執行prepareStatement(sql)時出現空指針異常NullPointerExceptio

在JAVA執行數據庫SELECT語句前，必定要記得先connect數據庫DriverManager.getConnection(urle,username, password );

否則會報空指針錯誤java.lang.NullPointerException

statement = conn.prepareStatement(sql);  
rs = statement.executeQuery(sql);