Thread: Need help with fixing an error dealing with interface implementation

Hi everyone, I wrote the following code for a Fraction class that implements an interface called StdMathOps which consists of 5 methods called add, sub, div mult, and toString. I am getting a compilation error which says "Fraction is not abstract and does not override abstract method mult(Object) in StdMathOps." I do not intend for Fraction to be an abstract class as it does implement each of the 5 methods in the interface, and am wondering if anyone can help me in identifying what needs to be corrected. Any help would be greatly appreciated.

Here is the interface:

 
public interface StdMathOps 
{
    public Object add(Object o);
    public Object sub(Object o);
    public Object div(Object o);
    public Object mult(Object o);
    @Override
    public String toString();
}

Here is the Fraction class:

 
public class Fraction implements StdMathOps
{
    private int num;
    private int den;
 
    public Fraction(int a, int b) 
    {
        num = a;
        den = b;
    }
 
    public Fraction() 
    {
        num = 1;
        den = 1;
    }
 
    @Override
    public String toString()
    {
        return num + "/" + den;
    }
 
    public double getDecimal()
    {
        double decimal = ((double)num / den);
        return decimal;
    }
 
    public String reduce()
    {   
        int n = num;
        int d = den;
 
        while(n != d)
        {
            if(n > d)
                n = n - d;
 
            if(d > n)
                d = d - n;
        }
        return((num / d) + "/" + (den / d));
    }
 
    public String toMixed()
    {
        int a = num;
        int b = den;
        int whole = num / den;
 
        while(a != b)
        {
            if(a > b)
                a = a - b;
            if(b > a)
                b = b - a;
        }
 
        Fraction g = new Fraction(num % den / b, den / b);
 
        if(whole > 0)
            return whole + " " + g.toString();
        else
            return g.toString();
    }
 
    public Fraction add(Fraction a)
    {
        int newNum = (((a.den * den) / a.den) * a.num) + (((den * a.den) / den) * num);
        int newDen = (a.den * den );
        Fraction x = new Fraction(newNum, newDen);
        return x;
    }
 
    public Fraction sub(Fraction a)
    {
        int newNum = ((num * a.den) - (a.num * den));
        int newDen = den * a.den;
        Fraction x = new Fraction (newNum, newDen);
        return x;
    }
 
    public Fraction div(Fraction a)
    {
        int newNum = num * a.den;
        int newDen = den * a.num;
        Fraction x = new Fraction (newNum, newDen);
        return x;
    }
 
    public Fraction mult(Fraction a)
    {
        int newNum = num * a.num;
        int newDen = den * a.den;
        Fraction x = new Fraction (newNum, newDen);
        return x;
    }
}

Hi jc,

You need to actually have the function definition match that of the interface. So you should change from Fraction in the parameters and return to Object.

Or, perhaps a smarter solution would be to use generics in the StdMathOps interface, such as the following. ( I'll let you work out the last part of the puzzle.... )

public interface StdMathOps<T>
{
    public T add(T o);
    public T sub(T o);
    public T div(T o);
    public T mult(T o);
    @Override
    public String toString();
}

If you would like more information about generics, then check this out: http://docs.oracle.com/javase/tutorial/java/generics/

Got it, thank you for your assistance.