What's wrong with this print-iterable method?

Question

private static void printIterable(Iterable iterable) {

    // ERROR: "Type mismatch: cannot convert from element type Object to Iterable"
    for (Iterable i : iterable) { 
        System.out.println(i);
    }

}

What the compiler is talking about? Its an Iterable, not an Object.

Don't use raw type in new code: http://stackoverflow.com/questions/2770321/what-is-a-raw-type-and-why-shouldnt-we-use-it — polygenelubricants, Jun 08 '10 at 15:49

Joachim Sauer · Accepted Answer · 2010-06-08T14:48:57.753

4

You try to do something for each Iterable inside the iterable. This would only make sense if iterable was an Iterable<? extends Iterable> (i.e. it would iterate over yet other Iterable objects).

But since you didn't specify a type argument for the argument, you only know that it will iterate over some kind of object (i.e. the base type Object is applicable).

You should try this:

for (Object o : iterable) { 
    System.out.println(o);
}

When read out loud it read as "For each Object o in iterable, print o". Replacing Object in that sentence with Iterable should illustrate what the problem was.

edited Jun 08 '10 at 14:48

answered Jun 08 '10 at 14:43

Joachim Sauer

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1

+1. It's easy to understand if you speak it out loud. Say you had `Iterable shoppingCart`. Then `for (Item o : shoppingCart) { total += o.getPrice(); } ` reads *for each Item "o" in shoppingCart add the price of o to the total.* – Mark Peters Jun 08 '10 at 14:46

score 3 · Answer 2 · answered Jun 08 '10 at 14:44

Your loop variable is not of type Iterable. It is supposed to have the type of collection elements. Since the parameter of type Iterable has no generic type argument, your items can be iterated as Object instances:

for (Object o : iterable) { 
    System.out.println(o);
}

score 1 · Answer 3 · answered Jun 08 '10 at 14:45

1

You are iterating over iterable. So the type of variable 'i' should be object not Iterable. If you want to have specific type there, use Java Generics.

answered Jun 08 '10 at 14:45

Vlad

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score 0 · Answer 4 · answered Jun 08 '10 at 16:09

As others have said, Your trying to iterate over iterators, which would make no sense at all. You need to use not Iterable in the for loop but object in your example. But a much better solution is

private static void printIterable(Iterable<String> iterable) {

    // ERROR: "Type mismatch: cannot convert from element type Object to Iterable"
    for (String i : iterable) { 
        System.out.println(i);
    }
}

(Replace string with the object you want)

What's wrong with this print-iterable method?

4 Answers4