java: abstract classes and its variables

Question

I am fairly new to java so I tried to implement an example and use abstract classes but my lack of OO knowledge makes me wonder why I cannot use a private variable from the abstract class in a "concrete" class extending it.

Here is my code:

abstract class Equation{
    private double[] c;//oefficient
    public static int degree(double[] coeff) {
        return coeff.length;
    }
    public abstract double[] solve();
}

class QuadraticEquation extends Equation{
    public double[] solve() {
        double[] solution;
        double discriminant = c[1]*c[1]-4*c[0]*c[2];
        if (discriminant < 0) {
            solution = new double[] {Double.NaN,Double.NaN};
        }
        else {
            solution = new double[] {(c[1]+Math.sqrt(discriminant))/(2*c[0])
                                    ,(c[1]-Math.sqrt(discriminant))/(2*c[0])};
        }
        return solution;
    }
}

The Error I get is

 c has private access in Equation

i could resolve this by making c a public variable, but I guess there is a better way to do this.

Bottom Line: How do I access the variable c.

Instead of documenting what c means with a comment, why not name your variable coefficient in the first place? Why is this variable defined in the base class since it doesn't use it at all? — JB Nizet
– JB Nizet, Commented Jun 29, 2013 at 22:34
(1) sorry I forgot renaming it; (2) it is used in the subclass for calculating the solution — epsilonhalbe
– epsilonhalbe, Commented Jun 29, 2013 at 22:42
If it's only used in the subclass, then declare it in the subclass. — JB Nizet
– JB Nizet, Commented Jun 29, 2013 at 22:44
but declaring it in every subclass seems like code duplication - and as every equation is determined by its coefficients, it seemed reasonable to put it in the superclass. — epsilonhalbe
– epsilonhalbe, Commented Jun 29, 2013 at 22:58

ATG · Accepted Answer · 2013-06-29 22:30:40Z

3

The solution you're looking for is to make c protected, so that it will have visibility to subclasses.

answered Jun 29, 2013 at 22:30

ATG

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Comments

nachokk · Accepted Answer · 2013-06-29 22:38:13Z

2

It's simple,

Modifier    Class   Package Subclass World
public       Y         Y       Y      Y
protected    Y         Y       Y      N
no modifier  Y         Y       N      N
private      Y         N       N      N

See the table, you have private then check if your subclass has access NO. You have to use protected, or no modifier if it's in the same package but if you change package then cannot acces anymore.

BTW use the most restrictive access level that makes sense for a particular member. Use private unless you have a good reason not to, you have a reason then you have to use another.

answered Jun 29, 2013 at 22:38

nachokk

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Comments

Tim · Accepted Answer · 2013-06-29 22:33:42Z

0

you can write an public Getter public double[] getC() { return c; } in your abstract class.

answered Jun 29, 2013 at 22:33

Tim

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1 Comment

JB Nizet Over a year ago

By that logic, why wouldn't we make everything public? Ever heard of encapsulation?

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