Java and fractions

Question

I'm trying to add three instance methods to the public interface of class 'Fraction' that all return a 'Fraction' as a result:

add, subtraction and multiplication. is it possible to change it from my current code into instance methods?

I just can't get it to work

Here is my code:

class Fraction {
    private Integer numerator;
    private Integer denumerator;

    public Fraction(Integer numerator, Integer denumerator) {

        int gcd = 1;

        for (int i = 1; i <= numerator && i <= denumerator; i++) {
            if (numerator % i == 0 && denumerator % i == 0)
                gcd = i;
        }
        this.numerator = numerator / gcd;
        this.denumerator = denumerator / gcd;
    }

    public Fraction(Integer numerator) {
        this.numerator = numerator;
        this.denumerator = 1;
    }

    public String toString() {

        return numerator + "/" + denumerator;
    }



    public static Fraction add(Fraction f1,Fraction f2){
        return new Fraction(f1.numerator*f2.denumerator+f2.numerator*f1.denumerator,f1.denumerator*f2.denumerator);
    }
    public static Fraction subtract(Fraction f1,Fraction f2){
        return new Fraction(f1.numerator*f2.denumerator-f2.numerator*f1.denumerator,f1.denumerator*f2.denumerator);
    }
    public static Fraction mul(Fraction f1,Fraction f2){
        return new Fraction(f1.numerator*f2.numerator,f1.denumerator*f2.denumerator);
    }
}

class Main
{
    public static void main(String[] arguments)
    {
        final Fraction HALF = new Fraction(1, 2);
        final Fraction THREE_FIFTH = new Fraction(3, 5);
        System.out.println(HALF.add(HALF, THREE_FIFTH).toString());
        System.out.println(THREE_FIFTH.subtract(HALF, THREE_FIFTH).toString());
        System.out.println(HALF.mul(HALF, THREE_FIFTH).toString());
    }
}

I'm not clear how a constructor and the `toString()` method can interact as you're mentioning. Doesn't really make sense. — Hovercraft Full Of Eels, Feb 15 '20 at 16:23
@HovercraftFullOfEels Updated my question. I was just trying to make changes in my constructor to make this to work. — , Feb 15 '20 at 16:23
just have `public class Fraction(int numerator) { this(numerator, 1); }`, no? — Hovercraft Full Of Eels, Feb 15 '20 at 16:24
Where exactly are you stuck? Just write another constructor? — Sweeper, Feb 15 '20 at 16:27
You're doing it wrong. Do it as I mentioned in my comment and as Jon Skeet does in the duplicate question. — Hovercraft Full Of Eels, Feb 15 '20 at 16:33
edit: as Jon Skeet does [here](https://stackoverflow.com/a/285184/522444) — Hovercraft Full Of Eels, Feb 15 '20 at 16:54
@HovercraftFullOfEels is it possible to change these into instance methods, though? — , Feb 15 '20 at 17:32
@UMLtotallydistilled: yes of course it is. The devil is in the details. You need to tell specifically what you're trying to do and show how you're trying to do it. — Hovercraft Full Of Eels, Feb 15 '20 at 17:33
@HovercraftFullOfEels I basically tried to add 3 methods like this with returning the numenator and denumerator — , Feb 15 '20 at 17:41

score 1 · Accepted Answer · answered Feb 15 '20 at 17:37

public static Fraction add(Fraction f1,Fraction f2){
  return new Fraction(f1.numerator*f2.denumerator+f2.numerator*f1.denumerator,
                      f1.denumerator*f2.denumerator);
}

is a class method (because of the static it does not need an instance to call "on").

Making it instance method would look like

public Fraction add(Fraction other){
  return new Fraction(this.numerator*other.denumerator+other.numerator*this.denumerator,
                      this.denumerator*other.denumerator);
}

of course you do not actually need to write the thiss there, just they emphasize that f1 became the current object, and f2 became the single argument.

Then you could use it as

Fraction HALF = new Fraction(1, 2);
Fraction THREE_FIFTH = new Fraction(3, 5);
System.out.println(HALF.add(THREE_FIFTH));

without repeating HALF (like HALF.add(HALF,THREE_FIFTH) in the original code).

Side comment: class methods (static stuff) can be referred via the name of the class, your original code would be more conventionally called in the form Fraction.add(...):

System.out.println(Fraction.add(HALF,THREE_FIFTH));

(System.out.println() knows that it should call toString() so you do not actually need to do that yourself)

@UMLtotallydistilled yes, `subtract()` and `mul()` could be made into instance methods in the same way as shown with `add()` — tevemadar, Feb 15 '20 at 18:32

Java and fractions

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