Generics In Java
June 06, 2020
Generic Terms:
| Term | Example |
|---|---|
| Parameterized type | List <String> |
| Actual type parameter | String |
| Generic type | List<E> |
| Formal type parameter | E |
| Unbounded wildcard type | List<?> |
| Raw type | List |
| Bounded type parameter | <E extends Number> |
| Recursive type bound | <T extends Comparable<T>> |
| Bounded wildcard type | List<? extends Number> |
| Generic method | static <E> List<E> asList(E[] a) |
| Type token | String.class |
Covariant: f is covariant if A ≤ B implies that f(A) ≤ f(B)
Arrays are covariant.
// Fails at runtime!
Object[] objectArray = new Long[1];
objectArray[0] = "I don't fit in"; // Throws ArrayStoreExceptionf is contravariant if A ≤ B implies that f(B) ≤ f(A)
Invariant: f is invariant if neither of the above holds
Generics, by contrast, are invariant.
// Won't compile!
List<Object> ol = new ArrayList<Long>(); // Incompatible types
ol.add("I don't fit in");If method is declared by:
Number[] method(ArrayList<Number> list) { ... }none of the following expressions will compile:
Integer[] result = method(new ArrayList<Integer>());
Number[] result = method(new ArrayList<Integer>());
Object[] result = method(new ArrayList<Object>());but
Number[] result = method(new ArrayList<Number>());
Object[] result = method(new ArrayList<Number>());will.
arrays are reified [JLS, 4.7]. This means that arrays know and enforce their element type at runtime.
Generics, by contrast, are implemented by erasure [JLS, 4.6]. This means that they enforce their type constraints only at compile time and discard (or erase) their element type information at runtime.
Because of erasure, the only parameterized types that are reifiable are unbounded wildcard types such as List<?> and Map<?,?>
// Why generic array creation is illegal - won't compile!
List<String>[] stringLists = new List<String>[1]; // (1)
List<Integer> intList = List.of(42); // (2)
Object[] objects = stringLists; // (3)
objects[0] = intList; // (4)
String s = stringLists[0].get(0); // (5)p.s: These notes were obtained from the book Effective Java 3rd Edition By Joshua Bloch and this stackoverflow answer.