Gsonでsnake_caseのjsonをcamelCaseのクラスで受け取る

MainActivity.java

        String json = "{id:3, age:30, first_name: 'hoge', last_name: 'fuga'}";
        Gson gson = new GsonBuilder().setFieldNamingPolicy(FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES).create();
        User user = gson.fromJson(json, User.class);

User.java

public class User {
    public int id;
    public int age;
    public String firstName;
    public String lastName;
}