OBJECTS AND CLASSES - Introduction to Java Programming: Comprehensive Version - page 333

for primitive variables. As shown in Figure 9.9, the assignment statement c1 = c2 copies
the reference of c2 into c1 for reference variables. After the assignment, variables c1 and c2
refer to the same object.

Object type assignment c1 = c2

Before:

After:

c1 c1

c2

c2

c2: Circle c1: Circle c2: Circle c1: Circle

radius = 9 radius = 5 radius = 9 radius = 5

F IGURE 9.9

Reference variable c2 is copied to variable c1 .

Note
As illustrated in Figure 9.9, after the assignment statement c1 = c2 , c1 points to the
same object referenced by c2 . The object previously referenced by c1 is no longer useful
and therefore is now known as garbage . Garbage occupies memory space, so the Java
runtime system detects garbage and automatically reclaims the space it occupies. This
process is called garbage collection .

garbage

garbage collection

Tip
If you know that an object is no longer needed, you can explicitly assign null to a refer-
ence variable for the object. The JVM will automatically collect the space if the object is
not referenced by any reference variable.

9.7

✓ ✓

Which operator is used to access a data field or invoke a method from an object?

Check

9.8

What is an anonymous object?

Point

9.9

What is NullPointerException ?

9.10

Is an array an object or a primitive type value? Can an array contain elements of an
object type? Describe the default value for the elements of an array.

9.11

What is wrong with each of the following programs?

1 public class ShowErrors {
2 public static void main(String[] args) {
3 ShowErrors t = new ShowErrors( 5 );
4 }
5 }

1 public class ShowErrors {
2 public static void main(String[] args) {
3 ShowErrors t = new ShowErrors();
4 t.x();
5 }
6 }

(a) (b)

1 public class ShowErrors {
2 public void method1() {
3 Circle c;
4 System.out.println( "What is radius "
5 + c.getRadius());
6 c = new Circle();
7 }
8 }

1 public class ShowErrors {
2 public static void main(String[] args) {
3 C c = new C( 5.0 );
4 System.out.println(c.value);
5 }
6 }
7
8 class C {
9 int value = 2 ;

10 }

(c) (d)