Logo Questions Linux Laravel Mysql Ubuntu Git Menu
 

deserializing generics with gson

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;
}
like image 254
Bick Avatar asked Dec 06 '10 08:12

Bick


People also ask

How do I deserialize JSON with Gson?

Deserialization – Read JSON using Gson. Deserialization in the context of Gson means converting a JSON string to an equivalent Java object. In order to do the deserialization, we need a Gson object and call the function fromJson() and pass two parameters i.e. JSON string and expected java type after parsing is finished ...

Is Gson better than Jackson?

ConclusionBoth Gson and Jackson are good options for serializing/deserializing JSON data, simple to use and well documented. Advantages of Gson: Simplicity of toJson/fromJson in the simple cases. For deserialization, do not need access to the Java entities.

Is Gson toJson thread safe?

Gson is typically used by first constructing a Gson instance and then invoking toJson(Object) or fromJson(String, Class) methods on it. Gson instances are Thread-safe so you can reuse them freely across multiple threads.


1 Answers

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);
like image 57
darioo Avatar answered Oct 03 '22 17:10

darioo