OBJECTS AND CLASSES - Introduction to Java Programming - page 315

When you compile TestCircleWithStaticMembers.java , the Java compiler automatically
compiles CircleWithStaticMembers.java if it has not been compiled since the last change.
Static variables and methods can be accessed without creating objects. Line 6 displays the
number of objects, which is 0 , since no objects have been created.
The main method creates two circles, c1 and c2 (lines 9, 18). The instance variable radius
in c1 is modified to become 9 (line 21). This change does not affect the instance variable radius
in c2 , since these two instance variables are independent. The static variable numberOfObjects
becomes 1 after c1 is created (line 9), and it becomes 2 after c2 is created (line 18).
Note that PI is a constant defined in Math , and Math.PI references the constant.
c1.numberOfObjects (line 27) and c2.numberOfObjects (line 30) are better
replaced by CircleWithStaticMembers.numberOfObjects . This improves
readability, because other programmers can easily recognize the static variable. You
can

also replace CircleWithStaticMembers.numberOfObjects with

CircleWithStaticMembers.getNumberOfObjects() .

Tip
Use ClassName.methodName(arguments) to invoke a static method and
ClassName.staticVariable to access a static variable. This improves readability,
because other programmers can easily recognize the static method and data in the class.

use class name

An instance method can invoke an instance or static method and access an instance or static
data field. A static method can invoke a static method and access a static data field. However,
a static method cannot invoke an instance method or access an instance data field, since static
methods and static data fields don't belong to a particular object. The relationship between
static and instance members is summarized in the following diagram:

invoke invoke

An instance method An instance method

access

access

An instance data field An instance data field

An instance method A static method

invoke invoke

A static method A static method

access access

A static data field A static data field

For example, the following code is wrong.

1 public class A {

2 int i = 5 ;

3 static int k = 2 ;

4
5 public static void main(String[] args) {

6 int j = i; // Wrong because i is an instance variable

7 m1(); // Wrong because m1() is an instance method

8 }
9
10 public void m1() {
11 // Correct since instance and static variables and methods
12 // can be used in an instance method
13 i = i + k + m2(i, k);
14 }
15
16 public static int m2( int i, int j) {

17 return ( int )(Math.pow(i, j));

18 }
19 }