Generic Bounded Wildcards

Wildcard arguments can be bounded in the same way that a type parameter can be bounded. A bounded wildcard is important when creating a generic type that will operate on a class hierarchy.

A bounded wildcard specifies either an upper bound or a lower bound for the type argument.

 
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;
  }
}
class Map<T extends TwoD> {
  T[] coords;

  Map(T[] o) {
    coords = o;
  }
}

public class Main {
  static void showXY(Map<?> c) {
    for (int i = 0; i < c.coords.length; i++){
      System.out.println(c.coords[i].x + " " + c.coords[i].y);
    }      
  }
  static void showXYZ(Map<? extends ThreeD> c) {
    for (int i = 0; i < c.coords.length; i++){
      System.out.println(c.coords[i].x + " " + c.coords[i].y + " "
          + c.coords[i].z);
    }
  }
  public static void main(String args[]) {
    TwoD td[] = { new TwoD(0, 0), new TwoD(-1, -2) };
    Map<TwoD> map = new Map<TwoD>(td);
    System.out.println("Contents of tdlocs.");

    showXY(map);
  }
}

In general, to establish an upper bound for a wildcard, use the following type of wildcard expression:

 
<? extends superclass>

superclass is the name of the class that serves as the upper bound. This is an inclusive clause.

You can specify a lower bound for a wildcard by adding a super clause to a wildcard declaration.

 
<? super subclass>

In this case, only classes that are superclasses of subclass are acceptable arguments. This is an exclusive clause, because it will not match the class specified by subclass.

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