How do I retain the default values of field in a deserialized object?

This is a known, currently open issue: https://github.com/google/gson/issues/513

Gson constructs the values of fields in deserialized objects with reflection, so it will set the values based on what's in the JSON only. Until Google provides a fix for this issue, there isn't that much you can do.

There are a number of workaround objects you have in the meantime:

1. Wrap the fields in getters, and lazily load the value. This is a good way (and my personal recommendation) to do it if a field is never ever allowed to be null, but they do need to be mutable.
2. Mark the default fields as final. This a good way to do it if they are immutable.
3. Create a custom ExclusionStrategy, and mark the particular fields that should be ignored using FieldAttributes
   • This is the most versatile of the options but also the most code.
4. Deserialize your POJO using just the fields that don't exist, and then compose that data structure with a new one that has the default values.

I agree that all of these have drawbacks, but as I said above, this is an open issue with Gson.