HashMap get an element and set into customized value type

Question

I have a problem while I set get an element and set into customized value type. Here is my customized type class User

public class User {

 private List<Integer> FriendList=new ArrayList<Integer>(); //Uid list
 private HashMap<String,String> CheckinMap =new HashMap<String, String>(); // location and time
 private int Uid;

 public User(int uid){
    this.Uid=uid;
 }
 public int getId(){
    return Uid;
 }
 public void setCheckins(HashMap<String,String> InMap){
    this.CheckinMap=(HashMap<String, String>) InMap;
 }
 public void setAFriend(int uid){
   this.FriendList.add(uid);
 }  
}

Then, I parse a JSON array to get data, I am sure that parsing JSON is fine here.

HashMap<Integer,User> UserMap =new HashMap();
HashMap<String,String> InMap =new HashMap();
     for(i=0;i<users.size();i++){
              JSONObject auser=(JSONObject)users.get(i);
              temp_uid=Integer.parseInt(auser.get("uid").toString());
              UserMap.put(temp_uid, new User(temp_uid));
              JSONArray checkins=(JSONArray)auser.get("check-ins");
              for(j=0;j<checkins.size();j++){
                  InMap.put( ((ArrayList) checkins.get(j)).get(0).toString(),  ((ArrayList) checkins.get(j)).get(1).toString());

              }
              UserMap.get(i).setCheckin(InMap);
              checkin_cnt=checkin_cnt+checkins.size();
     }

My Eclipse told me that an exception java.lang.NullPointerException is at the line. UserMap.get(i).setCheckin(InMap);

Can I only initialize Uid , but set FriendList and CheckinMap later? This sounds wired. Please give me some hints or suggestions. Thanks

Answer 1

From this code I should assume that auser's UID is always the same as it's index.

If my assumption is wrong, this is a reason, You should ask for UserMap.get(temp_uid) not UserMap.get(i)