Question

All,

I’m going through the Friedman & Felleisen book “A Little Java, A Few Patterns”. I’m trying to type the examples in DrJava, but I’m getting some errors. I’m a beginner, so I might be making rookie mistakes.

Here is what I have set-up:

public class ALittleJava {
  //ABSTRACT CLASS POINT
  abstract class Point {
    abstract int distanceToO();
  }
  class CartesianPt extends Point {
    int x;
    int y;
    int distanceToO(){
      return((int)Math.sqrt(x*x+y*y));
    }
    CartesianPt(int _x, int _y) {
    x=_x;
    y=_y;
    }
  }
  class ManhattanPt extends Point {
    int x;
    int y;
    int distanceToO(){
      return(x+y);
    }
    ManhattanPt(int _x, int _y){
      x=_x;
      y=_y;
    }
  }
}

And on the main’s side:

public class Main{
  public static void main (String [] args){
    Point y = new ManhattanPt(2,8);
    System.out.println(y.distanceToO());
  }
}

The compiler cannot find the symbols Point and ManhattanPt in the program.

If I precede each by ALittleJava., I get another error in the main, i.e.,

an enclosing instance that contains ALittleJava.ManhattanPt is required

I’ve tried to find ressources on the ‘net, but the book must have a pretty confidential following and I couldn’t find much.

Thank you all.

JDelage

Answer 1

Make your inner classes static, otherwise you’ll need to access your inner classes from an instance of ALittleJava, such as ALittleJava java = new ALittleJava(), which doesn’t seem to match your use case.

static inner classes cannot see the members of their enclosing class, and if classes are nonstatic, conversely, they CAN see the members of their enclosing class, and an enclosing instance is in fact required.