The Chain of Responsibility pattern allows a number of classes to attempt to handle a request, without any of them knowing about the capabilities of the other classes. It provides a loose coupling between these classes; the only common link is the request that is passed between them. The request is passed along until one of the classes can handle it.
The Chain of Responsibility forwards requests along a chain of classes, but the Command pattern forwards a request only to a specific module. It encloses a request for a specific action inside an object and gives it a known public interface. It lets you give the client the ability to make requests without knowing anything about the actual action that will be performed, and allows you to change that action without affecting the client program in any way.
Some programs benefit from having a language to describe operations they can perform. The Interpreter pattern generally describes defining a grammar for that language and using that grammar to interpret statements in that language.
The Iterator is one of the simplest and most frequently used of the design patterns. The Iterator pattern allows you to move through a list or collection of data using a standard interface without having to know the details of the internal representations of that data. In addition you can also define special iterators that perform some special processing and return only specified elements of the data collection.
When a program is made up of a number of classes, the logic and computation is divided logically among these classes. However, as more of these isolated classes are developed in a program, the problem of communication between these classes become more complex.
The State pattern is used when you want to have an enclosing class switch between a number of related contained classes, and pass method calls on to the current contained class. Design Patterns suggests that the State pattern switches between internal classes in such a way that the enclosing object appears to change its class. In Java, at least, this is a bit of an exaggeration, but the actual purpose to which the classes are put can change significantly.
Whenever you write a parent class where you leave one or more of the methods to be implemented by derived classes, you are in essence using the Template pattern. The Template pattern formalizes the idea of defining an algorithm in a class, but leaving some of the details to be implemented in subclasses. In other words, if your base class is an abstract class, as often happens in these design patterns, you are using a simple form of the Template pattern.
The Visitor pattern turns the tables on our object-oriented model and creates an external class to act on data in other classes. This is useful if there are a fair number of instances of a small number of classes and you want to perform some operation that involves all or most of them.