Java Reference
In-Depth Information
For convenience, the
System
class provides an
arraycopy
method that al-
lows you to assign the values from one array into another, instead of
looping through each of the array elementsthis is described in more de-
tail in "
Utility Methods
" on page
665
.
7.4.4. Arrays and Types
Arrays are implicit extensions of
Object
. Given a class
X
, classes
Y
and
Z
that extend
X
, and arrays of each, the class hierarchy looks something
like this:
This class relationship allows polymorphism for arrays. You can assign
an array to a variable of type
Object
and cast it back. An array of objects
of type
Y
is usable wherever an array of objects of its supertype
X
is
required. This seems natural but can require a run time check that is
sometimes unexpected. An array of
X
can contain either
Y
or
Z
refer-
ences, but an array of
Y
cannot contain references to
X
or
Z
objects.
The following code would generate an
ArrayStoreException
at run time on
either of its final two lines, which violate this rule:
Y[] yArray = new Y[3]; // a Y array
X[] xArray = yArray; // valid: Y is assignable to X
xArray[0] = new Y();
xArray[2] = new X(); // INVALID: can't store X in Y[]
xArray[1] = new Z(); // INVALID: can't store Z in Y[]
