Fractions

public class Fraction {
    private int numerator;
    private int denominator = 1;

    Fraction (int n, int d) {
        this.numerator = n;
        this.denominator = d;
        this.normalization();
    }
    Fraction (int n) {
        this.numerator = n;
    }


    String toDecimalFractions() {
        return Double.toString((double)this.numerator / (double)this.denominator);
    }

    void plus(Fraction obj) {
        this.denominator = this.denominator*obj.denominator;
        this.numerator = this.numerator*obj.denominator + obj.numerator*this.denominator;
        this.normalization();
    }

    void minus(Fraction obj) {
        this.denominator = this.denominator*obj.denominator;
        this.numerator = this.numerator*obj.denominator - obj.numerator*this.denominator;
        this.normalization();
    }

    void multiplyByK(int k) {
        this.numerator *= k;
        this.normalization();
    }

    @Override
    public String toString() {
        return this.numerator + "/" + this.denominator;
    }

    private void normalization() {
        this.numerator *= this.denominator / Math.abs(this.denominator);
        this.denominator = Math.abs(this.denominator);
        int nod = this.NOD(Math.abs(this.numerator), Math.abs(this.denominator));
        this.numerator /= nod;
        this.denominator /= nod;
    }

    private int NOD(int n1, int n2)
    {
        int div;
        if (n1 == n2)  return n1;
        int d = n1 - n2;
        if (d < 0) {
            d = -d;  div = NOD(n1, d);
        } else
            div = NOD(n2, d);
        return div;
    }
}