Most forums cite minor differences in speed, backup, etc. It's about time someone tell how the two differ when it comes to GUI data validation. Do this 2 Database always depend on java(or other software), or do they have the ability to create a user interface the accepts only valid input. Things like: positive numbers only, age between 1 to 100 only, email must be correct. I would be scared if my software accepts 500 years old for age.
Both offer native development tools that are roughly comparable and able to what you ask about.
Both also offer the ability for all main languages to interact with the RDBMS and so therefore the ability to to do the type of thing you discuss is as diverse as the options of a range of languages including Java, .Net, Ruby, Python, C++, VB, etc, etc
However what they don't really offer is a simple Access type 'forms and tables' type RAD tool. In simple terms the increased flexibility and power of both Oracle and DB2 comes at the price of simplicity.
Neither database DEPENDS on Java for implementing field level constraints. Data constraints can be implemented directly at the database level, and it is good practice to do so.
But you also need field level validation - users do not want to get constraint violation errors on insert.
As for tools that generate GUI applications from the database itself - I don't see that as an Oracle vs DB2 database question - it's more Oracle Apex vs IBMs equivalent - but even within Oracle you've got Forms (deceased), JDeveloper, Apex.