Java: How to extend a base class when other classes already extend the base?

Question

I want to extend a base class in apache commons email, the base class is Email. I simply want to add some throttling to the .send() method

3 other classes extend Email: HtmlEmail, SimpleEmail, and MultiPartEmail

There is no factory method for creating these 3 derived classes.

Is there a best way that I can extend this one method from the base Email class? All I can think of is to extend the 3 derived classes, override .send() in each, and have each of them call a common static method to accomplish the .send() throttling functionality.

Answer 1

It looks like you can use the decorator pattern and write e.g. a ThrottledEmail. It simply decorates another instance of Email (it can be ANY Email subclass) and it can @Override the send method to enforce some throttling. All other methods are simply delegated to the underlying Email instance.

This is similar to how a java.io.BufferedReader works, for example. It can decorate any Reader to give it a buffering feature. Other examples include java.util.Collections that provides utility methods such as Collection<T> synchronizedCollection(Collection<T>) which wraps ANY Collection<T> and decorates it with synchronization features.

Unless the base class is clearly documented to facilitate subclasses to @Override certain methods, you should generally favor composition (has-a) over inheritance (is-a) relationship.

See also

• Effective Java 2nd Edition, Item 16: Favor composition over inheritance
• Effective Java 2nd Edition, Item 17: Design and document for inheritance, or else prohibit it

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