deserializing generics with gson - json

Question

I am using GSON 1.4 and serializing an object with two generic arraylist<myObject> as follows
String data = Gson.toJson(object, object.class). When I desirialize it I do gson.fromJson(json, type);
sadly I get
java.lang.IllegalArgumentException: Can not set java.util.ArrayList
field ... to java.util.LinkedList
Why is that ? GSON doc notes that if I serialize with object.class parameter it supports generics. any idea? thanks.
my class is :
public class IndicesAndWeightsParams {
public List<IndexParams> indicesParams;
public List<WeightParams> weightsParams;
public IndicesAndWeightsParams() {
indicesParams = new ArrayList<IndexParams>();
weightsParams = new ArrayList<WeightParams>();
}
public IndicesAndWeightsParams(ArrayList<IndexParams> indicesParams, ArrayList<WeightParams> weightsParams) {
this.indicesParams = indicesParams;
this.weightsParams = weightsParams;
}
}
public class IndexParams {
public IndexParams() {
}
public IndexParams(String key, float value, String name) {
this.key = key;
this.value = value;
this.name = name;
}
public String key;
public float value;
public String name;
}

Answer 1

Gson has some limitations regarding collections because of Java's type erasure. You can read more about it here.
From your question I see you're using both ArrayList and LinkedList. Are you sure you didn't mean to use just List, the interface?
This code works:
List<String> listOfStrings = new ArrayList<String>();
listOfStrings.add("one");
listOfStrings.add("two");
Gson gson = new Gson();
String json = gson.toJson(listOfStrings);
System.out.println(json);
Type type = new TypeToken<Collection<String>>(){}.getType();
List<String> fromJson = gson.fromJson(json, type);
System.out.println(fromJson);
Update: I changed your class to this, so I don't have to mess around with other classes:
class IndicesAndWeightsParams {
public List<Integer> indicesParams;
public List<String> weightsParams;
public IndicesAndWeightsParams() {
indicesParams = new ArrayList<Integer>();
weightsParams = new ArrayList<String>();
}
public IndicesAndWeightsParams(ArrayList<Integer> indicesParams, ArrayList<String> weightsParams) {
this.indicesParams = indicesParams;
this.weightsParams = weightsParams;
}
}
And using this code, everything works for me:
ArrayList<Integer> indices = new ArrayList<Integer>();
ArrayList<String> weights = new ArrayList<String>();
indices.add(2);
indices.add(5);
weights.add("fifty");
weights.add("twenty");
IndicesAndWeightsParams iaw = new IndicesAndWeightsParams(indices, weights);
Gson gson = new Gson();
String string = gson.toJson(iaw);
System.out.println(string);
IndicesAndWeightsParams fromJson = gson.fromJson(string, IndicesAndWeightsParams.class);
System.out.println(fromJson.indicesParams);
System.out.println(fromJson.weightsParams);