<Spinner
    android:id="@+id/spinner"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
/>

import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Spinner;
import android.widget.Toast;

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.ValueEventListener;

import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {

    private Spinner spinner;
    private ArrayAdapter<String> spinnerAdapter;
    private List<String> veriListesi = new ArrayList<>();

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

        spinner = findViewById(R.id.spinner);
        spinnerAdapter = new ArrayAdapter<>(this, android.R.layout.simple_spinner_item, veriListesi);
        spinnerAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        spinner.setAdapter(spinnerAdapter);

        DatabaseReference databaseReference = FirebaseDatabase.getInstance().getReference("veriler");

        databaseReference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                veriListesi.clear();
                for (DataSnapshot veriSnapshot : dataSnapshot.getChildren()) {
                    String veri = veriSnapshot.getValue(String.class);
                    veriListesi.add(veri);
                }
                spinnerAdapter.notifyDataSetChanged();
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
                Toast.makeText(getApplicationContext(), "Veri okuma işlemi başarısız oldu.", Toast.LENGTH_SHORT).show();
            }
        });

        spinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
            @Override
            public void onItemSelected(AdapterView<?> parentView, View selectedItemView, int position, long id) {
                String secilenVeri = parentView.getItemAtPosition(position).toString();
                Toast.makeText(getApplicationContext(), "Seçilen veri: " + secilenVeri, Toast.LENGTH_SHORT).show();
            }

            @Override
            public void onNothingSelected(AdapterView<?> parentView) {
                           Toast.makeText(getApplicationContext(), "Hiç bir veri okunamadı.", Toast.LENGTH_SHORT).show();
            }
        });
    }
}