Question

I’m getting the error:

Exception in thread "main" java.lang.NullPointerException
at com.deanchester.minos.utils.DatabaseManager.createTables(DatabaseManager.java:59)
at com.deanchester.minos.tests.testAddContestantMethod.main(testAddContestantMethod.java:21)

I create a connection using this method:

private static final String driver = "org.apache.derby.jdbc.EmbeddedDriver";
private static final String protocol = "jdbc:derby:";
private static final Properties props = new Properties();


public static Connection getDatabaseConnection() {
    try {
        if(conn==null || conn.isClosed()) {
            try {
                Class.forName(driver).newInstance();
                conn = DriverManager.getConnection(protocol+"minos;create:true;",props);
            } catch (InstantiationException e) {
                e.printStackTrace();
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    } catch (SQLException e) {
        e.printStackTrace();
    }
    return conn;
}

I create tables using the following method:

 private static final String tablesSQL = "CREATE TABLE `contestants` (`id` int(11) NOT NULL AUTO_INCREMENT,`first_name` varchar(100) DEFAULT NULL,`last_name` varchar(100) DEFAULT NULL, `entry` varchar(100) DEFAULT NULL, PRIMARY KEY (`id`)) ENGINE=InnoDB DEFAULT CHARSET=latin1;CREATE TABLE `finalTimes` ( `id` int(11) NOT NULL AUTO_INCREMENT,`contestant` int(11) DEFAULT NULL,`time` int(11) DEFAULT NULL,PRIMARY KEY (`id`), KEY `contestant` (`contestant`), CONSTRAINT `finaltimes_ibfk_1` FOREIGN KEY (`contestant`) REFERENCES `contestants` (`id`)) ENGINE=InnoDB DEFAULT CHARSET=latin1;CREATE TABLE `heatTimes` (`id` int(11) NOT NULL AUTO_INCREMENT,`contestant` int(11) DEFAULT NULL,`time` int(11) DEFAULT NULL,PRIMARY KEY (`id`),CONSTRAINT `heattimes_ibfk_1` FOREIGN KEY (`id`) REFERENCES `contestants` (`id`)) ENGINE=InnoDB DEFAULT CHARSET=latin1;";    
public static void createTables(){
    try {
        PreparedStatement ps = conn.prepareStatement(tablesSQL);
        ps.executeUpdate();
        System.out.println("Tables Created");
    } catch (SQLException e) {
        e.printStackTrace();  
    }
}

So why am I getting this error? My create tables SQL came straight from a dump of a MYSQL database. I moved from MYSQL because my software will be run on standalone machines and I wanted nothing else to be installed this is why apache derby is a good choice.

Edit
Here is my Little test class:

public class testAddContestantMethod {
public static void main(String[] args){
    Contestant cont = new Contestant("Dean","Chester","Mazey");

    DatabaseManager.createTables();
    cont.writeContestantToDatabase();
    DatabaseManager.shutdownDatabase();
  }
}

Answer 1

If there’s an exception in getDatabaseConnection, you’re simply printing it out – which means it’s perfectly possible that there was an exception when you tried to create the connection, leaving conn as null, leading to the NullPointerException in createTables.

Additionally, you haven’t shown anything actually calling getDatabaseConnection to start with.

I suggest you make getDatabaseConnection propagate the exception if there is one, and then call it from createTables.

(In general, simply printing out an exception and then pretending it didn’t happen isn’t a suitable error-handling strategy.)