Parameterized Polymorphism & Type Bounds

Generics let a class or method operate across many types while preserving compile-time guarantees. They are how a reusable container can be flexible without falling back to unsafe Object storage everywhere.

Type Safety and Cast Elimination

Without generics, a collection can accept any object and returns only Object. Every read requires a cast, and wrong assumptions fail at runtime.

Object value = cache.get("user:1");
User user = (User) value;

With generics, the API carries the type contract:

Cache<String, User> cache = new Cache<>();
User user = cache.get("user:1");

The compiler rejects mismatched key/value usage before the program runs.

Type Erasure

In Java, generic parameters exist mainly for compile-time validation. During compilation, the type annotations are erased into their bounds, usually Object if no tighter bound exists. The compiler then injects casts at call sites where needed.

This preserves compatibility with older runtimes, but it also means generic type parameters are not usually available as concrete runtime classes without extra metadata.

Wildcard Variance

Variance decides whether a generic structure is safe to read from, write into, or both.

? extends T means the actual type is T or a subtype. This is a producer contract. You can safely read values as T, but you cannot safely insert new T values, because the real list might be a more specific subtype.

? super T means the actual type is T or an ancestor. This is a consumer contract. You can safely insert T, because every permitted runtime list can hold it.

The practical rule is PECS: producer extends, consumer super.

Lab 4.1: The Invariant Read/Write Trap

The initial code tries to insert an Integer into a List<? extends Number>. The compiler rejects it because the runtime list might actually be List<Double>. Change the boundary to a lower-bounded consumer contract.

Lab 4.2: Building a Universal Type Storage Engine

The starting cache uses raw Object references and forces casts on every read. Parameterize the cache as Cache<K, V> so the API preserves key and value types.