Informatica has its own transformation langugage that you can use in your expressions, filter and such. It is quite versatile and not at all difficult to learn, if you're familiar with any of the most popular programming languages of today. SQL is probably the one it resembles the most.
You will definitely need to learn at least the basics of this langugage to be able to use Informatica efficiently.
(Good understanding of SQL is pretty much a requirement anyway if you're going to use Informatica (or any ETL tool) - of course this depends on what your source and target systems are.)
It is also possible to create Java transformations, although those should be fairly rarely needed. Usually the built-in transformations combined with Informatica's own transformation language will provide more than enough options.
reference material regarding informatica
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