String null object reference [duplicate]

Answer 1

As the error message says:

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

So you're calling something.equals(...) somewhere, and something is null. The stack trace tells you that the problem is on:

MessageActivity$3.onDataChange(MessageActivity.java:134)

So on line 134 of MessageActivity.java, inside the onDataChange method. While I can't be certain what line 134 is, there is only one onDataChange method in the code you shared, and only one call to equals in there:

reference.addValueEventListener ( new ValueEventListener () {
    @Override
    public void onDataChange (@NonNull DataSnapshot dataSnapshot) {
        for(DataSnapshot snapshot:dataSnapshot.getChildren ()){
            User user=snapshot.getValue (User.class);

            assert user!=null;
            assert fuser!=null;
            if(!fuser.getUid ().equals ( user.getFirstName () )){
                username.setText ( user.getFirstName () );
            }
        }
        readMessages ( fuser.getUid (),userid );
    }

So the problem seems to come from this line:

if(!fuser.getUid().equals ( user.getFirstName () )){

Specifically: fuser.getUid() seems to return null, which means you're calling null.equals(...) and that's what the error message says.

It's fairly uncommon for FirebaseAuth.getInstance().getCurrentUser() to return a user object, and then for getUid() to return null, but that does seem to be what's happening for you.

Answer 2

• try to cache the error and suppress it when ever its null.
• check for the lines (.equals)
• Create a local variable and initialize default values (something like String myid=0, userid="n/a")
• Issue from readMessage method and click listeners - your values are empty
• Log the values DataSnapShot and check it