public void returInfoClient(userClient) {

    Connection mConn = (new ConnectionFactory()).getConnection();

    String sql = "SELECT nameClient, userClient, descriptionClient, passwordClient, birtDate
                  FROM Client
                  WHERE userClient LIKE 'userClient%';" //quero que o userClient venha do parametro da função
                  + "values(?);";

    try {
        PreparedStatement stmt = mConn.prepareStatement(sql);
        stmt.setString(1, userClient); //quero usar o userClient do parametro da função para o like do select
        stmt.execute();
        stmt.close();
        System.out.println("retornando informações author");
        connection.close();
    } catch (SQLException e) {
        e.printStackTrace();
    }
}

String sql = "SELECT nameClient, userClient, descriptionClient, passwordClient, birtDate FROM Client WHERE userClient LIKE '?%';"

public void returInfoClient(TipoDoObjeto userClient) {

public void returInfoClient(userClient) {

    Connection mConn = (new ConnectionFactory()).getConnection();

    String sql = "SELECT nameClient, userClient, descriptionClient, passwordClient, birtDate  FROM Client  WHERE userClient LIKE ?";

    try {
        PreparedStatement stmt = mConn.prepareStatement(sql);
        stmt.setString(1,  userClient + "%");
        stmt.execute();
        stmt.close();
        System.out.println("retornando informações author");
        connection.close();
    } catch (SQLException e) {
        e.printStackTrace();
    }
}