General Purpose Decision Tree Algorithm Code Implementations

Are there any well-designed, general purpose decision tree implementations for iPhone or Java? I know with LINQ it would be quite trivial, but with Objective C and J开发者_StackOverflow社区ava, it would be much more complex.

Basically, I want to drill down a set of objects based off any number of qualifications or attributes in my apps.

You could try Weka. The API is somewhat obtuse and makes simple things complicated, but it's a very good machine learning library and it even comes with a GUI front end if you want to play around with the classes interactively before writing code that uses them programmatically.