How do I Gson-serialize an object extending a Collection?

Question

I have this class which I want to serialize:

class EnrichedList extends ArrayList<String> {
    long timestamp = System.currentTimeMillis();
}

The out-of-the-box Gson serializer ignores the timestamp field because it's in a collection class. I know I can write a TypeAdapter and manually handle the class, and it might be the solution I go for, but I want to explore my options. Are there any better option which scales and maintains better? Think of the scenario when a new developer enters the team and adds a field/property to the EnrichedClass without knowing he has to update the TypeAdapter simultaneously.

Evidence for my issue:

class EnrichedList extends ArrayList<String> {
    long timestamp = System.currentTimeMillis();
}

class SimpleObject {
    long timestamp = System.currentTimeMillis();
}


@Test
public void serializeIt() {
    EnrichedList list = new EnrichedList();
    list.add("Fred");
    list.add("Flintstone");
    System.out.println("Enriched list: " + new Gson().toJson(list));
    System.out.println("Simple object: " + new Gson().toJson(new SimpleObject()));
}

Produces this output:

Enriched list: ["Fred","Flintstone"]
Simple object: {"timestamp":1418739904470}

Desired output (example):

Enriched list: {"timestamp":1418739904470, "list": ["Fred","Flintstone"]}
Simple object: {"timestamp":1418739904470}

The JSON you want is invalid. – Sotirios Delimanolis Dec 16 '14 at 14:34 — Sotirios Delimanolis, Dec 16 '14 at 14:34

score 3 · Accepted Answer · answered Dec 16 '14 at 14:39

So given your edit, with corrected JSON of

{"timestamp":1418739904470, "list": ["Fred","Flintstone"]}

you have to ask yourself where the key list comes from. What if your collection was a Set? What if it was something else? Gson doesn't have such defaults.

The most appropriate solution, in my opinion, is to use composition over inheritance (obviously, only if that's possible).

class EnrichedList {
    long timestamp = System.currentTimeMillis();
    ArrayList<String> list = new ArrayList<String>();
}

Gson will have no trouble serializing that to the JSON you want.

How do I Gson-serialize an object extending a Collection?

1 Answers1