Firebase 从 DataSnapShot 检索存储的 ArrayList 给出 NullPointerException

Question

My firebase database looks likes this

The blue rectangle is the data I want to retrieve.

Modal class for the blue rectangle looks like this

public class TripToCompany implements Serializable {

String tripDate;
String companyName;
String vehicleNo;
boolean isFinished;
String firstPickUp;
String inTime;
ArrayList<EmpToCompany> emptoCompanyList;

public TripToCompany() {
}

public TripToCompany(String tripDate, String companyName, String vehicleNo, boolean isFinished, String firstPickUp, String inTime, ArrayList<EmpToCompany> emptoCompanyList) {
    this.tripDate = tripDate;
    this.companyName = companyName;
    this.vehicleNo = vehicleNo;
    this.isFinished = isFinished;
    this.firstPickUp = firstPickUp;
    this.inTime = inTime;
    this.emptoCompanyList = emptoCompanyList;
}

public TripToCompany(String tripDate, String companyName, String vehicleNo) {
    this.tripDate = tripDate;
    this.companyName = companyName;
    this.vehicleNo = vehicleNo;
    this.isFinished = false;
    this.inTime = "-";
    this.emptoCompanyList = new ArrayList<>();
}

public String getTripDate() {
    return tripDate;
}

public void setTripDate(String tripDate) {
    this.tripDate = tripDate;
}

public String getCompany() {
    return companyName;
}

public void setCompany(String companyName) {
    this.companyName = companyName;
}

public String getVehicleNo() {
    return vehicleNo;
}

public void setVehicleNo(String vehicleNo) {
    this.vehicleNo = vehicleNo;
}

public boolean isFinished() {
    return isFinished;
}

public void setFinished(boolean isFinished) {
    this.isFinished = isFinished;
}

public String getFirstPickUp() {
    return firstPickUp;
}

public void setFirstPickUp(String firstPickUp) {
    this.firstPickUp = firstPickUp;
}

public String getInTime() {
    return inTime;
}

public void setInTime(String inTime) {
    this.inTime = inTime;
}

public ArrayList<EmpToCompany> getEmptoCompanyList() {
    return emptoCompanyList;
}

public void setEmptoCompanyList(ArrayList<EmpToCompany> emptoCompanyList) {
    emptoCompanyList = emptoCompanyList;
}

public void addEmpToCompanyList(EmpToCompany etc) {
    if (emptoCompanyList.size() == 0) {
        firstPickUp = etc.getCabInTime();
    }
    emptoCompanyList.add(etc);

  }
}

I am fetching the data from firebase using the standard query. Here is the code

    FirebaseDatabase defaultDatabase = FirebaseDatabase.getInstance();
    DatabaseReference mRef = defaultDatabase.getReference("data");
    Query query = mRef.child("toComp/" + companyId + "/" + fromDate + "/" + shiftTimeId + "/");
    query.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot ds) {

            if (ds.exists()) {
                System.out.println("exist");
                for (DataSnapshot singleSnapshot : ds.getChildren()) {
                    TripToCompany trpObj = (TripToCompany) singleSnapshot.getValue(TripToCompany.class);
                    ArrayList<EmpToCompany>ee= trpObj.getEmptoCompanyList();
                    System.out.println("Company Name: "+trpObj.getCompany()); //successfully retrived
                    System.out.println("Employee Count: " + ee.size()); //unable to fetch it. Gives NullPointerException

                }
            } else {
                System.out.println("error");
            }
        }

        @Override
        public void onCancelled(DatabaseError de) {}
    });

I am able to get the remaining data successfully from firebase. The data in red rectangle is the ArrayList, and I am unable to fetch it(show in red rectangle in the image). I have printed in the console using Sysout and I am unable to get ArrayList data. It returns NullPointerException . How can I fetch that ArrayList in the TripToCompany Object?

Answer 1

To solve this, please change the following lines of code:

public void setEmptoCompanyList(ArrayList<EmpToCompany> emptoCompanyList) {
    emptoCompanyList = emptoCompanyList;
}

to

public void setEmptoCompanyList(ArrayList<EmpToCompany> emptoCompanyList) {
    this.emptoCompanyList = emptoCompanyList;
//    ^
}

You should assign the value of the local emptoCompanyList variable to the member class ( this ) field, not to the same variable.

Answer 2

In your JSON case from the picture I see, you need to change this code:

ArrayList< EmpToCompany> emptoCompanyList;

to:

ArrayList< String> emptoCompanyList;

But if you want arraylist of EmpToCompany class, you need to show what you have inside the red rectangle for keys 0 and 1 .... and I will help you :)