Java Reference
In-Depth Information
Example 5.2-3. Assignment Conversion for Array Types
class Point { int x, y; }
class ColoredPoint extends Point { int color; }
class Test {
public static void main(String[] args) {
long[] veclong = new long[100];
Object o = veclong; // okay
Long l = veclong; // compile-time error
short[] vecshort = veclong; // compile-time error
Point[] pvec = new Point[100];
ColoredPoint[] cpvec = new ColoredPoint[100];
pvec = cpvec; // okay
pvec[0] = new Point(); // okay at compile time,
// but would throw an
// exception at run time
cpvec = pvec;
// compile-time error
}
}
In this example:
• The value of
veclong
cannot be assigned to a
Long
variable, because
Long
is a
class type other than
Object
. An array can be assigned only to a variable of a
compatible array type, or to a variable of type
Object
,
Cloneable
or
java.io.Serializable
.
• The value of
veclong
cannot be assigned to
vecshort
, because they are arrays of
primitive type, and
short
and
long
are not the same primitive type.
• The value of
cpvec
can be assigned to
pvec
, because any reference that could
be the value of an expression of type
ColoredPoint
can be the value of a vari-
able of type
Point
. The subsequent assignment of the new
Point
to a compon-
ent of
pvec
then would throw an
ArrayStoreException
(if the program were oth-
erwise corrected so that it could be compiled), because a
ColoredPoint
array
cannot have an instance of
Point
as the value of a component.
• The value of
pvec
cannot be assigned to
cpvec
, because not every reference
that could be the value of an expression of type
ColoredPoint
can correctly be
the value of a variable of type
Point
. If the value of
pvec
at run time were a
reference to an instance of
Point[]
, and the assignment to
cpvec
were allowed,
a simple reference to a component of
cpvec
, say,
cpvec[0]
, could return a
Point
,
and a
Point
is not a
ColoredPoint
. Thus to allow such an assignment would al-
