CLASSES

Field Modifiers

8.3.1

appear in a field declaration, it is customary, though not required, that they appear

in the order consistent with that shown above in the production for

FieldModifier

.

8.3.1.1

static

Fields

If a field is declared

static

, there exists exactly one incarnation of the field, no

matter how many instances (possibly zero) of the class may eventually be created.

A

static

 field, sometimes called a

class variable

, is incarnated when the class is

initialized ( 12.4).

A field that is not declared

static

 (sometimes called a non 

static

 field) is

called an

instance variable

. Whenever a new instance of a class is created, a new

variable associated with that instance is created for every instance variable

declared in that class or any of its superclasses.

The example program:

class Point {

int x, y, useCount;

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

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

}

class Test {

public static void main(String[] args) {

Point p = new Point(1,1);

Point q = new Point(2,2);

p.x = 3; p.y = 3; p.useCount++; p.origin.useCount++;

System.out.println("(" + q.x + "," + q.y + ")");

System.out.println(q.useCount);

System.out.println(q.origin == Point.origin);

System.out.println(q.origin.useCount);

}

}

prints:

(2,2)

0

true

1

showing that changing the fields

x

,

y

, and

useCount

 of

p

 does not affect the fields

of

q

, because these fields are instance variables in distinct objects. In this example,

the class variable

origin

 of the class

Point

 is referenced both using the class

name as a qualifier, in

Point.origin

, and using variables of the class type in

field access expressions ( 15.10), as in

p.origin

 and

q.origin

. These two ways

of accessing the

origin

 class variable access the same object, evidenced by the

fact that the value of the reference equality expression ( 15.20.3):

145