Generic Wildcard Arguments

To create a generic method, you use the wildcard argument. The wildcard argument is specified by the ?, and it represents an unknown type.

 
class Calculator<T extends Number> {
  T[] nums;
  Calculator(T[] o) {
    nums = o;
  }
  double average() {
    double sum = 0.0;
    for (int i = 0; i < nums.length; i++){
      sum += nums[i].doubleValue();
    }
    return sum / nums.length;
  }
}
public class Main {
  boolean sameAvg(Calculator<?> ob) {
    if (1.2 == ob.average())
      return true;
    return false;
  }

  public static void main(String args[]) {
  }
}

Calculator<?> matches any Stats object, allowing any two Stats objects to have their averages compared.

The following program demonstrates this:

 
class Calculator<T extends Number> {
  T[] nums;
  Calculator(T[] o) {
    nums = o;
  }
  double average() {
    double sum = 0.0;
    for (int i = 0; i < nums.length; i++)
      sum += nums[i].doubleValue();
    return sum / nums.length;
  }
  boolean sameAvg(Calculator<?> ob) {
    if (average() == ob.average())
      return true;
    return false;
  }
}
public class Main {
  public static void main(String args[]) {
    Integer inums[] = { 1, 2, 3, 4, 5 };
    Calculator<Integer> iob = new Calculator<Integer>(inums);

    Double dnums[] = { 1.1, 2.2, 3.3, 4.4, 5.5 };
    Calculator<Double> dob = new Calculator<Double>(dnums);

    if (iob.sameAvg(dob))
      System.out.println("are the same.");
    else
      System.out.println("differ.");
  }
}

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