I keep having this issue whenever I write a class where when I try to initialize an object belonging to that class like so

class name = new class();

I keep receiving the error “non-static variable this cannot be referenced from a static context”

For example with this simple class

public class num
{
    private int num;
        
    public num(int num)
    {
       this.num = num;
    }
    public int getNum()
    {
       return num;
    }

}
    public static void main(String[] args)
    {
        num x = new num(10);
        System.out.println(x.getNum());
    }

I realize simply adding static before the class keyword at line 1 would solve the issue but I would like to know why would I need to do that? Is it an error in my code or just a general requirement when writing classes?