CommentActivity

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.text.TextUtils;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

import com.firebase.ui.database.FirebaseRecyclerAdapter;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.Query;
import com.google.firebase.database.ServerValue;
import com.google.firebase.database.ValueEventListener;

public class CommentActivity extends AppCompatActivity {

    DatabaseReference mDatabase, mRef, mUserRef;
    FirebaseAuth mAuth;

    RecyclerView recyclerView;
    LinearLayoutManager mManager;
    FirebaseRecyclerAdapter<Comment, CommentViewHolder> mAdapter;

    String postKey;
    String fullName, userId;
    long timestamp;

    TextView tvNoComment;
    Button btnSend;
    EditText etComment;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_comment);

        Bundle extras = getIntent().getExtras();
        postKey = extras.getString("postKey");

        tvNoComment = (TextView) findViewById(R.id.tvNoComment);
        btnSend = (Button) findViewById(R.id.btnSend);
        etComment = (EditText) findViewById(R.id.etComment);

        recyclerView = (RecyclerView) findViewById(R.id.commentList);
        recyclerView.setHasFixedSize(true);
        mManager = new LinearLayoutManager(this);
        recyclerView.setLayoutManager(mManager);

        btnSend.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                sendComment();
            }
        });

//        if (fullName.equals(null)){
//            btnSend.setEnabled(false);
//        }
    }

    private void sendComment() {
        final String content = etComment.getText().toString();

        if (TextUtils.isEmpty(content)){
            etComment.setError("Komentar harus diisi!");
        } else {
            final String comKey = mRef.push().getKey();
            mRef.child(comKey).child("timestamp").setValue(ServerValue.TIMESTAMP);

//            Query comCount = mDatabase.child("reports").child(postKey).child("commentCount")
            Query query = mRef.child(comKey).child("timestamp");
            query.addValueEventListener(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                    if (dataSnapshot.getValue() != null) {
                        timestamp = dataSnapshot.getValue(long.class);
                        Comment com = new Comment(postKey, content, fullName, timestamp);
                        mRef.child(comKey).setValue(com);
                    } else {
                        tvNoComment.setVisibility(View.VISIBLE);
                    }
                }

                @Override
                public void onCancelled(DatabaseError databaseError) {

                }
            });
            etComment.setText("");

        }
    }

    @Override
    protected void onStart() {
        super.onStart();

        mDatabase = FirebaseDatabase.getInstance().getReference();
        mAuth = FirebaseAuth.getInstance();
        mRef = mDatabase.child("comments").child(postKey);

        fullName = getFullName();
        System.out.println("fullName2: " + fullName);

        mAdapter = new FirebaseRecyclerAdapter<Comment, CommentViewHolder>(
                Comment.class,
                R.layout.item_comment,
                CommentViewHolder.class, mRef
        ) {
            @Override
            protected void populateViewHolder(CommentViewHolder viewHolder, Comment model, int position) {
                viewHolder.bindToComment(model);
                tvNoComment.setVisibility(View.GONE);
            }
        };
        recyclerView.setAdapter(mAdapter);

    }
}