public class MainActivity extends AppCompatActivity {
    private EditText miEditTextLoginNombre, miEditTextLoginPass;
    private Button miBotonLoginEntrar;
    private DatabaseReference alumnos;
    Snackbar sBar;
    boolean isAdmin;

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

        miEditTextLoginNombre = (EditText) findViewById(R.id.editTextLoginNombre);
        miEditTextLoginPass = (EditText) findViewById(R.id.editTextLoginPass);
        miBotonLoginEntrar = (Button) findViewById(R.id.buttonLoginEntrar);

        alumnos = FirebaseDatabase.getInstance().getReference().child(FirebaseReferences.ALUMNOS_REF);

        isAdmin = false;
        miBotonLoginEntrar.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                String usuario = miEditTextLoginNombre.getText().toString();
                String contra = miEditTextLoginPass.getText().toString();

                    if(usuario.isEmpty() || contra.isEmpty()){
                        sBar = Snackbar.make(view, "Hay campos vacíos", Snackbar.LENGTH_LONG);
                        sBar.show();
                    }else{
                        comprobarUsuario();
                    }

            }
        });
    }

    void comprobarUsuario(){
        AlertDialog.Builder builder;

        alumnos.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                for(DataSnapshot snap: dataSnapshot.getChildren()){
                    Alumno a = snap.getValue(Alumno.class);
                    if(a.getDni_alum().equals(miEditTextLoginNombre) && a.getClave().equals(miEditTextLoginPass)){
                        isUser = true;
                        break;
                    }
                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });

      if(isAdmin == true){
            Toast.makeText(getApplicationContext(), "Has iniciado sesión como Admin", Toast.LENGTH_SHORT).show();
        }
    }
}