Inner classes and implementation of ArrayList - Data Structures and Problem Solving Using Java - page 588

102 public Iterator<AnyType> iterator( )
103 { return new ArrayListIterator( 0 ); }
104
105 public ListIterator<AnyType> listIterator( int idx )
106 { return new ArrayListIterator( idx ); }
107
108 // This is the implementation of the ArrayListIterator
109 private class ArrayListIterator implements ListIterator<AnyType>
110 {
111 private int current;
112 private int expectedModCount = modCount;
113 private boolean nextCompleted = false;
114 private boolean prevCompleted = false;
115
116 ArrayListIterator( int pos )
117 {
118 if( pos < 0 || pos > size( ) )
119 throw new IndexOutOfBoundsException( );
120 current = pos;
121 }
122
123 public boolean hasNext( )
124 {
125 if( expectedModCount != modCount )
126 throw new ConcurrentModificationException( );
127 return current < size( );
128 }
129
130 public boolean hasPrevious( )
131 {
132 if( expectedModCount != modCount )
133 throw new ConcurrentModificationException( );
134 return current > 0;
135 }

figure 15.15
ArrayList
implementation
(part 3)

when an instance of the iterator is created (immediately prior to calling the
constructor); modCount is a shorthand for ArrayList.this.modCount . The two
Boolean instance members that follow are flags used to verify that a call to
remove is legal.
The ArrayListIterator constructor is declared package visible; thus it is
usable by the ArrayList . Of course it could be declared public, but there is no
reason to do so and even if it were private, it would still be usable by ArrayList .
Package visible, however, seems most natural in this situation. Both hasNext and
hasPrevious verify that there have been no external structural modifications
since the iterator was created, throwing an exception if the ArrayList modCount
does not match the ArrayListIterator expectedModCount .