EXPRESSIONS

Simple Assignment Operator

=

15.25.1

N

If

T

 is a reference type, then

SC

 may not be the same as

T

, but rather a type

that extends or implements

TC

. Let

RC

 be the class of the object referred to

by the value of the right hand operand at run time.

The compiler may be able to prove at compile time that the array compo 

nent will be of type

TC

 exactly (for example,

TC

 might be

final

). But if the

compiler cannot prove at compile time that the array component will be of

type

TC

 exactly, then a check must be performed at run time to ensure that

the class

RC

 is assignment compatible ( 5.2) with the actual type

SC

 of the

array component. This check is similar to a narrowing cast ( 5.4,  15.15),

except that if the check fails, an

ArrayStoreException

 is thrown rather

than a

ClassCastException

. Therefore:

O

If class

RC

 is not assignable to type

SC

, then no assignment occurs and an

ArrayStoreException

 is thrown.

O

Otherwise, the reference value of the right hand operand is stored into the

selected array component.

The rules for assignment to an array component are illustrated by the following

example program:

class ArrayReferenceThrow extends RuntimeException { }

class IndexThrow extends RuntimeException { }

class RightHandSideThrow extends RuntimeException { }

class IllustrateSimpleArrayAssignment {

static Object[] objects = { new Object(), new Object() };

static Thread[] threads = { new Thread(), new Thread() };

static Object[] arrayThrow() {

throw new ArrayReferenceThrow();

}

static int indexThrow() { throw new IndexThrow(); }

static Thread rightThrow() {

throw new RightHandSideThrow();

}

static String name(Object q) {

String sq = q.getClass().getName();

int k = sq.lastIndexOf( . );

return (k < 0) ? sq : sq.substring(k+1);

}

static void testFour(Object[] x, int j, Object y) {

String sx = x == null ? "null" : name(x[0]) + "s";

String sy = name(y);

371