<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <!--ListView for displaying 
        our list of courses-->
    <ListView
        android:id="@+id/idLVCourses"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</RelativeLayout>

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

import android.os.Bundle;
import android.widget.ArrayAdapter;
import android.widget.ListView;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;

import com.google.firebase.database.ChildEventListener;
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 java.util.ArrayList;

public class MainActivity extends AppCompatActivity {

    // creating variables for our list view.
    private ListView coursesLV;

    // creating a new array list.
    ArrayList<String> coursesArrayList;

    // creating a variable for database reference.
    DatabaseReference reference;

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

        // initializing variables for listviews.
        coursesLV = findViewById(R.id.idLVCourses);

        // initializing our array list
        coursesArrayList = new ArrayList<String>();

        // calling a method to get data from
        // Firebase and set data to list view
        initializeListView();
    }

    private void initializeListView() {
        // creating a new array adapter for our list view.
        final ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_dropdown_item_1line, coursesArrayList);

        // below line is used for getting reference
        // of our Firebase Database.
        reference = FirebaseDatabase.getInstance().getReference();

        // in below line we are calling method for add child event 
        // listener to get the child of our database.
        reference.addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {
                // this method is called when new child is added to 
                // our data base and after adding new child
                // we are adding that item inside our array list and
                // notifying our adapter that the data in adapter is changed.
                coursesArrayList.add(snapshot.getValue(String.class));
                adapter.notifyDataSetChanged();
            }

            @Override
            public void onChildChanged(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {
                // this method is called when the new child is added.
                // when the new child is added to our list we will be 
                // notifying our adapter that data has changed.
                adapter.notifyDataSetChanged();
            }

            @Override
            public void onChildRemoved(@NonNull DataSnapshot snapshot) {
                // below method is called when we remove a child from our database.
                // inside this method we are removing the child from our array list 
                // by comparing with it's value.
                // after removing the data we are notifying our adapter that the 
                // data has been changed.
                coursesArrayList.remove(snapshot.getValue(String.class));
                adapter.notifyDataSetChanged();
            }

            @Override
            public void onChildMoved(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {
                // this method is called when we move our 
                // child in our database.
                // in our code we are note moving any child.
            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {
                // this method is called when we get any 
                // error from Firebase with error.
            }
        });
        // below line is used for setting 
        // an adapter to our list view.
        coursesLV.setAdapter(adapter);
    }
}