Gson - deserialize json with nested map - json

Question

I am trying to deserialize this type of json
{
"_embedded": {
"list": [
{
"000000": {
"date": "2015-07-10 14:29:15"
}
},
{
"111111": {
"date": "2015-07-11 14:29:15"
}
}
]
}
}
I manage to get a list inside my embedded object but the list entries are empty.
My Embedded class looks like this
public class Embedded {
#SerializedName("list")
private List<ListEntry> entries;
}
But I am not able to deserialize the list's entries. I think the problem is the fact that the map is nested inside an object that doesn't have a name.
public class ListEntry {
private Map<String, ListEntryInfo> map;
}

Answer 1

The initial problem is the way you declared your hierarchy. A ListEntry is a Map<String, ListEntryInfo> but does not have a Map<String, ListEntryInfo>. To make it work you have three options:
declare the ListEntry class as class ListEntry extends HashMap<String, ListEntryInfo> {}, which is a bad idea in my opinion
get rid of the ListEntry class and declare the entries list like this #SerializedName("list") List<Map<String, ListEntryInfo>> entries;
use the approach I initially described below, by implementing a custom deserializer
As said before what you could do is to write a custom deserializer, so that you have a more typed list of entries.
As a ListEntry instance has only one ListEntryInfo value mapped to a key, I would change the structure to this:
class ListEntry {
private String key;
private ListEntryInfo value;
public ListEntry(String key, ListEntryInfo value) {
this.key = key;
this.value = value;
}
public String toString() {
return key + " -> " + value;
}
}
class ListEntryInfo {
//assuming we store the date as a String for simplicity
#SerializedName("date")
private String date;
public ListEntryInfo(String date) {
this.date = date;
}
public String toString() {
return date;
}
}
Now you need to write a deserializer to create a new ListEntry instance when you deserialize the JSON:
class ListEntryDeserializer implements JsonDeserializer<ListEntry> {
#Override
public ListEntry deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
Iterator<Map.Entry<String, JsonElement>> ite = json.getAsJsonObject().entrySet().iterator();
//you may want to throw a custom exception or return an "empty" instance there
Map.Entry<String, JsonElement> entry = ite.next();
return new ListEntry(entry.getKey(), context.deserialize(entry.getValue(), ListEntryInfo.class));
}
}
This deserializer will read each ListEntry. As it's composed of a single key-value pair (in the first case the String "000000" is mapped to one ListEntryInfo and so on), we fetch the key and deserialize the corresponding ListEntryInfo with JsonDeserializationContext instance.
The final step, is to register it within the parser:
Gson gson = new GsonBuilder().registerTypeAdapter(ListEntry.class, new ListEntryDeserializer()).create();
Running it on your example, you should get:
[000000 -> 2015-07-10 14:29:15, 111111 -> 2015-07-11 14:29:15]