JAVA beginner's question. Why is the result zero?

package Calc;

import java.util.Scanner;

class Calculator{
    int a,b;
    void setOprands(int a,int b) {
        this.a = a;
        this.b = b;
    }
    int sum() {
        return this.a+this.b;
    }
    void plus() {

    }
    void minus() {
    }
    void run() {
        plus();
        minus();
    }
}
class plus extends Calculator{
    void plus() {
        System.out.println("++"+sum());
    }

}

class minus extends Calculator{
    void minus() {
        System.out.println("--"+sum());
    }
}

public class CalculatorConstructorDemo4 {

    public static void main(String[] args) {
        Calculator Calc = new Calculator();
        Calculator plus1 = new plus();
        Calculator minus1 = new minus();
        Scanner a = new Scanner(System.in);
        Scanner b = new Scanner(System.in);
        //Calc.setOprands(a.nextInt(), b.nextInt());
        Calc.setOprands(10, 20);
        plus1.plus();
        minus1.minus();
    }

}