Behavioral Patterns - Iterator Pattern

The Iterator pattern is one, which allows you to navigate through a collection of data using a common interface without knowing about the underlying implementation.

Iterator should be implemented as an interface. This allows the user to implement it anyway its easier for him/her to return data.

We use iterators quite frequently in everyday life. For example, remote control of TV. Any remote control we use, either at home/hotel or at a friend's place, we just pick up the TV remote control and start pressing Up and Down or Forward and Back keys to iterate through the channels.

What sort of interface can Iterator be in case of Remote Controls?

/** * Iterator interface has method declarations for iterating through * TV channels. All remote controls implement Iterator. */ public interface Iterator {
	public Channel nextChannel(int currentChannel); public Channel prevChannel(int currentChannel);
}// End of interface

The channel iterator is common for all the remote controls. It's like a specification implemented by all the remote control manufacturing companies.

/** * ChannelSurfer is a part of remote control which implements the Iterator * interface. This class overrides the nextChannel and prevChannel methods. */ public ChannelSurfer implements Iterator {
	/** * nextChannel – method which takes the current channel number * and returns the next channel. */ public Channel nextChannel (int currentChannel) {
		Channel channel = new Channel(currentChannel+1); return channel;
	}
	/** * prevChannel – method which takes the current channel number * and returns the previous channel. */ public Channel prevChannel (int currentChannel) {
			Channel channel = new Channel(currentChannel-1); return channel;
	}
}// End of class

/** * RemoteControl class is the actual remote control and it behaves and makes * use of ChannelSurfer. */ public class RemoteControl {
	private ChannelSurfer surfer; private Settings settings; public RemoteControl() {
		surfer = new ChannelSurfer(); settings = new Settings();
	}
	/** * getProgram returns the program for that channel. * */ public getProgram(ChannelSurfer surfer) {
		return new Program(surfer.nextChannel());
	}
}// End of class

We all know that every channel is associated to a program and it's basically the program and not the channel number which a user wants to see. And so, the implementation which returns a program for channels surfed.

This tells us that we can apply some logic before returning the elements through iterator. We can set rules. The Iterator here, can also be programmed to return the 'programs' straight away rather than returning the channels.

The common Java iterator is Enumeration which has implicit
hasMoreElements()
and nextElement() methods.

The benefits of Iterator are about their strength to provide a common interface for iterating through collections without bothering about underlying implementation.