How to skip null values in json text produced by gson, if TypeAdapter converts null-value?

Question

I created a TypeAdapter for the Type Instant by writing milliseconds. How can I handle the case of the instant-field to be null. I do not want to write a kind of special entry like -1L, which I later could convert to null during reading (see example below). I want to skip the entry in the produced json-text as it is the default case.

Example:

gsonBuilder.registerTypeAdapter(Instant.class, new TypeAdapter<Instant>(){
            @Override
            public void write(final JsonWriter jsonWriter, final Instant instant) throws IOException {
                if (instant != null) {
                    jsonWriter.value(instant.toEpochMilli());
                } else {
                    jsonWriter.value(-1L);
                }
            }

            @Override
            public Instant read(final JsonReader jsonReader) throws IOException {
                long millisEpoch = jsonReader.nextLong();
                if (millisEpoch == -1) {
                    return null;
                } else {
                    return Instant.ofEpochMilli(millisEpoch);
                }
            }
        });

Answer 1

It's easy, I think you can modify your TypeAdapter to check if the instant value is null and not write it to the JSON output

let me explain with a simple example, first, in write method, I check if the Instant value is null and write a null value instead of the epoch milliseconds, that will skip the null value in the JSON output and in the read method, I still check for the special case of -1L to convert it to null

gsonBuilder.registerTypeAdapter(Instant.class, new TypeAdapter<Instant>() {
    @Override
    public void write(final JsonWriter jsonWriter, final Instant instant) throws IOException {
        if (instant != null) {
            jsonWriter.value(instant.toEpochMilli());
        } else {
            jsonWriter.nullValue();
        }
    }

    @Override
    public Instant read(final JsonReader jsonReader) throws IOException {
        long millisEpoch = jsonReader.nextLong();
        if (millisEpoch == -1) {
            return null;
        } else {
            return Instant.ofEpochMilli(millisEpoch);
        }
    }
});

good luck!