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?

Code from AltBufferedWriter.java:

public Writer append(CharSequence csq) throws IOException {
    /* thrown an exception as this method is new and 
     subclasses need to override it */
    throw new UnsupportedOperationException();
}
 

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

Code from BufferedWriterThrowingExceptionTest.java:

try {
    bufferedWriter.append(what);
} catch (UnsupportedOperationException ex) {
    bufferedWriter.write(what.toString());
}
 

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:

Code from AltBufferedWriter.java:

if (csq == null) {
    write("null");
} else {
    write(csq.toString());
}
return this;
 

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:

Code from BufferedWriterOnCDImageTest.java:

CountingWriter writer = new CountingWriter();
CDSequence cdImage = new CDSequence();
BufferedWriter bufferedWriter = new BufferedWriter(writer);
bufferedWriter.append(cdImage);
assertEquals(
    "Correct number of writes delegated", 
    cdImage.length(), writer.getCharacterCount()
);
 

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.