ClassCastException for empty transformations

[sgaragan]

If a transformation returns an empty transformation (not null and not an empty list, equivalent of { } for JSON), a ClassCastException is thrown and the following comes out in the logs:

class java.util.HashMap cannot be cast to class java.util.List (java.util.HashMap and java.util.List are in module java.base of loader 'bootstrap')

The issue seems to be at com.swisscom.daisy.cosmos.candyfloss.transformations.Transformer:35 which is the transformList() method. There is an unsafe cast at line 36:

var transformed = (List<Map<String, Object>>) chainr.transform(input.json());

The code checks to see if this returns null but not an empty HashMap. Not entirely sure why an empty HashMap gets returned instead of null but that is somewhat separate from this

Thanks,
Sean

[ahassany]

Hello @sgaragan thank you for the feedback. This is actually by design, should've been documented better though. Cosmos Candyfloss is designed as one to many transformer, it expects the output of the Jolt transformation to be a List not a single element.

[sgaragan]

Hello @sgaragan thank you for the feedback. This is actually by design, should've been documented better though. Cosmos Candyfloss is designed as one to many transformer, it expects the output of the Jolt transformation to be a List not a single element.

Hey Ahmed,
The problem is that there are times when transformations seem to return an empty HashMap object (in other words, a JSON object that is just { }). This results in an exception and spams the log file with error messages. To get past this for the unit tests, we updated the code in question to look for an empty HashMap and to return an empty List instead but that was just for testing, we still use the version out of GitHub for production