Question

I am developing a library and I don’t want the user to create an instance of a class directly with the new keyword, instead I have special methods to create objects. So is it possible if the user instantiates a class using the new keyword, that the compiler gives error.
For Example:

public class MyClass {
    private MyClass(int i) {  }

    public MyClass createMyClassInt(int i) {
        return new MyClass(i);
    }

    private MyClass(double d) { }

    public MyClass createMyClassDouble(double d) {
        return new MyClass(d);
    }
}

So when the user tries to instantiate MyClass myClass = new MyClass();, the compiler will give an error.

Answer 1

Your code is nearly fine – all you need is to make your create methods static, and give them a return type:

public static MyClass createMyClassInt(int i) {
  return new MyClass(i);
}

public static MyClass createMyClassDouble(double d) {
  return new MyClass(d);
}

The methods have to be static so that the caller doesn’t already have to have a reference to an instance in order to create a new instance 🙂

EDIT: As noted in comments, it’s probably also worth making the class final.