Why can't I instantiate and create Object without main method? (Stack Overflow Error)

The presence of main method is not relevant here. The scope in which you have declared the variables, however, is very important.

Have you walked through what happens in the first version of the code?

Create new instance of Overloads
  -> ov2 = Create new instance of Overloads
      -> ov2 = Create new instance of Overloads
         -> ov2 = Create new instance of Overloads

and so on. The variable ov2 is in scope of the class, thus it is initialized whenever an instance of the class is instantiated. This will never terminate until you run out of memory and get the stack overflow. Run it with a debugger for a clearer view.

The second version of the code only instantiates one instace of Overloads, in the scope of the main method. Thus creating one instance does not lead to the newly created instance creating new instance and so on..

You can do like this

public class Overloads {
    String uniqueID;
    static Overloads ov2 = new Overloads();

    public static void main(String[] args) {
        System.out.println("IN MAIN");
    }

    public void setUniqueID(String theID) {
        // II lots of validation code, and then:
        uniqueID = theID;
        System.out.println(uniqueID);
    }
}

this will create shared instance of Overloads, instantiation will be done only once, when class loaded