The Java Language Specification, Third Edition

4.1 The Kinds of Types and Values TYPES, VALUES, AND VARIABLES
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1. At run-time, classes and interfaces are loaded by the Java virtual machine
using class loaders. Each class loader defines its own set of classes and interfaces.
As a result, it is possible for two loaders to load an identical class or
interface definition but produce distinct classes or interfaces at run-time.
2. Type arguments and type variables (§4.4) are not reified at run-time. As a
result, different parameterized types (§4.5) are implemented by the same class
or interface at run time. Indeed, all invocations of a given generic type declaration
(§8.1.2, §9.1.2 )share a single run-time implementation.
A consequence of (1) is that code that compiled correctly may fail at link time
if the class loaders that load it are inconsistent. See the paper Dynamic Class
Loading in the Java Virtual Machine, by Sheng Liang and Gilad Bracha, in Proceedings
of OOPSLA ’98, published as ACM SIGPLAN Notices, Volume 33,
Number 10, October 1998, pages 36-44, and The Java Virtual Machine Specification,
Second Edition for more details.
A consequence of (2) is the possibility of heap pollution (§4.12.2.1). Under
certain conditions, it is possible that a variable of a parameterized type refers to an
object that is not of that parameterized type. The variable will always refer to an
object that is an instance of a class that implements the parameterized type. See
(§4.12.2) for further discussion.
4.1 The Kinds of Types and Values
There are two kinds of types in the Java programming language: primitive types
(§4.2) and reference types (§4.3). There are, correspondingly, two kinds of data
values that can be stored in variables, passed as arguments, returned by methods,
and operated on: primitive values (§4.2) and reference values (§4.3).
Type:
PrimitiveType
ReferenceType
There is also a special null type, the type of the expression null, which has no
name. Because the null type has no name, it is impossible to declare a variable of
the null type or to cast to the null type. The null reference is the only possible
value of an expression of null type. The null reference can always be cast to any
reference type. In practice, the programmer can ignore the null type and just pretend
that null is merely a special literal that can be of any reference type.
DRAFT