Why can't e.g. List be passed to a method that takes a List as parameter?

Question

Simple example:

public class Person
{
    String name;
}

public class VIP extends Person
{
    String title;
}

And then doing:

public static void main(String[] args)
{
    Person p = new Person();
    p.name = "John";

    VIP vip = new VIP();
    vip.name = "Bob";
    vip.title = "CEO";

    List<Person> personList = new ArrayList<Person>();
    List<VIP> vipList = new ArrayList<VIP>();

    personList.add(p);
    personList.add(vip);

    vipList.add(vip);

    printNames(personList);
    printNames(vipList);
}

public static void printNames(List<Person> persons)
{
    for (Person p : persons)
        System.out.println(p.name);
}

gives an error on "printNames(vipList)" (required List<Person> found List<VIP>).

Does this mean that although VIP is a Person, List<VIP> is not a List<Person>?

Yes, that's what it means. Duplicate: http://stackoverflow.com/questions/2033912/c-variance-problem-assigning-listderived-as-listbase — Lasse V. Karlsen, Jan 25 '10 at 15:17
How is this a duplicate? That post asks about C#, this one about Java. The fact that they happen to work the same way doesn't mean that they have to. — danben, Jan 25 '10 at 15:21
@Lasse - that's a C# question, this is a Java question. However, I know this question has been asked numerous times about Java, too. — Paul Tomblin, Jan 25 '10 at 15:21
Oops, didn't notice the Java tag, my apologies, but it *is* a duplicate because all the reasons why it isn't possible in C# are exactly the same for Java. In short, if you were allowed to downcast (but not convert, note the distinction) a List{Child} to a List{Base}, the compiler would not be able to prevent you trying to add Base instances to the list. So while it is a duplicate, I should've picked a different duplicate question. The answer by BalusC is the right one, however, which is the same solution for both Java and C#, add a constraint to the method. — Lasse V. Karlsen, Jan 25 '10 at 22:13

score 11 · Answer 1 · answered Jan 25 '10 at 15:17

11

That's right. A list of bananas is not a list of fruit. Otherwise you could insert any fruit in a list of bananas. e.g.

List<Fruit> lf = new List<Banana>();
lf.add(new Apple());

would result in unexpected or counterintuitive results.

answered Jan 25 '10 at 15:17

Brian Agnew

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Try with List extends Fruits> lf = new List(); That way Java will know that it can be a list of "some child" and will not allow using lf.add(new Apple()) but will allow the asigment of the list. – helios Jan 25 '10 at 15:22

score 9 · Accepted Answer · answered Jan 25 '10 at 15:21

9

You're just prohibited by the rules of Generics. If you're rather interested in how to "fix" this behaviour, just change the printNames() method to take a List<? extends Person> argument instead.

answered Jan 25 '10 at 15:21

BalusC

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You can almost be certain that the best tutorials are been provided by the vendor itself :) You can find them by just Googling "[keyword] tutorial site:sun.com". For example: http://google.com/search?q=generics+tutorial+site:sun.com It's already the 1st hit. – BalusC Jan 25 '10 at 15:46

score 1 · Answer 3 · answered Jan 25 '10 at 16:08

1

Bjarne Stroustrup, inventor of C++, explains it rather well:

http://www2.research.att.com/~bs/bs_faq2.html#conversion

Yes, I know I am late for this party, but better than never, right..

answered Jan 25 '10 at 16:08

Armen Michaeli

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Why can't e.g. List be passed to a method that takes a List as parameter?

3 Answers3