15.25.2

Compound Assignment Operators

EXPRESSIONS

value of this component is saved and then the right hand operand is evaluated.

If this evaluation completes abruptly, then the assignment expression com 

pletes abruptly for the same reason and no assignment occurs. (For a simple

assignment operator, the evaluation of the right hand operand occurs before

the checks of the array reference subexpression and the index subexpression,

but for a compound assignment operator, the evaluation of the right hand

operand occurs after these checks.)

Otherwise, consider the array component selected in the previous step, whose

value was saved. This component is a variable; call its type

S

. Also, let

T

 be

the type of the left hand operand of the assignment operator as determined at

compile time.

N

If

T

 is a primitive type, then

S

 is necessarily the same as

T

.

O

The saved value of the array component and the value of the right hand

operand are used to perform the binary operation indicated by the com 

pound assignment operator. If this operation completes abruptly (the only

possibility is an integer division by zero see  15.16.2), then the assign 

ment expression completes abruptly for the same reason and no assign 

ment occurs.

O

Otherwise, the result of the binary operation is converted to the type of the

array component and the result of the conversion is stored into the array

component.

N

If

T

 is a reference type, then it must be

String

. Because class

String

 is a

final

 class,

S

 must also be

String

. Therefore the run time check that is

sometimes required for the simple assignment operator is never required for

a compound assignment operator.

O

The saved value of the array component and the value of the right hand

operand are used to perform the binary operation (string concatenation)

indicated by the compound assignment operator (which is necessarily

+=

). If this operation completes abruptly, then the assignment expression

completes abruptly for the same reason and no assignment occurs.

O

Otherwise, the

String

 result of the binary operation is stored into the

array component.

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

following example program:

class ArrayReferenceThrow extends RuntimeException { }

class IndexThrow extends RuntimeException { }

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