The RuntimeTypeAdapterFactory code example (https://github.com/google/gson/blob/master/extras/src/main/java/com/google/gson/typeadapters/RuntimeTypeAdapterFactory.java) depends on the internal API class com.google.gson.internal.Streams, which is not exported when depending on gson in a JPMS project using module path.

Unless there's some easy fix available, currently one has to make a copy of Streams, LazilyParsedNumber, and TypeAdapters classes from the internal packages in order to use this provided class.