Generics and Classes

Generics parameterize types: Box stores T. The compiler checks assignments and removes casts. At runtime, due to type erasure, often only the raw form exists.

Advantage: fewer ClassCastExceptions, clearer APIs. Limitation: no new T[], no instanceof T with type parameter.

Example: List accepts add("a"), but not add(Integer).

Typed repositories, event bus handlers, collection APIs. Generics shift class errors to compile time and are a prerequisite for readable library boundaries in large codebases.

Depth

A type parameter describes a family of classes with the same structure but different element types. The compiler checks insertion and removal consistently, which eliminates many runtime conversions.

Box<Integer> is not a subtype of Box<Number>. This invariance prevents, for example, writing a Double into a box that is actually an integer box via a supposedly Box<Number>.

Static members belong to the class and not to a specific type argument instantiation. Therefore, they cannot directly use the type parameter of the instance class. Methods can instead declare their own type parameters.

Difficulty levels

1. Use a class type parameter in fields and methods.
2. Justify invariance based on an unsafe write access.
3. Distinguish between class and method type parameters.

Pitfalls

A single letter is not a concrete runtime type. Additionally, an unchecked cast only removes the compiler check, not the possibility of a later type error.