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