ABSTRACT CLASSES AND INTERFACES - Introduction to Java Programming: Comprehensive Version - page 515

11
12

public int getId() {

13 return id;

14 }
15
16

public double getArea() {

17 return area;

18 }
19
20

public java.util.Date getWhenBuilt() {

21 return whenBuilt;

22 }
23
24 @Override /** Override the protected clone method defined in
25 the Object class, and strengthen its accessibility */

26 public Object clone() throws CloneNotSupportedException { This exception is thrown if

House does not implement
Cloneable

27 return super .clone();

28 }
29
30 @Override // Implement the compareTo method defined in Comparable
31 public int compareTo(House o) {

32 if (area > o.area)

33 return 1 ;

34 else if (area < o.area)

35 return -1 ;

36 else

37 return 0 ;

38 }
39 }

The House class implements the clone method (lines 26-28) defined in the Object class.
The header is:

protected native Object clone() throws CloneNotSupportedException;

The keyword native indicates that this method is not written in Java but is implemented
in the JVM for the native platform. The keyword protected restricts the method to be
accessed in the same package or in a subclass. For this reason, the House class must over-
ride the method and change the visibility modifier to public so that the method can be
used in any package. Since the clone method implemented for the native platform in the
Object class performs the task of cloning objects, the clone method in the House class
simply invokes super.clone() . The clone method defined in the Object class may throw
CloneNotSupportedException .
The House class implements the compareTo method (lines 31-38) defined in the
Comparable interface. The method compares the areas of two houses.
You can now create an object of the House class and create an identical copy from it, as
follows:

CloneNotSupportedException

House house1 = new House( 1 , 1750.50 );
House house2 = (House)house1.clone();

house1 and house2 are two different objects with identical contents. The clone method in
the Object class copies each field from the original object to the target object. If the field is of
a primitive type, its value is copied. For example, the value of area ( double type) is copied
from house1 to house2 . If the field is of an object, the reference of the field is copied. For
example, the field whenBuilt is of the Date class, so its reference is copied into house2 ,
as shown in Figure 13.6. Therefore, house1.whenBuilt == house2.whenBuilt is true,
although house1 == house2 is false. This is referred to as a shallow copy rather than a shallow copy