Firebase database exception: Attempt to invoke virtual method 'java.lang...' on a null object reference

I already searched everywhere and tried their methods on how to invoke a virtual method but the result is still the same. Please help me, I don't know where my mistake is.

When I run my app on a phone, it throws the following Exception:

Attempt to invoke virtual method 'java.lang.Stringmypackage.studentInformation.getUserID()' on a null object reference

@Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); //FirebaseAuth firebaseAuth = FirebaseAuth.getInstance(); FirebaseUser user = firebaseAuth.getCurrentUser(); if (user != null) { userID = user.getUid(); } //Realtime Firebase firebaseDatabase = FirebaseDatabase.getInstance(); DatabaseReference newData = firebaseDatabase.getReference(); //Display data newData.addValueEventListener(new ValueEventListener() { @Override public void onDataChange(@NonNull DataSnapshot dataSnapshot) { try { showData(dataSnapshot); } catch (Exception e) { Toast.makeText(getBaseContext(), e.getMessage(), Toast.LENGTH_LONG).show(); } } @Override public void onCancelled(@NonNull DatabaseError databaseError) { } }); } 

Method showData()

 private void showData(DataSnapshot dataSnapshot) { for (DataSnapshot ds : dataSnapshot.getChildren()) { studentInformation studentData = new studentInformation(); studentData.setUserID(ds.child(userID).getValue(studentInformation.class).getUserID()); studentData.setStudentFullName(ds.child(userID).getValue(studentInformation.class).getStudentFullName()); studentData.setStudentID(ds.child(userID).getValue(studentInformation.class).getStudentID()); studentData.setEmail(ds.child(userID).getValue(studentInformation.class).getEmail()); studentData.setCampus(ds.child(userID).getValue(studentInformation.class).getCampus()); studentData.setCourse(ds.child(userID).getValue(studentInformation.class).getCourse()); studentData.setIntake(ds.child(userID).getValue(studentInformation.class).getIntake()); } } 

Class studentInformation()

package mypackage; import com.google.firebase.database.IgnoreExtraProperties; @IgnoreExtraProperties public class studentInformation { private String userID; private String studentFullName; private long studentID; private String email; private String campus; private String course; private String intake; studentInformation() { } studentInformation(String userID, String studentFullName, long studentID, String email, String campus, String course, String intake) { this.userID = userID; this.studentFullName = studentFullName; this.studentID = studentID; this.email = email; this.campus = campus; this.intake = intake; } public String getStudentFullName() { return studentFullName; } public void setStudentFullName(String studentFullName) { this.studentFullName = studentFullName; } public long getStudentID() { return studentID; } public void setStudentID(long studentID) { this.studentID = studentID; } public String getEmail() { return email; } public void setEmail(String email) { this.email = email; } public String getCampus() { return campus; } public void setCampus(String campus) { this.campus = campus; } public String getCourse() { return course; } public void setCourse(String course) { this.course = course; } public String getIntake() { return intake; } public void setIntake(String intake) { this.intake = intake; } public String getUserID() { return userID; } public void setUserID(String userID) { this.userID = userID; } } 

Firebase Database: I export it and I changed from json to txt.

