MainActivity.java

public class MainActivity extends Activity implements View.OnClickListener {
    private Button btnLogar, btnCadastrar;
    private EditText edtLogin, edtSenha;
    private DbHelper dbhelper;

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

        initViews();
        initDbHelper();
    }
  
  public void initDbHelper(){
      dbhelper = new DbHelper(getApplicationContext()); 
  }
  
  public void initViews(){
    btnCadastrar = (Button)findViewById(R.id.btnCadastrar);
    btnCadastrar.setOnClickListener(this);
    btnLogar = (Button)findViewById(R.id.btnLogar);
    btnLogar.setOnClickListener(this);           
    edtLogin = (EditText)findVviewById(R.id.edtLogin);
    edtSenha = (EditText)findVviewById(R.id.edtSenha);
  }
  
  @Override
    public void onClick(View v) {
        switch (v.getId()) {
          case R.id.btnLogar:
            try{
              if(dbhelper.entrar(edtLogin.getText().toString(), edtSenha.getText().toString()))
                alert("Entrou!");
              else
                alert("Usuário não encontrado!");
            }catch(Exception e){
              alert(e.getMessage());
            }            
            break;
            case R.id.btnCadastrar:
                //Ir para tela de cadastro CadastroActivity
                break;
        }
    }

    private void alert(String s) {
        Toast.makeText(this, s, Toast.LENGTH_LONG).show();
    }
}