Why is one of the variables in an object being changed within a constructor?

Question

I apologise for the poorly worded question - it is hard to explain consisely.

I have written 3 classes - one to represent complex numbers, one to represent polynomials with complex coefficients and the final to perform Newton-Raphson algorithm on said polynomials to find estimates of roots.

The problem is, when i create an instance of the Newton class, with this being the constructor:

public Newton(Polynomial p) {
       this.f = p;
       this.fp = this.f.derivative();

it has no problem with setting f to p, but then when

this.fp = this.f.derivative();

is run it changes this.f to this.f.derivative(); too.

this is the code for the derivative:

public Polynomial derivative() {
        ArrayList<Complex> derivativeCoeff = coeff;
        if (derivativeCoeff.size() == 1){
            return new Polynomial();
        }
        derivativeCoeff.remove(0);
        for (int i = 1; i <= derivativeCoeff.size() ;i++){
            derivativeCoeff.set(i-1,derivativeCoeff.get(i-1).multiply(i));

        }
        Polynomial test = new Polynomial(derivativeCoeff);
        return test;  
    }

This is the main for Newton:

ArrayList<Complex> coeff = new ArrayList<Complex>();
        coeff.add(new Complex(-1.0,0.0));
        coeff.add(new Complex());
        coeff.add(new Complex());
        coeff.add(new Complex(1.0,0.0));

        Polynomial p = new Polynomial(coeff);
        System.out.println("the polynomial being used = "+p);
        Newton n = new Newton(p);

        System.out.println("f = "+n.getF());
        System.out.println("fp = "+n.getFp());

and this is the output I get:

the polynomial being used = (-1.0+0.0i)+(0.0+0.0i)X+(0.0+0.0i)X^2+(1.0+0.0i)X^3
f = (0.0+0.0i)+(0.0+0.0i)X+(3.0+0.0i)X^2
fp = (0.0+0.0i)+(0.0+0.0i)X+(3.0+0.0i)X^2

when f should be equal to p.

Please ask if any more information or code is required. I am not experienced with asking questions on SO and would appreciate any help I can get.

No, it is not. Should it be? We recieve the skeleton and just have to fill in the functions, and it was not static initially... — Deema Bowgen, Mar 15 '20 at 17:03

Andy Turner · Accepted Answer · 2020-03-15T18:20:19.470

ArrayList<Complex> derivativeCoeff = coeff;

Doesn't copy the list: updating derivativeCoeff updates coeff too.

Take a copy:

ArrayList<Complex> derivativeCoeff = new ArrayList<>(coeff);

Or, you can remove the first element before taking the copy (obviously checking for emptiness first):

ArrayList<Complex> derivativeCoeff = new ArrayList<>(coeff.subList(1, coeff.size()));

Or you can copy and update the elements directly with a stream.

ArrayList<Complex> derivativeCoeff =
    IntStream.rangeClosed(1, coeff.size() - 1)
        .mapToObj(i -> coeff.get(i).multiply(i))
        .collect(Collectors.toCollection(ArrayList::new));

Or even

return
    IntStream.rangeClosed(1, coeff.size() - 1)
        .mapToObj(i -> coeff.get(i).multiply(i))
        .collect(
            Collectors.collectingAndThen(
                Collectors.toCollection(ArrayList::new)),
                Polynomial::new));

score -1 · Answer 2 · answered Mar 15 '20 at 17:32

-1

To fix this, just change
ArrayList<Complex> derivativeCoeff = coeff; to ArrayList<Complex> derivativeCoeff = new ArrayList(coeff);

This has happened because Java is call by reference and coeff is a field of object Polynomial and when you mutate your Polynomial object it changes the state of coeff and change is reflected everywhere it was referenced.

answered Mar 15 '20 at 17:32

Mukesh Gupta

24
3

"This has happened because Java is call by reference" [No, it's not](https://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value). But anyway, that's irrelevant, as this is an assignment, not a method call. – Andy Turner Mar 15 '20 at 18:07

Why is one of the variables in an object being changed within a constructor?

2 Answers2