The Java Language Specification, Third Edition

TYPES, VALUES, AND VARIABLES Variables of Reference Type 4.12.2
4.12 Variables
A variable is a storage location and has an associated type, sometimes called its
compile-time type, that is either a primitive type (§4.2) or a reference type (§4.3).
A variable’s value is changed by an assignment (§15.26) or by a prefix or postfix
++ (increment) or -- (decrement) operator (§15.14.2, §15.14.3, §15.15.1,
§15.15.2).
Compatibility of the value of a variable with its type is guaranteed by the
design of the Java programming language, as long as a program does not give rise
to unchecked warnings (§4.12.2.1). Default values are compatible (§4.12.5) and
all assignments to a variable are checked for assignment compatibility (§5.2), usually
at compile time, but, in a single case involving arrays, a run-time check is
made (§10.10).
4.12.1 Variables of Primitive Type
A variable of a primitive type always holds a value of that exact primitive type.
4.12.2 Variables of Reference Type
A variable of a class type T can hold a null reference or a reference to an instance
of class T or of any class that is a subclass of T. A variable of an interface type can
hold a null reference or a reference to any instance of any class that implements
the interface.
DISCUSSION
DRAFT
Note that a variable is not guaranteed to always refer to a subtype of its declared type, but
only to subclasses or subinterfaces of the declared type. This is due to the possibility of
heap pollution discussed below.
If T is a primitive type, then a variable of type “array of T ” can hold a null reference
or a reference to any array of type “array of T ”; if T is a reference type,
then a variable of type “array of T ” can hold a null reference or a reference to any
array of type “array of S” such that type S is a subclass or subinterface of type T.
In addition, a variable of type Object[] can hold an array of any reference type.
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