15.25.2

Compound Assignment Operators

EXPRESSIONS

Threads[throw]=throw => IndexThrow

Threads[throw]=StringBuffer => IndexThrow

Threads[1]=throw => RightHandSideThrow

Threads[1]=StringBuffer => ArrayStoreException

Threads[throw]=throw => IndexThrow

Threads[throw]=Thread => IndexThrow

Threads[1]=throw => RightHandSideThrow

Threads[1]=Thread => Okay!

Threads[throw]=throw => IndexThrow

Threads[throw]=StringBuffer => IndexThrow

Threads[9]=throw => RightHandSideThrow

Threads[9]=StringBuffer => IndexOutOfBoundsException

Threads[throw]=throw => IndexThrow

Threads[throw]=Thread => IndexThrow

Threads[9]=throw => RightHandSideThrow

Threads[9]=Thread => IndexOutOfBoundsException

The most interesting case of the lot is the one thirteenth from the end:

Threads[1]=StringBuffer => ArrayStoreException

which indicates that the attempt to store a reference to a

StringBuffer

 into an

array whose components are of type

Thread

 throws an

ArrayStoreException

.

The code is type correct at compile time: the assignment has a left hand side of

type

Object[]

 and a right hand side of type

Object

. At run time, the first actual

argument to method

testFour

 is a reference to an instance of  array of

Thread

and the third actual argument is a reference to an instance of class

StringBuffer

.

15.25.2   Compound Assignment Operators

All compound assignment operators require both operands to be of primitive type,

except for

+=

, which allows the right hand operand to be of any type if the left 

hand operand is of type

String

.

A compound assignment expression of the form

E1 op

=

E2

 is equivalent to

E1 = (T )((E1) op (E2))

, where

T

 is the type of

E1

, except that

E1

 is evaluated

only once. Note that the implied cast to type

T

 may be either an identity conver 

sion ( 5.1.1) or a narrowing primitive conversion ( 5.1.3). For example, the fol 

lowing code is correct:

short x = 3;

x += 4.6;

and results in

x

 having the value

7

 because it is equivalent to:

short x = 3;

x = (short)(x + 4.6);

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