Java generic: Bounded Wildcard arguments

  
/*
Java 2, v5.0 (Tiger) New Features
by Herbert Schildt
ISBN: 0072258543
Publisher: McGraw-Hill/Osborne, 2004
*/

 
// Two-dimensional coordinates. 
class TwoD { 
  int x, y; 
 
  TwoD(int a, int b) { 
    x = a; 
    y = b; 
  } 
} 
 
// Three-dimensional coordinates. 
class ThreeD extends TwoD { 
  int z; 
   
  ThreeD(int a, int b, int c) { 
    super(a, b); 
    z = c; 
  } 
} 
 
// Four-dimensional coordinates. 
class FourD extends ThreeD { 
  int t; 
 
  FourD(int a, int b, int c, int d) { 
    super(a, b, c); 
    t = d;  
  } 
} 
 
// This class holds an array of coordinate objects. 
class Coords<T extends TwoD> { 
  T[] coords; 
 
  Coords(T[] o) { coords = o; } 
} 
 
// Demonstrate a bounded wildcard. 
public class BoundedWildcard { 
  static void showXY(Coords<?> c) { 
    System.out.println("X Y Coordinates:"); 
    for(int i=0; i < c.coords.length; i++) 
      System.out.println(c.coords[i].x + " " + 
                         c.coords[i].y); 
    System.out.println(); 
  } 
 
  static void showXYZ(Coords<? extends ThreeD> c) { 
    System.out.println("X Y Z Coordinates:"); 
    for(int i=0; i < c.coords.length; i++) 
      System.out.println(c.coords[i].x + " " + 
                         c.coords[i].y + " " + 
                         c.coords[i].z); 
    System.out.println(); 
  } 
 
  static void showAll(Coords<? extends FourD> c) { 
    System.out.println("X Y Z T Coordinates:"); 
    for(int i=0; i < c.coords.length; i++) 
      System.out.println(c.coords[i].x + " " + 
                         c.coords[i].y + " " + 
                         c.coords[i].z + " " + 
                         c.coords[i].t); 
    System.out.println(); 
  } 
 
  public static void main(String args[]) { 
    TwoD td[] = { 
      new TwoD(0, 0), 
      new TwoD(7, 9), 
      new TwoD(18, 4), 
      new TwoD(-1, -23) 
    }; 
 
    Coords<TwoD> tdlocs = new Coords<TwoD>(td);     
 
    System.out.println("Contents of tdlocs."); 
    showXY(tdlocs); // OK, is a TwoD 
//  showXYZ(tdlocs); // Error, not a ThreeD 
//  showAll(tdlocs); // Erorr, not a FourD 
 
    // Now, create some FourD objects. 
    FourD fd[] = { 
      new FourD(1, 2, 3, 4), 
      new FourD(6, 8, 14, 8), 
      new FourD(22, 9, 4, 9), 
      new FourD(3, -2, -23, 17) 
    }; 
 
    Coords<FourD> fdlocs = new Coords<FourD>(fd);     
 
    System.out.println("Contents of fdlocs."); 
    // These are all OK. 
    showXY(fdlocs);  
    showXYZ(fdlocs); 
    showAll(fdlocs); 
  } 
}

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