No built-in type adapter for Iterable

What steps will reproduce the problem?
See the attached file.

What is the expected output?
[{"a":1},{"a":2}]
[{"a":1},{"a":2}]
[1, 2]
[1, 2]

What do you see instead?
[{"a":1},{"a":2}]
{}
[1, 2]
Exception in thread "main" java.lang.RuntimeException: Unable to invoke no-args 
constructor for java.lang.Iterable<com.ash2k.test1.App$A>. Register an 
InstanceCreator with Gson for this type may fix this problem.
    at com.google.gson.internal.ConstructorConstructor$8.construct(ConstructorConstructor.java:166)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:163)
    at com.google.gson.Gson.fromJson(Gson.java:755)
    at com.google.gson.Gson.fromJson(Gson.java:721)
    at com.google.gson.Gson.fromJson(Gson.java:670)
    at com.ash2k.test1.App.main(App.java:31)
Caused by: java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at com.google.gson.internal.UnsafeAllocator$1.newInstance(UnsafeAllocator.java:48)
    at com.google.gson.internal.ConstructorConstructor$8.construct(ConstructorConstructor.java:163)
    ... 5 more
Caused by: java.lang.InstantiationException: java.lang.Iterable
    at sun.misc.Unsafe.allocateInstance(Native Method)
    ... 11 more

What version of the product are you using? On what operating system?
2.1 on Ubuntu 11.10 x64

Original issue reported on code.google.com by [e-mail] on 1 Apr 2012 at 7:38

Attachments:

• App.java

[jlapolla]

@inder123 To answer your question: yes, absolutely.

From a user’s perspective this is consistent with the rest of Gson. When I ask Gson to deserialize some JSON to a List<Integer>, I don’t much care which specific implementation of the List<Integer> interface Gson chooses to produce. It could be a LinkedList<Integer> or an ArrayList<Integer>. But as long as it’s something that implements List<Integer> (and it’s reasonably performant) I’m happy, and my code works.

In this case, Gson does exactly what you suggested: it picks an appropriate subclass of List<Integer> for me. It would be great if it did the same for an Iterable<Integer> as well.

I haven’t dug into the Gson source code, so forgive me if this is technically impossible for some reason. But, from an outsider’s perspective, this seems totally feasible.

I know this is an older issue, but I’m still having to work around this issue in my current project, specifically when serializing an Iterable<Integer>. As @ear…@gmail.com pointed out in his fourth paragraph on 8 Nov 2012, the workaround requires me to iterate over my Iterable<Integer> one more time, which is less than ideal.

[inder123]

Hi Folks. What is the intended behavior here? Do you want us to pick a random sub-class of Iterable (such as List) and deserialize the collection into it?