Why does the Java extends baseclass need the main(string[]) method

Question

this is the code of baseclass:

class Base
{
    public double size;
    public String name;
    public Base(double size,String name)
    {
        this.size=size;
        this.name=name;
    }
}

this is the code of subclass:

 public class Sub extends Base
{
    public String color;
    public Sub(double size,String name,String color)
    {
        super(size,name);
        this.color=color;
    }
    public static void main(String[] args)
    {
        var s=new Sub(5.6,"Te`enter code here`st","Green");
        System.out.println(s.size+"---"+s.name+"---"+s.color);
    }
}

Start compiling: javac Sub.java ;is good. Start running: java Sub.java ; in class not find main(String[]) method what's going on?