java 里面的 Gson 问题

public class TestGson {

public static void main(String args[])
{

    String jsonAvatar=""; //你的json数据
    User user=new User();
    user.setUser_id(3);
    user.setUsername("Tony");
    user.setUrlname("comq");
    user.setAvatar(jsonAvatar);
    Gson gson=new Gson();
    String result=gson.toJson(user);
    System.out.println(result);
 }

}
public class User {

private int user_id;

private String username;

private String urlname;

private String avatar; 

public int getUser_id() {
    return user_id;
}

public void setUser_id(int user_id) {
    this.user_id = user_id;
}

public String getUsername() {
    return username;
}

public void setUsername(String username) {
    this.username = username;
}

public String getUrlname() {
    return urlname;
}

public void setUrlname(String urlname) {
    this.urlname = urlname;
}

public String getAvatar() {
    return avatar;
}

public void setAvatar(String avatar) {
    this.avatar = avatar;
}
}