Firebase 从 DataSnapShot 检索存储的 ArrayList 给出 NullPointerException

Question

我的 firebase 数据库看起来像这样

蓝色矩形是我要检索的数据。

蓝色矩形的模态类看起来像这样

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);

  }
}

我正在使用标准查询从 firebase 获取数据。 这是代码

    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) {}
    });

我能够从firebase成功获取剩余数据。 红色矩形中的数据是ArrayList，我无法获取它（在图像中以红色矩形显示）。 我已使用Sysout在控制台中打印，但无法获取 ArrayList 数据。 它返回NullPointerException 。 如何在TripToCompany对象中获取该 ArrayList？

Answer 1

要解决此问题，请更改以下代码行：

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

至

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

您应该将本地emptoCompanyList变量的值分配给成员类（ this ）字段，而不是同一变量。

Answer 2

在我看到的图片中的 JSON 案例中，您需要更改此代码：

ArrayList<EmpToCompany> emptoCompanyList;

到：

ArrayList<String> emptoCompanyList;

但是，如果您想要 EmpToCompany 类的数组列表，则需要显示键 0 和 1 的红色矩形内的内容......我会帮助您:)