8.3.2

Initialization of Fields

CLASSES

class Test {

static int i = 0, j = 0;

static synchronized void one() { i++; j++; }

static synchronized void two() {

System.out.println("i=" + i + " j=" + j);

}

}

This prevents method

one

 and method

two

 from being executed concurrently, and

furthermore guarantees that the shared values of

i

 and

j

 are both updated before

method

one

 returns. Therefore method

two

 never observes a value for

j

 greater

than that for

i

; indeed, it always observes the same value for

i

 and

j

.

Another approach would be to declare

i

 and

j

 to be

volatile

:

class Test {

static volatile int i = 0, j = 0;

static void one() { i++; j++; }

static void two() {

System.out.println("i=" + i + " j=" + j);

}

}

This allows method

one

 and method

two

 to be executed concurrently, but guaran 

tees that accesses to the shared values for

i

 and

j

 occur exactly as many times,

and in exactly the same order, as they appear to occur during execution of the pro 

gram text by each thread. Therefore, method

two

 never observes a value for

j

greater than that for

i

, because each update to

i

 must be reflected in the shared

value for

i

 before the update to

j

 occurs. It is possible, however, that any given

invocation of method

two

 might observe a value for

j

 that is much greater than the

value observed for

i

, because method

one

 might be executed many times between

the moment when method

two

 fetches the value of

i

 and the moment when

method

two

 fetches the value of

j

.

See  17 for more discussion and examples.

A compile time error occurs if a

final

 variable is also declared

volatile

.

8.3.2   Initialization of Fields

If a field declarator contains a

variable initializer

, then it has the semantics of an

assignment ( 15.25) to the declared variable, and:

148