我有以下JSON表示盐请求的服务器响应：

{
    "USER":
    {
        "E_MAIL":"email",
        "SALT":"salt"
    },
    "CODE":"010"
}

我尝试使用以下POJO映射它：

public class SaltPOJO {
    private String code = null;
    private User user = null;

    @Override
    public String toString() {
        return this.user.toString();
    }

    public String getCode() {
        return code;
    }
    public void setCode(String code) {
        this.code = code;
    }
    public User getUser() {
        return user;
    }
    public void setUser(User user) {
        this.user = user;
    }

    public class User {
        private String e_mail = null;
        private String salt = null;

        @Override
        public String toString() {
            return this.e_mail + ": " + this.salt;
        }
        public String getE_mail() {
            return e_mail;
        }
        public void setE_mail(String e_mail) {
            this.e_mail = e_mail;
        }
        public String getSalt() {
            return salt;
        }
        public void setSalt(String salt) {
            this.salt = salt;
        }
    }
}

现在每次我这样做：

Gson gson = new Gson();
SaltPOJO saltPojo = gson.fromJson(json.toString(), SaltPOJO.class);

Log.v("Bla", saltPojo.toString());

该saltPojo.toString()为空。如何使用Gson将JSON映射到POJO？我的变量顺序对Gson映射重要吗？