Question

I have a List of objects, let’s say:

List<Timestamp>

Each “Timestamp” object includes other objects, in particular it has a “Tag” object.

class Timestamp {
    String time;
    ...
    Tag tag;
    ...
}

Now, every Tag object is identified by an ID of type “Integer”.

class Tag {
    Integer id;
    ...
}

For a few reasons, I have to write the JSON representation of the whole timestamp list into a file by using the Gson library. In some cases I need the decimal representation of the ID of each Tag, while in other cases I need IDs in hexadecimal format.

How can I “switch” between the two formats? Consider that to write the whole list of Timestamp objects I use the following instruction:

ps.println(gson.toJson(timestamps));

and I can’t add other fields/types/objects in the Tag class because the JSON representation would be different.

Answer 1

I think this is the answer:

1. write a custom gson serializer for the Tag class.
2. Add a flag variable to Tag that indicates when to output the id in hex and when to toutput the id in decimal.
3. Create a toString() method on the Tag class that pays attention to the newly added flag.

Custom Serializer (variation from the gson doc example)

private class TagSerializer implements JsonSerializer<Tag>
{
  public JsonElement serialize(Tag src, Type typeOfSrc, JsonSerializationContext context)
  {
    return new JsonPrimitive(src.toString());
  }
}

Register the custom serializer

GsonBuilder gson = new GsonBuilder();
gson.registerTypeAdapter(Tag.class, new TagSerializer());

Tag Updates

boolean displayIdInHex = false;

public void setDisplayIdInDecimal()
{
  displayIdInHex = false;
}

public void setDisplayIdInHex()
{
  displayIdInHex = true;
}

public String toString()
{
  ... stuff ...
  if (displayIdInHex)
  {
    ... output id in hex.
  }
  else
  {
    ... output id in decimal.
  }
}

TimeStamp Updates
public void setDisplayIdInDecimal()
{
tag.setDisplayIdInDecimal();
}

public void setDisplayIdInHex()
{
  tag.setDisplayIdInHex();
}