错误：在类Calculate中找不到主方法，请将main方法定义为：public static void main（String [] args）

我需要帮助主方法，我收到此错误：

I need help with the main method, I'm getting this error:

Error: Main method not found in class Calculate, please define the main method as:
    public static void main(String[] args)

这是代码：

class Calculate {

private double fn;
private double sn;
private char op;

public void setNumber(double fnum, double snum){
    this.fn = fnum;
    this.sn = snum;
}
public double getNumber1(){
    return fn;
}
public double getNumber2(){
    return sn;
}
public void setOper(char oper){
    this.op = oper;
}
public char getOper(){
    return op;
}
public void getAnswer(){
    double ans;
    switch (getOper()){
        case 'a': {
            ans = add(getNumber1(), getNumber2());
            ansOutput(ans);
            break;
        }case 'b': {
            ans = sub (getNumber1(), getNumber2());
            ansOutput(ans);
            break;
        }case 'c': {
            ans = mul (getNumber1(), getNumber2());
            ansOutput(ans);
            break;
        }case 'd': {
            ans = div (getNumber1(), getNumber2());
            ansOutput(ans);
            break;
        }default:
            System.out.println("--------------------------");
            System.out.println("Invalid choice of operator");
            System.out.println("--------------------------");
        }
    }
    public static double add(double x,double y){
        return x + y;
    }
    public static double sub(double x, double y){
        return x - y;
    }
    public static double mul(double x, double y){
        return x * y;
    }
    public static double div(double x, double y){
        return x / y;
    }

    public static void ansOutput(double x){
        System.out.println("----------- -------");
        System.out.printf("the answer is %.2f\n", x);
        System.out.println("-------------------");
    }
}