Jdbc addresses this mismatch through the wasnull

JDBC addresses this mismatch through the wasNull( ) method. As its name implies, wasNull( ) returns true if the last value fetched was
SQL NULL . For calls returning a Java object, the value will generally be NULL when an SQL NULL is read from the database. In these instances,
wasNull( ) may appear somewhat redundant. For primitive datatypes, however, a valid value—such as 0—may be returned on a fetch. The
wasNull( ) method gives you a way to see if that value was NULL in the database.
13.2.2 Error Handling and Clean Up
All JDBC method calls can throw SQLException or one of its subclasses if something happens during a database call. Your code should be set
up to catch this exception, deal with it, and clean up any database resources that have been allocated. Each of the JDBC classes mentioned so
far has a close( ) method associated with it. Practically speaking, however, you only really need to make sure you close things whose calling
processes might remain open for a while. In the examples we have seen so far, you only really need to close your database connections. Closing
the database connection closes any statements and result sets associated with it automatically. If you intend to leave a connection open for any
period of time, however, it is a good idea to close the statements you create using that connection when you finish with them. In the JDBC
examples you have seen, this clean up happens in a finally clause. You do this since you want to make sure to close the database connection
no matter what happens.
only for RuBoard - do not distribute or recompile

only for RuBoard - do not distribute or recompile
13.3 Dynamic Database Access
So far, we have dealt with applications in which you know exactly what needs to be done at compile time. If this were the only kind of database
support that JDBC provided, no one could ever write tools like the mysql interactive command-line tool that determines SQL calls at runtime and
executes them. The JDBC Statement class provides the execute( ) method for executing SQL that can be either a query or an update.
Additionally, ResultSet instances provide runtime information about themselves in the form of an interface called ResultSetMetaData , which
you can access via the getMetaData( ) call in the ResultSet .
13.3.1 Metadata
The term metadata sounds officious, but it is really nothing more than extra data about some object that would otherwise waste resources if it
were actually kept in the object. For example, simple applications do not need the name of the columns associated with a ResultSet —the
programmer probably knew that when the code was written. Embedding this extra information in the ResultSet class is thus not considered by
JDBC's designers to be part of the core of ResultSet functionality. Data such as the column names, however, is very important to some
database programmers—especially those writing dynamic database access. The JDBC designers provide access to this extra information—the
metadata—via the ResultSetMetaData interface. This class specifically provides: