marcus green mock #3 q.57

[Maria Garcia]

  class Base {}
class Agg extends Base{
        public String getFields(){
         String name =  "Agg";
        return name;
        }
}

public class Avf{
public static void main(String argv[]){
        Base a = new Agg();
        //Here
        }
} 
1) System.out.println(a.getFields());
2) System.out.println(a.name);
3) System.out.println((Base) a.getFields());
4) System.out.println( ((Agg) a).getFields());
 

answer is 4. The object is Agg() but why is it looking for getField method in Base?

[Jose Botella]

Maria,
it is not possible in Java to invoke a method on a variable whose (declared/compile) type has not declared or inherited that method. This is why Java is a type safe language. Every expression has a type, and you can invoke only the methods corresponding to this type: you cannot treat an object as it were of another (unrelated) type.
Base a = new Agg();
the declared/compile type of the variable a is Base. However the type of the object pointed by a is Agg.

1) System.out.println(a.getFields());
Error. Base did not declared or inherited getFields
2) System.out.println(a.name);
Error. The same as above.
3) System.out.println((Base) a.getFields());
Error. The cast does not apply to a, but to the return of a.getFields. Thus error because of 1)
4) System.out.println( ((Agg) a).getFields());
OK. The declared/compile type of a is Agg. This is what the cast carries out. Now in the class Agg the invoked method is declared, that makes the compiler happy.

[Maria Garcia]

Thanks Jose.