ABSTRACT CLASSES AND INTERFACES - Introduction to Java Programming: Comprehensive Version - page 516

deep copy , meaning that if the field is of an object type, the object's reference is copied rather
than its contents.

deep copy

house1: House house1: House

Memory Memory

id = 1
area = 1750.50
whenBuilt

1

id = 1
area = 1750.50
whenBuilt

1

1750.50 1750.50

whenBuilt: Date whenBuilt: Date

reference reference

date object
contents

date object
contents house2 =

house1.clone()

house2 =
house1.clone()

house2: House house2: House

Memory Memory

id = 1
area = 1750.50
whenBuilt

id = 1
area = 1750.50
whenBuilt

1 1

whenBuilt: Date

1750.50 1750.50

date object
contents

reference reference

(a) (b)

F IGURE 13.6

(a) The default clone method performs a shallow copy. (b) The custom clone method performs a deep

copy.

To perform a deep copy for a House object, replace the clone() method in lines 26-28
with the following code:

deep copy

public Object clone() throws CloneNotSupportedException {
// Perform a shallow copy
House houseClone = (House) super .clone();
// Deep copy on whenBuilt
houseClone.whenBuilt = (java.util.Date)(whenBuilt.clone());
return houseClone;

}

or

public Object clone() {
try {
// Perform a shallow copy
House houseClone = (House) super .clone();
// Deep copy on whenBuilt
houseClone.whenBuilt = (java.util.Date)(whenBuilt.clone());
return houseClone;

}
catch (CloneNotSupportedException ex) {
return null ;

}
}

Now if you clone a House object in the following code:

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

house1.whenBuilt == house2.whenBuilt will be false . house1 and house2 contain
two different Date objects, as shown in Figure 13.6b.