OBJECTS AND CLASSES - Introduction to Java Programming: Comprehensive Version - page 337

Enter point1's x-, y-coordinates: 1.5 5.5
Enter point2's x-, y-coordinates: -5.3 -4.4
p1 is Point2D [x = 1.5, y = 5.5]
p2 is Point2D [x = -5.3, y = -4.4]
The distance between p1 and p2 is 12.010412149464313

This program creates two objects of the Point2D class (lines 15-16). The toString()
method returns a string that describes the object (lines 17-18). Invoking p1.distance(p2)
returns the distance between the two points (line 20).

9.14

✓ ✓

How do you create a Date for the current time? How do you display the current time?

Check

9.15

How do you create a Point2D ? Suppose p1 and p2 are two instances of Point2D ?
How do you obtain the distance between the two points?

Point

9.16

Which packages contain the classes Date , Random , Point2D , System , and Math ?

9.7 Static Variables, Constants, and Methods
A static variable is shared by all objects of the class. A static method cannot access
instance members of the class.

Key
Point

The data field radius in the circle class is known as an instance variable . An instance vari-
able is tied to a specific instance of the class; it is not shared among objects of the same class.
For example, suppose that you create the following objects:

Static vs. instance
instance variable

Circle circle1 = new Circle();
Circle circle2 = new Circle( 5 );

VideoNote
Static vs. instance

The radius in circle1 is independent of the radius in circle2 and is stored in a differ-
ent memory location. Changes made to circle1 's radius do not affect circle2 's radius ,
and vice versa.
If you want all the instances of a class to share data, use static variables , also known as
class variables . Static variables store values for the variables in a common memory location.
Because of this common location, if one object changes the value of a static variable, all
objects of the same class are affected. Java supports static methods as well as static variables.
Static methods can be called without creating an instance of the class.
Let's modify the Circle class by adding a static variable numberOfObjects to count the
number of circle objects created. When the first object of this class is created, numberOfOb-
jects is 1 . When the second object is created, numberOfObjects becomes 2 . The UML
of the new circle class is shown in Figure 9.13. The Circle class defines the instance vari-
able radius and the static variable numberOfObjects , the instance methods getRadius ,
setRadius , and getArea , and the static method getNumberOfObjects . (Note that static
variables and methods are underlined in the UML class diagram.)
To declare a static variable or define a static method, put the modifier static in the
variable or method declaration. The static variable numberOfObjects and the static method
getNumberOfObjects() can be declared as follows:

static variable

static method

static int numberOfObjects;

declare static variable

static int getNumberObjects() {
return numberOfObjects;

define static method

}