NullPointerException when creating tables in Derby Embedded mode

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.)