a Rational class implementation which corresponds to Algs4 1.2.16

Rational.java

public class Rational {
    private long n;
    private long d;
    public Rational (long numerator, long denominator) {
        n = numerator;
        d = denominator;
    }
    public long gcd(long p, long q) {
        if (q == 0) return p;
        long r = p % q;
        return gcd(q, r);
    }
    private void simplify() {
        long r = gcd(n, d);
        n = n / r;
        d = d / r;
    }
    public Rational plus(Rational b) {
        Rational result = new Rational(n, d);
        result.n = result.n + b.n;
        result.simplify();
        return result;
    }
    public Rational minus(Rational b) {
        Rational result = new Rational(n, d);
        result.n = result.n - b.n;
        result.simplify();
        return result;
    }
    public Rational times(Rational b) {
        Rational result = new Rational(n, d);
        result.n = result.n * b.n;
        result.d = result.d * b.d;
        result.simplify();
        return result;
    }
    // a / b, not b / a
    public Rational divides(Rational b) {
        Rational result = new Rational(n, d);
        result.n = result.n * b.d;
        result.d = result.d * b.n;
        result.simplify();
        return result;
    }
    public boolean equals(Rational b) {
        if (b.n == n && b.d == d) {
            return true;
        } else {
            return false;
        }
    }
    public String toString() {
        if (d == 1 || d == 0) {
            return String.format("%d", n);
        } else {
            return String.format("%d/%d", n, d);
        }
    }
    public static void main(String[] args) {
        Rational a = new Rational(3, 4);
        Rational b = new Rational(1, 4);
        StdOut.println(a.plus(b));
        StdOut.println(a.minus(b));
        StdOut.println(a.times(b));
        StdOut.println(a.divides(b));
        StdOut.println(a.minus(b).equals(a.minus(b)));
    }
}

plus and minus are just wrong. Please be careful.