Java Programming

I'm a student learning Java, and doing this class is proving unexpectedly difficult.

public class Rational {
    
    private int num;
    private int denom;
    
    /** Creates a new instance of Rational */
    public Rational(int num, int denom) {
        this.num = num;
        this.denom = denom;
        int d = gcd(int num, int denom);
    }
    
    private int gcd(int num, int denom){
        while (denom > 0) {
            int q = num/denom;
            int r = num - q*denom;
            num = denom;
            denom = r;
        }
        return(num);
    }
    
    
    
    public Rational add(Rational r){
        return new Rational((this.num*r.denom+this.denom*r.num),this.denom*r.denom);
    }
    
    public Rational sub(Rational r){
        return add(r.neg());
    }
    
    public Rational mult(Rational r){
        return new Rational(this.num*r.num,this.denom*r.denom);
    }
    
    public Rational div(Rational r){
        return mult(r.inv());
    }
    
    public Rational neg(){
        return new Rational(this.num*-1,this.denom);
    }
    
    public Rational inv(){
        return new Rational(this.denom,this.num);
    }
    
    public String toString(){
        return (num +  "/" + denom);
    }
    
}

Main

public static void main(String[] args) {
        Rational r1 = new Rational(1,2);
        Rational r2 = new Rational(3,4);
        Rational r3 = r1.add(r2);
        Rational r4 = r1.sub(r2);
        Rational r5 = r1.mult(r2);
        Rational r6 = r1.div(r2);
        System.out.println(r3);
        System.out.println(r4);
        System.out.println(r5);
        System.out.println(r6);
        }

gcd(int num, int denom) gets a "; expected" error and a ".class expected" error. In addition, I cannot get the GCD to apply to the program and reduce the fraction.
How do I get the GCD to apply itself in the program?