MainActivity.java

package com.venvw.firebasetest;

import android.content.DialogInterface;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.BaseAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AlertDialog;
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 com.google.firebase.database.GenericTypeIndicator;
import com.google.firebase.database.ValueEventListener;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.UUID;

public class MainActivity extends AppCompatActivity {

    private EditText nameEditText;
    private EditText salaryEditText;
    private ListView workerListView;

    private DatabaseReference workersReference;
    private HashMap<String, Worker> workerHashMap;
    private List<Worker> workerList;

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

        nameEditText            = findViewById(R.id.nameEditText);
        salaryEditText          = findViewById(R.id.salaryEditText);
        Button addWorkerButton  = findViewById(R.id.addWorkerButton);
        workerListView          = findViewById(R.id.workerListView);

        addWorkerButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Worker worker = new Worker(
                        UUID.randomUUID().toString(),
                        nameEditText.getText().toString(),
                        Integer.valueOf(salaryEditText.getText().toString())
                );
                workersReference.child(worker.getUuid()).setValue(worker);
            }
        });

        workerListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                final String uuid = workerList.get(position).getUuid();
                AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);
                builder.setTitle("Attention")
                        .setMessage("Do you want to remove worker?")
                        .setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                            @Override
                            public void onClick(DialogInterface dialog, int which) {
                                workersReference.child(uuid).removeValue();
                            }
                        })
                        .setNegativeButton("No", new DialogInterface.OnClickListener() {
                            @Override
                            public void onClick(DialogInterface dialog, int which) {
                                dialog.cancel();
                            }
                        })
                        .setOnDismissListener(new DialogInterface.OnDismissListener() {
                            @Override
                            public void onDismiss(DialogInterface dialog) {

                            }
                        })
                        .create().show();
            }
        });

        FirebaseDatabase database = FirebaseDatabase.getInstance();
        workersReference = database.getReference("workers");
        workersReference.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                workerHashMap = dataSnapshot.getValue(new GenericTypeIndicator<HashMap<String, Worker>>() {});
                if(workerHashMap == null) {
                    workerHashMap = new HashMap<>();
                }

                workerList = new ArrayList<Worker>();
                workerListView.setAdapter(new ArrayAdapter<>(getApplicationContext(),
                        android.R.layout.simple_list_item_1, workerList));
                updateWorkerList();

                workersReference.addChildEventListener(new ChildEventListener() {
                    @Override
                    public void onChildAdded(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
                        workerHashMap.put(dataSnapshot.getKey(), dataSnapshot.getValue(Worker.class));
                        updateWorkerList();
                    }

                    @Override
                    public void onChildChanged(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
                        workerHashMap.remove(dataSnapshot.getKey());
                        workerHashMap.put(dataSnapshot.getKey(), dataSnapshot.getValue(Worker.class));
                        updateWorkerList();
                    }

                    @Override
                    public void onChildRemoved(@NonNull DataSnapshot dataSnapshot) {
                        workerHashMap.remove(dataSnapshot.getKey());
                        updateWorkerList();
                    }

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

                    }

                    @Override
                    public void onCancelled(@NonNull DatabaseError databaseError) {
                        Toast.makeText(getApplicationContext(), "Failed to load", Toast.LENGTH_LONG).show();
                    }
                });
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
                Toast.makeText(getApplicationContext(), "Failed to load", Toast.LENGTH_LONG).show();
            }
        });
    }

    private void updateWorkerList() {
        workerList.clear();
        if(workerHashMap != null && !workerHashMap.isEmpty()) {
            workerList.addAll(workerHashMap.values());
        }
        ((BaseAdapter) workerListView.getAdapter()).notifyDataSetChanged();
    }
}