Deserialise a generic list in Gson

Question

I want to write a generic function which deserialise a generic type List with Gson here is the code:

private  List GetListFromFile(String f         


        

Answer 1

There is no way to do it without passing actual type of T (as Class<T>) to your method.

But if you pass it explicitly, you can create a TypeToken for List<T> as follows:

private <T> List<T> GetListFromFile(String filename, Class<T> elementType) {
    ...
    TypeToken<ArrayList<T>> token = new TypeToken<ArrayList<T>>() {};
    List<T> something = gson.fromJson(data, token.getType());
    ...
}

See also:

• TypeToken

Answer 2

The approach of use TypeToken won't work.

new TypeToken<ArrayList<T>>() 

is not possible because of how generics (type erasure) and reflection works. The whole TypeToken hack works because Class#getGenericSuperclass() does the following

Returns the Type representing the direct superclass of the entity (class, interface, primitive type or void) represented by this Class.

If the superclass is a parameterized type, the Type object returned must accurately reflect the actual type parameters used in the source code.

In other words, if it sees ArrayList<T>, that's the ParameterizedType it will return and you won't be able to extract the compile time value that the type variable T would have had.

Type and ParameterizedType are both interfaces. You can provide an instance of your own implementation.

So, you have two options:

Option 1: implement java.lang.reflect.ParameterizedType yourself and pass it to Gson.

private static class ListParameterizedType implements ParameterizedType {

    private Type type;

    public ListParameterizedType(Type type) {
        this.type = type;
    }

    @Override
    public Type[] getActualTypeArguments() {
        return new Type[] {type};
    }

    @Override
    public Type getRawType() {
        return ArrayList.class;
    }

    @Override
    public Type getOwnerType() {
        return null;
    }

    // implement equals method too! (as per javadoc)
}

Then simply:

Type type = new ListParameterizedType(clazz);
List<T> list = gson.fromJson(json, type);

Note that as per javadoc, equals method should also be implemented.

Option 2: Parse the list manually and then for each element use Gson

public <T> List<T> listEntity(Class<T> clazz)
        throws WsIntegracaoException {
    try {
        // Consuming remote method
        String strJson = getService().listEntity(clazz.getName());

        JsonParser parser = new JsonParser();
        JsonArray array = parser.parse(strJson).getAsJsonArray();

        List<T> lst =  new ArrayList<T>();
        for(final JsonElement json: array){
            T entity = GSON.fromJson(json, clazz);
            lst.add(entity);
        }

        return lst;

    } catch (Exception e) {
        throw new WsIntegracaoException(
                "WS method error [listEntity()]", e);
    }
}