Generics in Java

Generics in Java parameterize classes, interfaces and methods by reference types so reusable code can retain compile-time type checks. For example, List<String> describes a list whose elements are strings, and retrieving one has static type String. Java generic types are generally invariant: List<Integer> is not a subtype of List<Number>. Wildcards express related producer or consumer views. Type erasure means a parameterized class usually has one shared runtime class rather than a fresh class for each type argument. Primitive values use wrapper types in ordinary generic collections.^[ref]

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A method declaring its own type parameters, for example <T> List<T> singletonList(T value). The type parameter connects argument and result types and permits compile-time checking without casts at ordinary call sites. A method mentioning only a generic class's type parameter need not itself be a generic method.

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Even when S is a subtype of T, neither G<S> nor G<T> is generally a subtype of the other. This prevents unsafe writes into mutable generic containers. Wildcards express restricted views.

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In Java, type erasure implements generic code by replacing a type parameter with its bound, or Object if unbounded, while inserting required casts and bridge methods. Thus List<String> and List<Integer> share a runtime list class even though the compiler checks different element types. Some generic type information remains in metadata for reflection, so erasure does not mean every trace of generic declarations disappears. Restrictions such as the inability to directly create new T() follow from the absence of a distinct reified runtime class for arbitrary type arguments.^[ref]

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