Gson合并JsonObject引发java.lang.UnsupportedOperationException

Question

In some special cases I want jsonObjects to be merged in a single object. I read both objects from a file

new JsonParser().parse(new FileReader(definitionFile)).getAsJsonObject();

and both files have the same structure. I try to merge the files with a following code:

public JsonObject merge(JsonObject firstObject, JsonObject secondObject) throws Exception {
    firstObject.getAsJsonObject(BaseGenerator.MEMBER_METHODS).entrySet()
            .addAll(secondObject.getAsJsonObject(BaseGenerator.MEMBER_METHODS).entrySet());
    return firstObject;
}

But exception is thrown while trying to merge these objects: java.lang.UnsupportedOperationException

How can I merge those objects?

Answer 1

The exception is because the EntrySet implementation for GSON library does not implement the add function.

I haven't touch GSON lib for a long time. But, maybe you could try something like this

new HashSet(firstObject.getAsJsonObject(BaseGenerator.MEMBER_METHODS).entrySet()).addAll(secondObject.getAsJsonObject(BaseGenerator.MEMBER_METHODS).entrySet())`

Answer 2

It's easy. Just don't merge them using the entrySet() method since sets returned by that method do not implement Set.add / Set.addAll .

private static void mergeInto(final JsonObject o1, final JsonObject o2) {
    for ( final Entry<String, JsonElement> e : o2.entrySet() ) {
        o1.add(e.getKey(), e.getValue());
    }
}

Also, that method above returns nothing in order to explicitly indicate that the arguments may change their internal state -- the first parameter actually (that is usually a big surprise to anyone who's expecting something new from a methods that returns something). I would create a new JsonObject instance in order not to touch the arguments state, something like:

private static JsonObject merge(final JsonObject o1, final JsonObject o2) {
    final JsonObject merged = new JsonObject();
    mergeInto(merged, o1);
    mergeInto(merged, o2);
    return merged;
}