When a new class is defined, the data elements are defined, but the methods associated with the class are defined on a separate stage. Methods are implemented in a manner similar to the one used for S3 classes. A function is defined, and the way the function reacts depends on its arguments. If a method is used to change one of the data components of an object, then it must return a copy of the object, just as we saw with S3 classes.

The creation of new methods is discussed in two steps. We will first discuss how to define a method for a class where the method does not yet exist. Next, we will discuss some predefined methods that are available and how to extend them to accommodate a new class.