Interfaces and Nested Classes - Multimedia Introduction to Programming Using Java - page 347

Guideline for using an inner class . If the purpose of a class In
is simply to support another class Out —meaning that In is used
only in Out and in no other part of the program— and if In needs
to reference non-static components of Out , then make In an inner
class of Out .

An inner class that is an iterator
We provide a second example of a useful inner class, having to do with iter-
ators. Consider a class Bank that maintains a set of bank accounts, as outlined in
Fig. 12.15. The accounts are stored in array segment bank[0..size-1] . Con-
sider writing an Iterator that will produce the accounts, but in reverse order.
We show it as a separate class, also in Fig. 12.15.

Activity
12-6.3

import java.util.*;
/** An instance is an array of bank accounts */
public class Bank {
// Class invariant: the accounts are in bank[0..size - 1]
private BankAccount[] bank;
private int size;
/** A (reverse) iterator for bank accounts */
private class BAIterator implements Iterator {
/** bank[0..n-1] remains to be enumerated */
private int n= size;
/** = " there is another account to enumerate " */
private int hasNext()
{ return n > 0; }
/** = the next item to be enumerated */
public Object next() {
n= n - 1;
return bank[n];

}
/** remove is not implemented */
public void remove() {}

}
/** An iterator that enumerates bank accounts in reverse order */
public Iterator iterator()
{ return new BAIterator(); }

}
Figure 12.16: Class Bank, with BAIterator as an inner class