Separate APIs for Clients and Providers

From APIDesign

The Writer

This chapter analyses the Writer example. Do you know that in JDK 1.5 new methods has been added into the Writer class to work with CharSequences? Of course, they could not be left abstract, as that would not be compatible. They needed some implementation. Of course, there are multiple choices to use. So, which one would you choose?

does not exists: writer.throw

Possible, but useless, for clients calling the Writer.append method. They would always have to code very defensively, as shown here:

does not exists: writer.throw.client

Of course, this is a very horrible API. You want to guarantee to clients calling into Writer that reasonable implementation of the append method is provided:

does not exists: writer.super

This looks good, as clients can always rely on the append method to be implemented. Even by writers written against JDK 1.4 version of the Writer. This is not bad, however it is also not very efficient, as it always converts the whole CharSequence to String. In case of big sequences, like onces representing whole CD, this is a way to ask for OutOfMemoryErrors:

does not exists: writer.countcd

There must be more effective way to do this! Yes, it is. Please read the chapter 8 of TheAPIBook to learn what to do and how to prevent situations like this.