{ "main" : { "home" : { "announcement-date" : "jkljlkjdaj", "campus" : { "unikl-miit" : { "course" : { "bcem" : { "courseName" : "BCEM" } } }, "unikl-mimet" : { "course" : { "bse" : { "courseName" : "BSE" } } } } } }, "student" : { "6LKSe5oqfwgrkEV57NXQekDvNhl1" : { "campus" : "UniKL MIIT", "course" : "Diploma In Animation", "email" : "[email protected]", "intake" : "55", "studentFullName" : "haMan", "studentID" : 52213116153, "userID" : "6LKSe5oqfwgrkEV57NXQekDvNhl1" }, "evPFfkE1VtObUlBNz8pizSNKt662" : { "campus" : "UniKL MIIT", "course" : "Bachelor of Multimedia Technology (Hons) in Interactive Multimedia Design (BIMD)", "email" : "[email protected]", "intake" : "69", "studentFullName" : "Adam", "studentID" : 52213116190, "userID" : "evPFfkE1VtObUlBNz8pizSNKt662" }, "rcUgnUBqd4Xb3AxmngE3OD1BBpd2" : { "campus" : "UniKL MIIT", "email" : "[email protected]", "intake" : "52/49/4", "studentFullName" : "jwjajs", "studentID" : 52213116153, "userID" : "rcUgnUBqd4Xb3AxmngE3OD1BBpd2" }, "z46FjWQE8TRYiJxIexMkrp6gSYv2" : { "campus" : "UniKL MIIT", "email" : "[email protected]", "intake" : "16/9/18", "studentFullName" : "Eisom", "studentID" : 52213116132, "userID" : "z46FjWQE8TRYiJxIexMkrp6gSYv2" } } } 

Questions[Update]:

//FirebaseAuth firebaseAuth = FirebaseAuth.getInstance(); FirebaseUser user = firebaseAuth.getCurrentUser(); if (user != null) { userID = user.getUid(); DatabaseReference usersRef = FirebaseDatabase.getInstance().getReference("student"); DatabaseReference currentUserRef = usersRef.child(userID); currentUserRef.addValueEventListener(new ValueEventListener() { @Override public void onDataChange(@NonNull DataSnapshot dataSnapshot) { studentData = dataSnapshot.getValue(studentInformation.class); //studentID I delcared outside the onCreate. Does the 'studentID' store from Firebase? studentFullName = studentData.getStudentFullName();//This one textViewEmail.setText(studentData.getEmail());//Its working, display } @Override public void onCancelled(@NonNull DatabaseError databaseError) { throw databaseError.toException(); // don't ignore errors } }); } //At 'onCreate' textViewStudentName.setText(studentFullName); //Can I do like this? 

Updated 16/06/2018 QUESTION:

I want to make the value from Firebase Database inside new firebaseDatabaseCampus.

I made this one, and apps crashed. Please help me.

@Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { //Inflate the layout for this fragment View viewMain = inflater.inflate(R.layout.fragment_campus, container, false); recyclerView = viewMain.findViewById(R.id.recycleViewCampus); recyclerView.setHasFixedSize(true); recyclerView.setLayoutManager(new LinearLayoutManager(getContext()));//Ini penting nak display firebaseAuth = FirebaseAuth.getInstance();//Ini penting firebaseDatabase = FirebaseDatabase.getInstance(); //So retrieve data userID = firebaseAuth.getUid(); databaseReferenceStudent = firebaseDatabase.getReference().child("student").child(userID); databaseReferenceStudent.addValueEventListener(new ValueEventListener() { //So sini kita ambik value campus ~ UniKL MIIT @Override public void onDataChange(@NonNull DataSnapshot dataSnapshot) { studentData = new studentInformation();//Ini declare studentData = dataSnapshot.getValue(studentInformation.class); //1. So student.getCampus is here databaseReferenceCampus = firebaseDatabase.getReference().child("campus").child(studentData.getCampus()).child("post"); } @Override public void onCancelled(@NonNull DatabaseError databaseError) { } }); return viewMain; } //On Start @Override public void onStart() { super.onStart(); FirebaseRecyclerAdapter<campus, CampusViewHolder> firebaseRecyclerAdapterCampus = new FirebaseRecyclerAdapter<campus, CampusViewHolder>( campus.class, R.layout.campus_row, CampusViewHolder.class, databaseReferenceCampus //So the value getCampus is here? ) { @Override protected void populateViewHolder(CampusViewHolder viewHolder, campus model, int position) { viewHolder.setDetails(getContext(), model.getImage(), model.getTitle(), model.getDate(), model.getShortDescription()); } }; //setAdapter recyclerView.setAdapter(firebaseRecyclerAdapterCampus); }