8.3.1

Field Modifiers

CLASSES

q.origin==Point.origin

is

true

. Further evidence is that the incrementation:

p.origin.useCount++;

causes the value of

q.origin.useCount

 to be

1

; this is so because

p.origin

 and

q.origin

 refer to the same variable.

8.3.1.2

final

Fields

A field can be declared

final

, in which case its declarator must include a variable

initializer or a compile time error occurs. Both class and instance variables

(

static

 and non 

static

 fields) may be declared

final

.

Any attempt to assign to a

final

 field results in a compile time error. There 

fore, once a

final

 field has been initialized, it always contains the same value. If

a

final

 field holds a reference to an object, then the state of the object may be

changed by operations on the object, but the field will always refer to the same

object. This applies also to arrays, because arrays are objects; if a

final

 field

holds a reference to an array, then the components of the array may be changed by

operations on the array, but the field will always refer to the same array.

Declaring a field

final

 can serve as useful documentation that its value will

not change, can help to avoid programming errors, and can make it easier for a

compiler to generate efficient code.

In the example:

class Point {

int x, y;

int useCount;

Point(int x, int y) { this.x = x; this.y = y; }

final static Point origin = new Point(0, 0);

}

the class

Point

 declares a

final

 class variable

origin

. The

origin

 variable

holds a reference to an object that is an instance of class

Point

 whose coordinates

are (0, 0). The value of the variable

Point.origin

 can never change, so it always

refers to the same

Point

 object, the one created by its initializer. However, an

operation on this

Point

 object might change its state for example, modifying its

useCount

 or even, misleadingly, its

x

 or

y

 coordinate.

8.3.1.3

transient

Fields

Variables may be marked

transient

 to indicate that they are not part of the per 

sistent state of an object. If an instance of the class

Point

:

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