Hitting a nullPointerExecption in my android project

Question

Hi Iam trying to make a chat app. In this process Iam trying to retrieve messages stored in database but Iam getting a Null Object reference exception.

The code I used to add messages to database is:

    img_btn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {


            String msg = text_send.getText().toString();
            if(!msg.equals(""))
                sendMessage(fuser.getUid(), currUserId, msg);
            else
                Toast.makeText(MessageActivity.this, "Enter Something", 
            Toast.LENGTH_SHORT).show();
         }
       });


  private void sendMessage(String sender,String receiver,String message)
  {
    DatabaseReference reference = 
    FirebaseDatabase.getInstance().getReference();
    HashMap<String,Object>hashMap = new HashMap<>();
    hashMap.put("sender",sender);
    hashMap.put("receiver",receiver);
    hashMap.put("message",message);

    reference.child("Chats").push().setValue(hashMap);

}

NOTE:The above part of code is working fine. My database is getting updated correctly every time I send message.

After this part I tried to display messages of users, using the following code

     DatabaseReference chatref = 
     FirebaseDatabase.getInstance().getReference().child("Chats");
     ArrayList<String> userchats = new ArrayList<>();

     listView = findViewById(R.id.listView);
     final ArrayAdapter <String> arrayAdapter = new ArrayAdapter<String> 
     (this,android.R.layout.simple_list_item_1,userchats);
     listView.setAdapter(arrayAdapter);

     chatref.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(@NonNull DataSnapshot dataSnapshot, 
        @Nullable String s) {

            String value = dataSnapshot.getValue().toString();
            try {
                JSONObject object = new JSONObject(value);
                message = object.getString("message");
            }
            catch (JSONException e) {
                e.printStackTrace();
            }
            userchats.add(message);
            arrayAdapter.notifyDataSetChanged();
        }

        @Override
        public void onChildChanged(@NonNull DataSnapshot dataSnapshot, 
         @Nullable String s) {

        }

        @Override
        public void onChildRemoved(@NonNull DataSnapshot dataSnapshot) {

        }

        @Override
        public void onChildMoved(@NonNull DataSnapshot dataSnapshot, 
        @Nullable String s) {

        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {

        }
        });

When I ran the app at this moment Iam getting error and my stacktrace is as follows:

         Process: com.kcw.firebasebasics, PID: 10943
         java.lang.NullPointerException: Attempt to invoke virtual method 
         'java.lang.String java.lang.Object.toString()' on a null object 
         reference at android.widget.ArrayAdapter.
         createViewFromResource(ArrayAdapter.java:401)
         at android.widget.ArrayAdapter.getView(ArrayAdapter.java:369)
         at android.widget.AbsListView.obtainView(AbsListView.java:2346)
         at android.widget.ListView.makeAndAddView(ListView.java:1875)

Finally a picture of my database:

Answer 1

Change this:

String value = dataSnapshot.getValue().toString();

into this:

String value = dataSnapshot.child("message").getValue(String.class);