Simple Java generics question

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;
 }

You shouldn't require a `LinkedList` because everything you do is iterating of the elements in `l`. Just use `Iterable extends Number>`. — whiskeysierra, Sep 26 '10 at 19:38

Péter Török · Accepted Answer · 2010-09-26T20:22:55.813

11

You should restrict your list to Numbers. That is the common superclass for Integer, Float, Double and other numeric types. It has no operators defined, only conversion methods to e.g. 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;
 }

edited Sep 26 '10 at 20:22

answered Sep 26 '10 at 19:00

Péter Török

114,404
31
268
329

This whole ` extends Number>` happened while I was off using Ruby. What is this construct called? Does this syntax `LinkedList>` say we have a LL of unknown type, and `LinkedList extends Number>` is a LL of unknown types each guaranteed to be subclassed from Number? – Tony Ennis Sep 26 '10 at 19:21
and why not `LinkedList ` ? – Tony Ennis Sep 26 '10 at 19:23
1

To answer your first question: Yes, using `LinkedList>` specifies a linked list containing elements of unknown type (they are at least objects) and `LinkedList extends Number>` may contain elments of any subtype of Number (and if nobody is doing strange reflection hacks, it gaaranteed). Why not using `LinkedList`? It ties the users of your api to a specific type. You can't pass in a `LinkedList`. In fact there is no need to even required LinkedList, the best way would be to require an `Iterable extends Number>`. – whiskeysierra Sep 26 '10 at 19:36
1

@Tony, this is called a _wildcard with extends_. `LinkedList extends Number>` matches any list of a subclass of `Number`. I.e. it can be a `LinkedList`, a `LinkedList`, a `LinkedList` etc. It is important to note that the latter are _not_ subclasses of `LinkedList` - that's why the wildcard is needed here. – Péter Török Sep 26 '10 at 19:37
@Willi, if you want to reflect to someone else's comment, it is recommended to include his nick preceded by a @ in your answer (e.g. @Tony) - otherwise he won't get notified about your comment. – Péter Török Sep 26 '10 at 19:58
@willi @Péter Török Thanks for the info! I'll get to googling on it. – Tony Ennis Sep 26 '10 at 22:23

score 1 · Answer 2 · answered Sep 26 '10 at 19:04

The only option would be to generalize on java.lang.Numberbut 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.

score 0 · Answer 3 · answered Sep 26 '10 at 19:02

0

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;
}

answered Sep 26 '10 at 19:02

Samuel García

2,199
14
21

Looks extremely similiar to Peter's answer. – whiskeysierra Sep 26 '10 at 19:12
Aham... So clever... did you ever think i c&p the question snippet and edited it? – Samuel García Sep 26 '10 at 19:44
I did, but the only thing you edited is not adding any value to the answer proposed by Peter. Using a generic method with only one parameter is useless. – whiskeysierra Sep 26 '10 at 19:59
In fact, if you see the post time, i posted it before. Actually, 3 minutes before... – Samuel García Sep 26 '10 at 20:39

score 0 · Answer 4 · answered Sep 26 '10 at 19:03

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>();. 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;
 }

Won't compile. The + operator is not defined for objects. – whiskeysierra Sep 26 '10 at 19:13 — whiskeysierra, Sep 26 '10 at 19:13

score 0 · Answer 5 · answered Sep 26 '10 at 21:03

0

Here's a couple of things to consider.

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

answered Sep 26 '10 at 21:03

BillRobertson42

12,602
4
40
57

Simple Java generics question

5 Answers5

Linked