Firebase Reading Data Error onDataChange method on null object reference

Question

I'm facing an error while reading data from firebase DB, I'm trying to allow login with MAC address :

Error:

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference

Data:

This is my code :

 DatabaseReference users = FirebaseDatabase.getInstance().getReference("users");
Query query = users.orderByChild("macAddress").equalTo(getMacAddr());
query.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        if(dataSnapshot.exists()) {
            User user = dataSnapshot.getValue(User.class);
            if(!(user.getEmail().equals(edtEmail.getText().toString()))){
                Snackbar.make(rootLayout, "Please Login With Phone You Registered With", Snackbar.LENGTH_SHORT).show();
                waitingdialog.dismiss();
                return;
            }
        }
        else{
            Snackbar.make(rootLayout, "Please Login With Phone You Registered With", Snackbar.LENGTH_SHORT).show();
            waitingdialog.dismiss();
            return;
        }
    }

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

getMacAddr() is a function that returns mac address of the device in a string

User.class code used in above code :

public class User {

private  String email,password,name,phone,macAddress;
private int isSecured;

public User(String email, String name, String password,String phone,String macAddress,int isSecured){

    this.email=email;
    this.name=name;
    this.password=password;
    this.phone=phone;
    this.macAddress=macAddress;
    this.isSecured=isSecured;
}
public User() {}

public int getIsSecured() {
    return isSecured;
}

public void setIsSecured(int isSecured) {
    this.isSecured = isSecured;
}

public String getEmail() {
    return email;
}

public void setEmail(String email) {
    this.email = email;
}

public String getPassword() {
    return password;
}

public void setPassword(String password) {
    this.password = password;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public String getMacAddress() {
    return macAddress;
}

public void setMacAddress(String macAddress) {
    this.macAddress = macAddress;
}

public String getPhone() {
    return phone;
}

public void setPhone(String phone) {
    this.phone = phone;
}
}

this is User.class that is used to create user object

Answer 1

I just found the error , i changed this line :

Query query = users.orderByChild("macAddress").equalTo(getMacAddr());

to

Query query=users.child(auth.getCurrentUser().getUid()).orderByChild("email");

auth is FirebaseAuth object

Answer 2

Assuming that the issue is indeed within the code that you have provided and given the error message

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference

I would think that the problem is on this line:

Query query = users.orderByChild("macAddress").equalTo(getMacAddr());

More specifically, it appears that users.orderByChild("macAddress") is returning null . I would look into how the users entry is defined in the Firebase database and make sure it is structured correctly. Maybe you can try placing a breakpoint on the problem line and examine the users object to verify that users.orderByChild("macAddress") is not returning null and make corrections to the Firebase db as needed.