within a method which is your getController method but strictly checking that not more than one object is returned from that method so that whenever that method is called from any other class it always returns the same object of that class.

public static StudentRecordController getController() {
if (object == null) { //checks here whether the object of the class has already been made or not.
object = new StudentRecordController();
}
return object;
}
3. To check the for the same object the reference varible we are using is "object" as mentioned above. That should be static and private so that whenever we access that particular variable it should always return the same object due to its single copy property.

private static StudentRecordController;