Bag Implementations That Use Arrays - Data Structures and Abstractions with Java - page 32

Design Decision: Should the method toArray return the array bag instead of a copy?
Suppose that we define toArray as follows:
public String[] toArray()
{
return bag;
} // end toArray
This simple definition would certainly return an array of the bag's contents to a client. For example, the
statement
String[] bagArray = myBag.toArray();
provides a reference to an array of the entries in myBag . A client could use the variable bagArray to
display the contents of myBag .
The reference bagArray , however, is to the array bag itself. That is, bagArray is an alias for the
private instance variable bag within the object myBag , and therefore it gives the client direct access
to this private data. Thus, a client could change the contents of the bag without calling the class's
public methods. For instance, if myBag is the full bag pictured in Figure 2-3, the statement
bagArray[2] = null ;
would change the entry Ted to null . Although this approach might sound good to you if the
intent is to remove Ted from the bag, doing so would destroy the integrity of the bag. In particu-
lar, the entries in the array bag would no longer be consecutive, and the count of the number of
entries in the bag would be incorrect.
Programming Tip
A class should not return a reference to an array that is a private data field.
Note: A variable whose declared data type is Object can reference an object of any data
type. A collection whose entries are referenced by variables of type Object can contain
objects of various unrelated classes. In contrast, a variable having a generic data type can ref-
erence only an object of specific data types. A collection whose entries are referenced by
variables of a generic type can contain only objects of classes related by inheritance. Gener-
ics enable you to restrict the data types of the entries in your collections.
Question 4 In the previous method toArray , does the value of numberOfEntries equal
bag.length in general?
Question 5 Suppose that the previous method toArray gave the new array result the same
length as the array bag . How would a client get the number of entries in the returned array?
Question 6 Suppose that the previous method toArray returned the array bag instead of
returning a new array such as result . If myBag is a bag of five entries, what effect would the
following statements have on the array bag and the field numberOfEntries ?
Object[] bagArray = myBag.toArray();
bagArray[0] = null ;
Question 7 The body of the method toArray could consist of one return statement if you
call the method Arrays.copyOf . Make this change to toArray .