When validation gets complex (e.g. there are many criteria to check), I like to build a list/stream/array (what ever the language offers) of tuples of predicates (functions from the object that gets validated to boolean) and strings (or functions from the object to string so I can have context in my error messages).

Then iterate over the tuples, if a predicate fails, return the associated error message and throw an error/display the message to the user.

In the end it looks something like this:

  var validators = Stream.of(
    Map.entry(user -> user != null, "User must be defined"),
    Map.entry(user -> user.firstName != null, "Missing first name"))

  validators.filter(e -> e.getKey().apply(userToBeValidated)).map(Map.Entry::getValue).getFirst()

(This example uses Map.entry for tuples as Java lacks native support for tuples)

This limits branching and you have all validation criteria neatly organized in the same location.