Trying To Make A Chat App In Android Studio But I Am Unable To Append The Texts

Chat.java :

My app only shows the last message typed and shows in the ChatWindow. My firebase Database gets the messages but i am unable to show the same to the user in my app. I want to show at least 5-10 previous messages to the user.

    public class Chat extends AppCompatActivity {
    private DatabaseReference Database;
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_chat);
        final UserLocalStore localStore = new UserLocalStore(Chat.this);
        Database = FirebaseDatabase.getInstance().getReference("Messages");
        final TextView Chat_Window=(TextView)findViewById(R.id.chat_window);
        Database.addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(@NonNull DataSnapshot dataSnapshot, @androidx.annotation.Nullable String s) {
                Log.e("DatasnapShot",dataSnapshot.getKey().toString());

                Chat_Window.setText(dataSnapshot.getValue().toString());
            }
            @Override
            public void onChildChanged(@NonNull DataSnapshot dataSnapshot, @androidx.annotation.Nullable String s) {

            }

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

            }

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

            }

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

            }
        });

        Button SendMessage = (Button)findViewById(R.id.send);
        SendMessage.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                String messageText = Chat_Window.getText().toString();            
         Database.push().setValue((localStore.getUserData().get(0).toString()));
                Database.push().setValue(ChatType.getText().toString());
                Chat_Window.append(ChatType.getText().toString());
                ChatType.setText("");
            }
        });
            Button Signout = (Button)findViewById(R.id.signout);
    {
        Signout.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
            UserLocalStore localStore=new UserLocalStore(Chat.this);
            localStore.LogOut();
            FirebaseAuth.getInstance().signOut();
            Intent intent = new Intent(Chat.this,MainActivity.class);
                startActivity(intent);
            }
        });
    } 
    }
}

My activity_chat.xml:

    <?xml version="1.0" encoding="utf-8"?>
    <androidx.constraintlayout.widget.ConstraintLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/linearLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

   <EditText
    android:id="@+id/chat_type"
    android:layout_width="260dp"
    android:layout_height="44dp"
    android:layout_marginBottom="2dp"
    android:ems="10"
    android:hint="Enter message to send..."
    android:inputType="textPersonName"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintStart_toStartOf="parent" />

   <Button
    android:id="@+id/send"
    android:layout_width="122dp"
    android:layout_height="47dp"
    android:layout_marginStart="29dp"
    android:text="Send"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintHorizontal_bias="0.0"
    app:layout_constraintStart_toEndOf="@+id/chat_type" />

    <TextView
    android:id="@+id/chat_window"
    android:layout_width="406dp"
    android:layout_height="566dp"
    android:layout_marginTop="24dp"
    android:layout_marginBottom="8dp"
    android:gravity="bottom"
    android:hint="Chat message wil appear here..."
    android:padding="5dp"
    app:layout_constraintBottom_toTopOf="@+id/chat_type"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintHorizontal_bias="1.0"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toBottomOf="@+id/signout" />

    <Button
    android:id="@+id/signout"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginStart="324dp"
    android:text="Sign Out"
    app:layout_constraintBottom_toTopOf="@+id/chat_window"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="@id/chat_window" />

    </androidx.constraintlayout.widget.ConstraintLayout>

A screentshot of the VirtualMobile: https://imgur.com/a/XUWgom7

Firebase Database structure:

Chat_Window.setText("");

Add this line after you initialize Chat_Window.

Chat_Window.setText(dataSnapshot.getValue().toString());

Change this line to -

Chat_Window.append("\n" + dataSnapshot.getValue().toString());