What is type erasure, and what can you not do because of it?

Generic type information exists at compile time and is erased in the bytecode, where a List of String is just a List with casts inserted by the compiler. So you cannot check instanceof against a parameterized type, create an array of a type parameter, or overload two methods that differ only by generic argument. The compiler adds bridge methods to keep polymorphism working, and unchecked casts can cause heap pollution.

Generics are a compile time feature. After compilation a List of String is just a List, with the compiler having inserted the casts, which is what kept generics backward compatible. The visible consequences are that I cannot ask if something is an instance of a parameterized type, I cannot instantiate an array of a type parameter without an unchecked cast, and I cannot overload a method on two different generic arguments because both erase to the same signature. It is also why an unchecked cast warning matters: a wrongly typed list can hold the wrong element and fail later at some unrelated cast, which is heap pollution and is genuinely annoying to trace. When I design an API I use bounded wildcards to make it usable, following the producer extends consumer super rule, so a method reading from a collection takes a wildcard extending the type. When I really need the type at runtime, for example telling a JSON library what to deserialize into, I pass a class object or use a type reference that captures the parameter through an anonymous subclass, since the type argument on a class is retained in the signature.