How to make object referencing to the same thing?

public class Number {
    private int j;
    public Number(){
        j = 3;
    }

    public Number(int m){
        j = m;
    }

    public void SetNum(int n){
        j = n;
    }

    public int GetNum(){
        return j;
        }

    }

/// (different file, same directory) 

    public class User {
        private Number jim;

        public User(){
                this.jim = new Number();
            }

            public static void main(String []args){
                User keith = new User();
                User justin = new User();
                ??????????????????????????
            }
     }

I want keith.jim = justin.jim and justin.jim = keith.jim. Such that if i do keith.jim.SetNum(34), both keith and justin's jim.j will be 34. How to do that? This idea will be implement in a bigger piece of code.

Also, user.java and Number.java must exist. The context cannot be changed,I can add new methods but I cannot alter the context in Number (E.G put j in user instead of Number). Each user must have a number object which every users is referencing to as well.

Jon Skeet
people
quotationmark

Others have suggested creating just one User object. An alternative would be to have two User objects, but with the same reference for the value of the jim field:

User keith = new User();
User justin = new User();
justin.jim = keith.jim;

This would be useful if you have other fields where keith and justin need to be different, but any change in the object that the value of the jim field refers to must be seen by both.

Both of these are valid options, depending on your requirements.

A few points, however:

  • This means the original value of justin.jim because pointless; you could consider adding a User constructor taking a Number reference for the initial value of the field, allowing you to write:

    User justin = new User(keith.jim);
  • Shared mutable state like this can become tricky to reason about, particularly in the face of multiple threads
  • As noted in comments, you should rename your Number class to be something that isn't in java.lang (or java.util etc, if possible).
  • You should follow Java naming conventions for your methods, using camelCase instead of PascalCase - I'd call them getValue and setValue, personally.

people

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