I am new in Android and working on an Android app which can retrieve data from firebase. There is a weird problem. I already successfully implemented the retrieving function for one data model of my program, and I used same codes just changed the variables but it doesn't work for another data model. I did many tests and I think the problem is in FirebaseHelper cuz there is no any data returned from the data snapshot. The error is
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
The codes are shown below:
The Activity that shows the retrieved data in a listview.
public class TimeTableActivity extends AppCompatActivity {
DatabaseReference db;
FirebaseHelper firebasehelper;
TimeTableAdapter adapter;
ListView lv_CourseList;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_time_table);
//GET INTENT
Intent intent = this.getIntent();
String majorID = intent.getStringExtra("MAJOR_ID");
lv_CourseList = (ListView) findViewById(R.id.lv_CourseList);
//INITIALIZE FIREBASE DB
db= FirebaseDatabase.getInstance().getReference();
firebasehelper=new FirebaseHelper(db);
//ADAPTER
adapter = new TimeTableAdapter(getApplicationContext(),firebasehelper.retrieveCourse(majorID, new CourseCallbacks() {
@Override
public void onCourseCallback(ArrayList<CourseInfo> courseInfos) {
lv_CourseList.setAdapter(adapter);
}
}));
lv_CourseList.setAdapter(adapter);
}
}
FirebaseHelper:
public class FirebaseHelper{
private DatabaseReference db;
private ArrayList<Major> majors = new ArrayList<>();
private ArrayList<CourseInfo> courseInfos = new ArrayList<>();
public FirebaseHelper(DatabaseReference db) {
this.db = db;
}
//Save the Major info. into db
public Boolean saveMajor(Major major)
{
Boolean saved = null;
if(major==null)
{
saved =false;
}else
{
try
{
db.child("Major").push().setValue(major);
saved =true;
}catch (DatabaseException e)
{
e.printStackTrace();
saved =false;
}
}
return saved;
}
//Save the Course info. into db
public Boolean saveCourse(CourseInfo courseInfo)
{
Boolean saved = null;
if(courseInfo==null)
{
saved =false;
}else
{
try
{
db.child("CourseInfo").push().setValue(courseInfo);
saved =true;
}catch (DatabaseException e)
{
e.printStackTrace();
saved =false;
}
}
return saved;
}
public ArrayList<Major> retrieveMajor(final MajorCallbacks majorCallbacks){
ChildEventListener childEventListener = new ChildEventListener() {
@Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
for (DataSnapshot ds : dataSnapshot.getChildren()) {
Major major = ds.getValue(Major.class);
if (major != null && major.getMajor_id() != null) {
majors.add(major);
}
}
majorCallbacks.onMajorCallback(majors);
}
@Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
for (DataSnapshot ds : dataSnapshot.getChildren()) {
Major major = ds.getValue(Major.class);
if (major != null && major.getMajor_id() != null) {
majors.add(major);
}
}
majorCallbacks.onMajorCallback(majors);
}
@Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
}
@Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
};
db.addChildEventListener(childEventListener);
if (!majors.isEmpty()){
db.removeEventListener(childEventListener);
}
return majors;
}
public ArrayList<CourseInfo> retrieveCourse(String majorID, final CourseCallbacks courseCallbacks){
ChildEventListener childEventListener = new ChildEventListener() {
@Override
public void onChildAdded(DataSnapshot dataSnapshot, String prevChildKey) {
for (DataSnapshot ds : dataSnapshot.getChildren()) {
CourseInfo courseInfo = ds.getValue(CourseInfo.class);
if (courseInfo != null && courseInfo.getCourse_id() != null) {
courseInfos.add(courseInfo);
}
}
courseCallbacks.onCourseCallback(courseInfos);
}
@Override
public void onChildChanged(DataSnapshot dataSnapshot, String prevChildKey) {
for (DataSnapshot ds : dataSnapshot.getChildren()) {
CourseInfo courseInfo = ds.getValue(CourseInfo.class);
if (courseInfo != null && courseInfo.getCourse_id() != null) {
courseInfos.add(courseInfo);
}
}
courseCallbacks.onCourseCallback(courseInfos);
}
@Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
}
@Override
public void onChildMoved(DataSnapshot dataSnapshot, String prevChildKey) {
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
};
db.child("CourseInfo").orderByChild("major_id").equalTo(majorID).addChildEventListener(childEventListener);
return courseInfos;
}
}
The retrieveMajor method works well even though it returns many null data and repetitive data, and I used the codes of retrieveMajor in retrieveCourse. It doesn't work, the courseInfos is always null. Even though I changed db.child("CourseInfo").orderByChild("major_id").equalTo(majorID).addChildEventListener(childEventListener);
to
db.addChildEventListener(childEventListener);
,still nothing is retrieved which means the problem is not the query (Probably the query is also wrong).
