10.10

Array Store Exception

ARRAYS

class Test {

public static void main(String[] args) {

ColoredPoint[] cpa = new ColoredPoint[10];

Point[] pa = cpa;

System.out.println(pa[1] == null);

try {

pa[0] = new Point();

} catch (ArrayStoreException e) {

System.out.println(e);

}

}

}

produces the output:

true

java.lang.ArrayStoreException

Here the variable

pa

 has type

Point[]

 and the variable

cpa

 has as its value a ref 

erence to an object of type

ColoredPoint[]

. A

ColoredPoint

 can be assigned

to a

Point

; therefore, the value of

cpa

 can be assigned to

pa

.

A reference to this array

pa

, for example, testing whether

pa[1]

 is

null

, will

not result in a run time type error. This is because the element of the array of type

ColoredPoint[]

 is a

ColoredPoint

, and every

ColoredPoint

 can stand in for

a

Point

, since

Point

 is the superclass of

ColoredPoint

.

On the other hand, an assignment to the array

pa

 can result in a run time error.

At compile time, an assignment to an element of

pa

 is checked to make sure that

the value assigned is a

Point

. But since

pa

 holds a reference to an array of

ColoredPoint

, the assignment is valid only if the type of the value assigned at

run time is, more specifically, a

ColoredPoint

.

Java checks for such a situation at run time to ensure that the assignment is

valid; if not, an

ArrayStoreException

 is thrown. More formally: an assignment

to an element of an array whose type is

A[]

, where

A

 is a reference type, is

checked at run time to ensure that the value assigned can be assigned to the actual

element type of the array, where the actual element type may be any reference

type that is assignable to

A

.

 At length burst in the argent revelry,

With plume, tiara, and all rich array . . .

 John Keats,

The Eve of St. Agnes

(1819)

200