Although SQL functions are useful, Java classes provide a more powerful and flexible way of implementing user-defined functions, with the additional advantage that you can move them from the database server to a client application if desired.
Any class method of an installed Java class can be used as a user-defined function anywhere a built-in function of the same data type is used.
Instance methods are tied to particular instances of a class, and so have different behavior from standard user-defined functions.
For more information on creating Java classes, and on class methods, see Java in the Database in the Programming guide.