Java接口和向下转换

Question

I have a Fraction class which implements an interface named FractionInterface . In FractionInterface I have specified a method FractionInterface add(FractionInterface secondFraction); . I am having trouble implementing this method in my Fraction class.

public class Fraction implements FractionInterface
{private int num; // Numerator 
private int den; // Denominator
    public int getNum()
{
    return this.num;
}   // end getNum

public int getDen()
{
    return this.den;
}   // end getDen
public FractionInterface add(FractionInterface secondFraction)
    {
     FractionInterface result = (num*secondFraction.getDen()+secondFraction.getNum()*den)/
     (den*secondFraction.getDen());
    }
}

I get an error saying "Type mismatch: cannot convert from int to FractionInterface.", I was given a hint to downcast the parameter secondFraction from FractionInterface to Fraction but i'm not sure how to do that. Could someone explain what I am doing wrong?

Answer 1

What do you expect this to do?

FractionInterface result = (num*secondFraction.getDen()+secondFraction.getNum()*den)/
 (den*secondFraction.getDen());

The right side is a number, the left side is an object. The compiler has no way to know how to convert a number into a Fraction object.

You need a constructor in your Fraction class that accepts a numerator and denominator:

public Fraction(int numerator, int denominator)  {
   num = numerator;
   den = denominator;
}

and then do this

FractionInterface result = new Fraction(THENUMERATOR, THEDEMONIMATOR);

which I think, for you, is

FractionInterface result = new Fraction(
   (num*secondFraction.getDen()+secondFraction.getNum()*den),
   (den*secondFraction.getDen());