Adapter:
public class TimeTableAdapter extends BaseAdapter {
Context context;
ArrayList<CourseInfo> courseInfos;
public TimeTableAdapter(Context context, ArrayList<CourseInfo> courseInfos) {
this.context = context;
this.courseInfos = courseInfos;
}
@Override
public int getCount() {
return courseInfos.size();
}
@Override
public Object getItem(int pos) {
return courseInfos.get(pos);
}
@Override
public long getItemId(int pos) {
return pos;
}
@Override
public View getView(int position, View convertView, ViewGroup viewGroup) {
LayoutInflater inflater = LayoutInflater.from(context);
if(convertView == null)
{
convertView= LayoutInflater.from(context).inflate(R.layout.model_timetable,viewGroup,false);
}
TextView tv_courseid= (TextView) convertView.findViewById(R.id.tv_courseid);
TextView tv_coursename= (TextView) convertView.findViewById(R.id.tv_coursename);
TextView tv_courseinstructor= (TextView) convertView.findViewById(R.id.tv_courseinstructor);
TextView tv_courseavailable= (TextView) convertView.findViewById(R.id.tv_courseavailable);
final CourseInfo courseInfo= (CourseInfo) this.getItem(position);
tv_courseid.setText(courseInfo.getCourse_id());
tv_coursename.setText(courseInfo.getCourse_name());
tv_courseinstructor.setText(courseInfo.getCourse_instructor());
tv_courseavailable.setText(courseInfo.getCourse_available());
return convertView;
}
}
Data model:
@IgnoreExtraProperties
public class CourseInfo {
public String course_id;
public String course_name;
public int course_section;
public String course_type;
public double course_crdhrs;
public String course_days;
public String course_times;
public String course_location;
public int course_max;
public int course_cur;
public int course_available;
public int course_wl;
public double course_per;
public String course_instructor;
public String course_description;
public String course_prerequire;
public String major_id;
public CourseInfo() {
}
public CourseInfo(String course_id, String course_name, int course_section, String course_type, double course_crdhrs, String course_days, String course_times, String course_location, int course_max, int course_cur, int course_available, int course_wl, double course_per, String course_instructor, String course_description, String course_prerequire, String major_id) {
this.course_id = course_id;
this.course_name = course_name;
this.course_section = course_section;
this.course_type = course_type;
this.course_crdhrs = course_crdhrs;
this.course_days = course_days;
this.course_times = course_times;
this.course_location = course_location;
this.course_max = course_max;
this.course_cur = course_cur;
this.course_available = course_available;
this.course_wl = course_wl;
this.course_per = course_per;
this.course_instructor = course_instructor;
this.course_description = course_description;
this.course_prerequire = course_prerequire;
this.major_id = major_id;
}
public String getCourse_id() {
return course_id;
}
public String getCourse_name() {
return course_name;
}
public int getCourse_section() {
return course_section;
}
public String getCourse_type() {
return course_type;
}
public double getCourse_crdhrs() {
return course_crdhrs;
}
public String getCourse_days() {
return course_days;
}
public String getCourse_times() {
return course_times;
}
public String getCourse_location() {
return course_location;
}
public int getCourse_max() {
return course_max;
}
public int getCourse_cur() {
return course_cur;
}
public int getCourse_available() {
return course_available;
}
public int getCourse_wl() {
return course_wl;
}
public double getCourse_per() {
return course_per;
}
public String getCourse_instructor() {
return course_instructor;
}
public String getCourse_description() {
return course_description;
}
public String getCourse_prerequire() {
return course_prerequire;
}
public String getMajor_id() {
return major_id;
}
}
If you need more codes or information, please comment and let me know. I will really appreciate if you can also help me solve the null data and repetitive data problem cuz it makes the listview show many blank and repetitive items.
