Generics & Type Safety

Generics provide compile-time type safety by parameterising classes and methods with type parameters, eliminating explicit casts and converting runtime ClassCastException into compile errors.

Bounded types restrict type parameters to a specific hierarchy, wildcards add flexibility with ? extends (producer/read) and ? super (consumer/write), and PECS is the rule that ties them together: Producer Extends, Consumer Super.

Type erasure means the Java compiler enforces generic type safety at compile time and then removes all type parameters from the bytecode, so at runtime List<String> and List<Integer> are both just List.

Raw types bypass generics and cause heap pollution where a parameterised variable holds the wrong type, leading to ClassCastException far from the bug; @SafeVarargs suppresses warnings on safe varargs methods with generics.

List<?> is read-only because the compiler does not know the actual element type, and the wildcard capture helper pattern uses a private generic method to capture ? as a named type T, enabling type-safe reads and writes.

Recursive generics (self-bounded types) is the pattern class X<T extends X<T>> where the type parameter references the declaring class, enabling type-safe fluent chains and self-referential methods like Enum's compareTo.

Collections.checkedList/Map/Set wraps a collection with runtime type checking on every insertion, catching heap pollution at add time instead of get time.