Can someone please explain why it doesn't work? The error is at `obj[0][0]=1;`. It says that GPA can't be converted to int, same thing for `String` variable assignment `s`.

```java
public class GPA {        
        public String s;
        public int n;
        public GPA[][] a;

        //constructor
        public GPA(GPA[][] a){}    

        public static void main(String[] args) {
            GPA[][] obj=new GPA[2][2];
            obj[0][0]=1; //error here
        }
}

```