You need to loop through the contents of a Collection.

Use a LoopingIterator to repeatedly iterate through the contents of a Collection. Pass an existing Collection to the constructor of LoopingIterator, and call iterator( ). The following code uses a LoopingIterator to retrieve five items from a List with three items:

List drivers = new ArrayList( );
drivers.add( "Chris" );
drivers.add( "Sean" );
drivers.add( "Kathy" );
LoopingIterator loopingIterator = new LoopingIterator( drivers );
for( int i = 1; i <= 5; i++ ) {
    String driver = (String) loopingIterator.next( );
    System.out.println( "Driver for Day " + i + ": " + driver );
}

The previous example simulates the selection of a driver in a car pool with three drivers. Five drivers are selected, and the LoopingIterator returns the first item in the List on the fourth day:

Driver for Day 1: Chris
Driver for Day 2: Sean
Driver for Day 3: Kathy
Driver for Day 4: Chris
Driver for Day 5: Sean

Keep in mind that a LoopingIterator never stops iterating; hasNext( ) always returns true in a LoopingIterator. If you are using a LoopingIterator with a while loop, be sure to code an exit condition. A LoopingIterator is appropriate for a situation in which a series of values must be repeatedly evaluated, or a loop of commands must be repeatedly executed. An application, for example, may have a series of components that need to be continuously updated, or a series of queues or buffers that need to be continuously tested or monitored—an event loop, for example.

The LoopingIterator also implements the Commons Collections interface ResettableIterator; this extension of the Iterator interface can be reset to start at the beginning of a Collection. When using a ResettableIterator, call reset() to jump back to the beginning of a Collection:

List items = new ArrayList( );
items.add( "Item 1" );
items.add( "Item 2" );
items.add( "Item 3" );
LoopingIterator iterator = new LoopingIterator( items );
// Print out two elements from the LoopingIterator
System.out.println( iterator.next( ) );
System.out.println( iterator.next( ) );
// Reset iterator to start of List
System.out.println( "Reset" );
iterator.reset( );
// Print out two elements from the LoopingIterator
System.out.println( iterator.next( ) );
System.out.println( iterator.next( ) );

This example iterates over the first two elements in a List before calling reset( ). After calling reset( ), the code calls next( ) twice, printing out the first two elements, as shown below:

Item 1
Item 2
Reset
Item 1
Item 2

Recipe 5.3 deals with another implementation of ResettableIterator, ArrayListIterator, which can be used to iterate over a subset of elements in an ArrayList.