The iterator design pattern

Surfing channels on a TV acts as a stress buster for me. Switching channels back and forth is so soothing. This process of visiting channels in an order is called iteration. When we are in a hotel room, away from home, the aired channels may be different. However, the process of iteration remains the same. We visit a channel, we may spend some time watching it, and if we feel bored again, we go to the next one.

In programming, visiting each element of a data structure is routinely required. To make itself iterable, the data structures produce an iterator object. The iterator object needs to obey a contract that is laid down by the interface. For example, if the data structure produces an instance of java.lang.Iterable, then we can use Java's foreach loop to visit each element, that is, we can iterate the data structure. Let's look at the following figure:

Figure 5.1: Iterator Abstraction

The following Java code snippet shows how we can iterate various data structures...