Exploring Advanced Language Features - Beginning Java 7 - page 281

Listing 3-57 teaches youhowarraymanipulation canlead toan ArrayStoreEx-
ception :
Listing 3-57. How an ArrayStoreException arises
class Point
{
int x, y;
}
class ColoredPoint extends Point
{
int color;
}
class ReificationDemo
{
public static void main(String[] args)
{
ColoredPoint[] cptArray = new ColoredPoint[1];
Point[] ptArray = cptArray;
ptArray[0] = new Point();
}
}
Listing 3-57 ' s main() method first instantiates a ColoredPoint array that can
store one element. In contrast to this legal assignment (the types are compatible), spe-
cifying ColoredPoint[] cptArray = new Point[1]; isillegal(andwon't
compile)becauseitwouldresultina ClassCastException atruntime—thearray
knows that the assignment is illegal.
Note Ifit'snotobvious, ColoredPoint[] cptArray = new Point[1];
is illegal because Point instances have fewer members (only x and y ) than Co-
loredPoint instances( x , y ,and color ).Attemptingtoaccessa Point instance's
nonexistent color fieldfromitsentryinthe ColoredPoint arraywouldresultin
amemoryviolation(becausenomemoryhasbeenassignedto color )andultimately
crash the JVM.
The second line ( Point[] ptArray = cptArray; ) is legal because of co-
variance (an array of supertype references is a supertype of an array of subtype refer-