Generics: type parameters, bounded types and wildcards (? extends, ? super)

Generics let you write a class or method once and have the compiler check the types it works with. When you write List<String>, the compiler rejects list.add(42) and lets you read elements without a cast. The angle-bracket names such as T, E, K and V are type parameters: placeholders that are replaced by real type arguments at each use. The diamond <> on the right side (new ArrayList<>()) asks the compiler to infer the type argument from the left side.