I am creating a social networking app using android studio and java, my problem is in comments functionality in the comments section of the app, whenever a user tries to post a comment his previous comments are replaced by new ones, please help!

Here’s the code where i think is the problem –

 protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_comments);


        Post_key = getIntent().getExtras().get("PostKey").toString();
        PostsRef = FirebaseDatabase.getInstance().getReference().child("Posts").child(Post_key).child("comments").push();
        String comment_push_id = PostsRef.getKey();

        mAuth = FirebaseAuth.getInstance();
        current_user_id = mAuth.getCurrentUser().getUid();

        UsersRef = FirebaseDatabase.getInstance().getReference().child("Users");



        CommentsList = findViewById(R.id.comments_list);
        CommentsList.setHasFixedSize(true);
        LinearLayoutManager linearLayoutManager = new LinearLayoutManager(this);
        linearLayoutManager.setReverseLayout(true);
        linearLayoutManager.setStackFromEnd(true);
        CommentsList.setLayoutManager(linearLayoutManager);

Here’s the related part of code which might be useful –

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

                UsersRef.child(current_user_id).addValueEventListener(new ValueEventListener() {
                    @Override
                    public void onDataChange(@NonNull DataSnapshot snapshot) {

                        if(snapshot.exists())
                        {
                            String userName = snapshot.child("username").getValue().toString();
                            ValidateComment(userName);

                            CommentInputText.setText("");
                        }


                    }

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

                    }
                });

            }
        });

    }

Here’s the part of code where a user inserts the comments into the firebase database –

 private void ValidateComment(String userName) {

        String commentText = CommentInputText.getText().toString();

        if(TextUtils.isEmpty(commentText))
        {
            Toast.makeText(this,"Please write text to comment...",Toast.LENGTH_LONG).show();
        }
        else
        {

            Calendar callForDate = Calendar.getInstance();
            SimpleDateFormat currentDate = new SimpleDateFormat("dd-MMMM-yyyy");
            final String saveCurrentDate = currentDate.format(callForDate.getTime());

            Calendar callForTime = Calendar.getInstance();
            SimpleDateFormat currentTime = new SimpleDateFormat("HH:mm");
            final String saveCurrentTime = currentTime.format(callForDate.getTime());

            final String RandomKey = current_user_id + saveCurrentDate + saveCurrentTime;

            HashMap commentsMap = new HashMap();
            commentsMap.put("uid",current_user_id);
            commentsMap.put("comment",commentText);
            commentsMap.put("date",saveCurrentDate);
            commentsMap.put("time",saveCurrentTime);
            commentsMap.put("username",userName);

           PostsRef.child(RandomKey).updateChildren(commentsMap).addOnCompleteListener(new OnCompleteListener() {
               @Override
               public void onComplete(@NonNull Task task) {

                   if(task.isSuccessful())
                   {
                       Toast.makeText(CommentsActivity.this,"comment posted successfully....",Toast.LENGTH_LONG).show();
                   }
                   else
                   {
                       Toast.makeText(CommentsActivity.this,"Error Occured, Try Again...",Toast.LENGTH_LONG).show();
                   }

               }
           });


        }

    }
}

In the database also some comments are not getting stored, while only the replaced ones are getting stored, please help