If you want to do this from PL/SQL, then you're in the wrong place.  If
you want to do it from Java, then you'll need to provide more
information.  For example, what kind of API is it?  SOAP?

Since Oracle 9i you can implement Java classes directly in the database
with:

create or replace and compile java source named foobar as
package foo.bar;

public class FooBar {

 String getFooBar(String foo) {

   return foo + "bar";
 }
}

To call the methods of your class from PL/SQL you have to write wrapper
procedures and functions like this:

function get_foobar(foo in varchar2)
 return varchar2
 as language java name
   'foo.bar.FooBar(java.lang.String) return oracle.sql.VARCHAR2';

Warning: All code is untested.

Have also a look at the Java Developer Guide from Oracle:

http://download-west.oracle.com/docs/cd/B14117_01/java.101/b12021/toc.htm

Hope that helps.

Cheers,

Jan