How to check if two constructors are equal to one another with three variables?

Question

I need to be able to check if variables a, b, and c from one constructor are equal to a, b, and c from another constructor (Quadratic q). I get the feeling that my copy constructor might be wrong and this could potentially be the problem as well. I'm trying to accomplish this through the last method. Ignore what I have in there now, I'm sure it's completely wrong. I'd appreciate any help I can get. This is what I have so far:

import java.util.Scanner;

public class Quadratic
{
    // instance variables - replace the example below with your own
    private double a;
    private double b;
    private double c;

    /**
     * Assignment constructor
     */
    public Quadratic(double aIn, double bIn, double cIn)
    {
        a=aIn; b=bIn; c=cIn;
    }

    /**
     * Copy constructor of class Quadratic for objects 
     */
    public Quadratic(Quadratic q)
    {
        a=q.a;b=q.b;c=q.c;
    }

    /**
     * Default constructor - uses Scanner class.
     */
    public Quadratic()
    {
       Scanner in = new Scanner(System.in);
       System.out.println("Enter a: ");
       double a=in.nextDouble();
       System.out.println("Enter b: ");
       double b= in.nextDouble();
       System.out.println("Enter c: ");
       double c= in.nextDouble();
    }

    /**
     * Returns an expression for the quadratic, i.e., 1.0x^2 + 3.0x + 2.0
     */
    public String toString()
    {
        return a+"x^2 + "+b+"x + " + c;
    }

    /**
     * Returns true if a, b, and c all match for this & q; false otherwise.
     */
    public boolean equals(Quadratic q)
    {

        if (Quadratic(q).equalsQuadratic(a,b,c))
            return true;
        else
            return false;
    }

}

Why do you have a copy constructor? It strikes me as a C++ idiom that you don't need in Java. — John Kugelman, Oct 27 '18 at 15:45
You want to be sure that `a,b,c` are not same with another `a,b,c`? — Oleg Cherednik, Oct 27 '18 at 15:45
@oleg.cherednik No, I want to be sure that a,b,c are the same as a,b,c from another constructor. So if two instances of that class have the same a,b,c values, it will return true. — J.Cole, Oct 27 '18 at 15:47
why don't you just override equals method and use a,b,c for comparison? — The Scientific Method, Oct 27 '18 at 15:50
@J.Cole So I guess you want to create two instances of `Quadratic` and compare them? If they have the same a, b, c then return `true`? If so, this has little to do with constructors. Or are you saying that you want to _test_ whether the copy constructor is working correctly, so you tried to write an `equals` method but failed? — Sweeper, Oct 27 '18 at 15:55
@Sweeper Yes, I am trying to compare two instances of Quadratic. How would I do that? — J.Cole, Oct 27 '18 at 16:05

Oleg Cherednik · Answer 1 · 2018-10-27T16:00:04.190

Hint. If you use IntelliJ IDEA, then right click and Generate... -> equals() and hashCode();

public final class Quadratic {

    private final double a;
    private final double b;
    private final double c;

    public Quadratic(double a, double b, double c) {
        this.a = a;
        this.b = b;
        this.c = c;
    }

    public Quadratic(Quadratic quadratic) {
        this(quadratic.a, quadratic.b, quadratic.c);
    }

    @Override
    public String toString() {
        return a + "x^2 + " + b + "x + " + c;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (!(obj instanceof Quadratic))
            return false;
        Quadratic quadratic = (Quadratic)obj;
        return Double.compare(quadratic.a, a) == 0 &&
                Double.compare(quadratic.b, b) == 0 &&
                Double.compare(quadratic.c, c) == 0;
    }

    @Override
    public int hashCode() {
        return Objects.hash(a, b, c);
    }
}

Use final class variable if it is possible.
Use this to access class variable, but not mehtod parameter
Scanner should be closed
Do not use Scanner in constructor, this is not belong to the class. Use it in the client code and then call constructor
double values should be compared with Double.compare
equals() + hashCode() - this is must have (well, hashCode() could be useless if you not plan to use this class as a key in hash map, but still - this should be automatic)
When implement equals() + hashCode() manually, do pay attention on specification.

@VinceEmigh cause quadratic.a will fail .. - It is correct code syntax, do learch documentation :-) — Oleg Cherednik, Oct 27 '18 at 16:13
@JohnKugelman https://stackoverflow.com/questions/8081827/how-to-compare-two-double-values-in-java — Oleg Cherednik, Oct 28 '18 at 19:08
That doesn't answer my question. `Double.compare()` still does an exact comparison, same as `==`. — John Kugelman, Oct 28 '18 at 20:58

How to check if two constructors are equal to one another with three variables?

1 Answers1