inherited overridden toString method does not use the proper attributes

Question

I'm currently trying to understand inheritance better. Therefore I wrote a simple class to handle vectors and then wanted to create a class for 2D vectors that inherits from the Vector class. Here is the code for the Vector class:

'''

    public class Vector {

private double[] coordinates;

public Vector() {
    this.coordinates = new double[0];
}

public Vector(int dim) {
    this.coordinates = new double[dim];
    for(int i=0;i<dim;i++) this.coordinates[i] = 0;
}

public Vector(double[] values) {
    this.coordinates = new double[values.length];
    for(int i=0;i<values.length;i++) this.coordinates[i]=values[i];
}

public void set(double[] values) {
    for(int i=0;i<Math.min(this.coordinates.length, values.length);i++) this.coordinates[i]=values[i];
}

public double[] get() {
    return this.coordinates;
}

public double norm() {
    double sqSum =0;
    for(double i:this.coordinates) sqSum += i*i;
    return Math.sqrt(sqSum);
}

public int getDim() {
    return this.coordinates.length;
}

public double skalarprodukt(Vector other) {
    if(this.getDim()!=other.getDim()) return Double.NaN;
    double sp = 0;
    for(int i=0;i<this.getDim();i++) sp += this.coordinates[i]*other.coordinates[i];
    return sp;
}

public boolean isOrthogonal(Vector other) {
    if(Math.abs(this.skalarprodukt(other))<0.000001) return true;
    return false;
}

public void add(Vector other) {
    if(this.getDim()== other.getDim()) {
        for(int i=0;i<this.getDim();i++) this.coordinates[i] += other.coordinates[i];
    }
}

@Override 
public String toString() {
    String ret = "(";
    for(int i=0; i<this.coordinates.length;i++) {
        ret += this.coordinates[i];
        if(i<this.coordinates.length-1) ret+=", ";
    }
    ret+=")";
    return ret;
}

    }

'''

and here for the Vector2d class:

'''

    public class Vector2d extends Vector {

private double[] coordinates = new double[2];


public Vector2d() {
    this.coordinates[0] = 0;
    this.coordinates[1] = 0;
}

public Vector2d(double x, double y) {
    this.coordinates[0] = x;
    this.coordinates[1] = y;
}

    }

'''

Now, if I invoked the toString method for a Vector object, it does what it should (i.e. the Vector (1,1) appears as "(1,1)" ), but if I invoke it for a Vector2d Object, the returned String is always "()", as if the coordinate tuple was empty. When I add the toString() method into the Vector2d class (with copy and paste), however, it works fine.

Can anyone explain to me why that is and how I can make it work? Preferably without just copying the methods into the subclass.

Thanks

Because you overwrite the coordinates array in Vector2d. You need to use the inherited values, and the getters and setters to access it because it is private. — NomadMaker, Jun 14 '20 at 10:02
Does this answer your question? https://stackoverflow.com/questions/4716040/do-subclasses-inherit-private-fields — Joni, Jun 14 '20 at 10:08

score 1 · Accepted Answer · answered Jun 14 '20 at 10:08

You private fields in Vector2d and Vector are independent. They are only visible in class which declared them.

Instead of declaring new field, declare it as protected in your superclass:

protected double[] coordinates;

you will then be able to assign to it in your subclass:

public Vector2d() {
    this.coordinates = new double[2];
    this.coordinates[0] = 0;
    this.coordinates[1] = 0;
}

Tashkhisi · Answer 2 · 2020-06-14T10:54:58.163

0

Something that you are doing is called variable hiding and variable hiding is not the same as method overriding.

While variable hiding looks like overriding a variable (similar to method overriding), it is not. Overriding is applicable only to methods while hiding is applicable to variables.

see this link for further information:

overriding-vs-hiding-java-confused

edited Jun 14 '20 at 10:54

answered Jun 14 '20 at 10:30

Tashkhisi

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inherited overridden toString method does not use the proper attributes

2 Answers2