简单的Java泛型问题

Question

I want to have a method which calculates the mean of a LinkedList of type Integer, Double and Float.

The problem is the sum += i; statement, since java says that the + operator isn't defined for type Object.

I could do a cast, but if the LinkedList was of type Float, for example, and the cast was to Integer, I would be not computing the correct mean.

What should I do? Thanks.

 public double mean (LinkedList<?> l)
 {
  double sum = 0;
  int n = 0;
  for (Object i : l)
  {
   n++;
   sum += i;
  }

  return sum / n;
 }

Answer 1

You should restrict your list to Number s. That is the common superclass for Integer , Float , Double and other numeric types. It has no operators defined, only conversion methods to eg double , but that is enough for you here:

 public double mean (LinkedList<? extends Number> l)
 {
  double sum = 0;
  int n = 0;
  for (Number i : l)
  {
   n++;
   sum += i.doubleValue();
  }

  return sum / n;
 }

Answer 2

The only option would be to generalize on java.lang.Number but it wouldn't actually help much as you can't unbox a Number to anything which can be applied to the primitive arithmetic operators. So you'll still have to check for each of the Number-types and call Number.doubleValue, intValue and so forth on the Number-object.

Answer 3

public <T extends Number> double mean (LinkedList<T> l) {
  double sum = 0;
  int n = 0;
  for (T i : l)  {
    n++;
    sum += i.doubleValue();
  }

  return sum / n;
}

Answer 4

In Java, generic collections are contravariant. This means that if you have parent class A and class B which extends A, then you cannot make something like List<A> list = new LinkedList<B>(); .

But there is a possibility to allow this type of substitution: you can use a construction like List<? extends A> list = new LinkedList<B>(); List<? extends A> list = new LinkedList<B>(); . For your case:

 public double mean (List<? extends Number> l)
 {
  double sum = 0;
  int n = 0;
  for (Object i : l)
  {
   n++;
   sum += i;
  }

  return sum / n;
 }

Answer 5

Here's a couple of things to consider.

1. Allow Collection instead of only lists
2. You don't need a separate variable 'n', just use l.size()
3. Watch out for empty/null input