What is type erasure, what does it cost you at runtime, and how do you work around it?

Generics are a compile-time-only feature: the compiler erases type parameters to their bounds (or Object) and inserts casts, so at runtime List<String> and List<Integer> are the same class.

Consequences are you can't do new T[], can't use instanceof List<String>, can't catch a generic exception type, and overloads that differ only by type parameter clash. The workarounds are passing a Class<T> token (Class.cast / Array.newInstance) or capturing the type with a super-type token (the TypeReference pattern Jackson uses).