Bounded Type Parameters

NumberBox.java

// The bounded type parameter dictates the kinds of Box instances we can create.
public class NumberBox<T extends Number> {
  private T _number;

  public NumberBox() { this(null); }
  public NumberBox(T toStore) {
    set(toStore);
  }

  public T get() { return _number; }
  public void set(T val) { _number = val; }
}

// The following are valid:
NumberBox<Integer> intBox = new NumberBox<Integer>();
NumberBox<Float> fltBox = new NumberBox<Float>();
NumberBox<Double> dblBox = new NumberBox<Double>();
NumberBox<Number> numBox = new NumberBox<Number>();

// The following are invalid:
NumberBox<String> strBox = new NumberBox<String>();
NumberBox<Object> objBox = new NumberBox<Object>();

Wildcards.java

// Now, we deal with wildcards
public class WildcardExample {

  public static void showValuesBad(NumberBox<Number> box) {
    System.out.println(box.get().doubleValue());
    System.out.println(box.get().intValue());
  }

  public static void showValuesGood(NumberBox<? extends Number> box) {
    System.out.println(box.get().doubleValue());
    System.out.println(box.get().intValue());
  }

  public static void putValuesBad(NumberBox<? extends Number> box) {
    box.set(new Integer(42));
  }
  
  public static void main(String[] args) {
    NumberBox<Integer> intBox = new NumberBox<Integer>(new Integer(4));
    NumberBox<Double> dblBox = new NumberBox<Double>(new Double(3.2));
    // should be invalid:
    showValuesBad(intBox);
    showValuesBad(dblBox);

    // should be valid:
    showValuesGood(intBox);
    showValuesGood(dblBox);

    // should be invalid:
    putValuesBad(intBox);
    putValuesBad(dblBox);
  }
}