Generics are a facility of generic programming that extend Java's type system to allow a type or method to operate on objects of various types while providing compile-time type safety. In particular, the Java collections framework supports generics to specify the type of objects stored in a collection instance.
Generics are implemented in Java through Type erasure, which means that during runtime the Type information specified in the instantiation of a generic class is not available. For example, the statement
List<String> names = new ArrayList<>(); produces a list object from which the element type
String cannot be recovered at runtime. However, if the list is stored in a field of type
List<String>, or passed to a method/constructor parameter of this same type, or returned from a method of that return type, then the full type information can be recovered at runtime through the Java Reflection API.
This also means that when casting to a generic type (e.g.:
(List<String>) list), the cast is an unchecked cast. Because the parameter
<String> is erased, the JVM cannot check if a cast from a
List<?> to a
List<String> is correct; the JVM only sees a cast for
List at runtime.