如何为Moshi中的空字段设置默认值

public class DefaultValueJsonAdapter<T> extends JsonAdapter<T> {
    private final T defaultValue;
    private final JsonAdapter<T> delegate;

    public DefaultValueJsonAdapter(T defaultValue, JsonAdapter<T> delegate) {
        this.defaultValue = defaultValue;
        this.delegate = delegate;
    }

    @Override
    public T fromJson(JsonReader reader) throws IOException {
        if (reader.peek() == JsonToken.NULL) {
            reader.nextNull();
            return defaultValue;
        }
        return delegate.fromJson(reader);
    }

    @Override
    public void toJson(JsonWriter writer, T value) throws IOException {
        if (value.equals(defaultValue)) {
            writer.nullValue();
        } else {
            delegate.toJson(writer, value);
        }
    }
}

public class MyClass {
    @JsonAdapter(DefaultValueJsonAdapter.class)
    private String fieldWithDefaultValue = "default value";
}

Moshi moshi = new Moshi.Builder()
        .add(new DefaultValueJsonAdapter<String>("default value", String.class))
        .build();
JsonAdapter<MyClass> adapter = moshi.adapter(MyClass.class);
MyClass myObject = adapter.fromJson(json);