Select usuarios para retorna se existe ou nao!

boolean flag = false;
        Connection conn = null;
        PreparedStatement stm = null;
        ResultSet rs = null;

        try {

            String sql = "select email from tb_usuario where email = ? ";

            AcessoBD bd = new AcessoBD();
            conn = bd.obtemConexao();
            stm = conn.prepareStatement(sql);

            stm.setString(1, email);

            rs = stm.executeQuery();

            if(rs != null){
               flag = true;
            }
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