Question

I’m trying to behaviorally test a Java object called StringReader. The code looks something like this:

public interface CharReader {
  public char readChar();
}    

public class ByteArrayCharReader implements CharReader {
  public ByteArrayCharReader(byte[] bytes);
  public char readChar();
}

public class StringReader {
  public String readString(byte[] bytes);
}

The protocol/responsibility of StringReader is:

• When readString is called, StringReader should repeatedly call CharReader.readChar() to read a character from the byte array until it encounters a null termination character or a max number of characters which is 20. The idea is that StringReader shouldn’t be bothered with all the integral stuff in correct unicode character decoding and such the responsibility is delegated to a CharReader.

I want to write a unit test for StringReader that expresses this protocol, but I can’t seem to figure out how.

First I thought I just wanted StringReader to internally create a ByteArrayCharReader and use that on the byte array it is handed in readString() but this seems to not make sense since I then wouldn’t be able to mock it from the outside.

It then seems I got a kind of chicken-and-egg problem. I want to inject a ByteArrayCharReader to StringReader, but ByteArrayCharReader must be constructed with a byte array that is only first available when readString is called – so how can I pass it if I can’t construct/mock it from the outside?. It seems like a silly problem, because in theory I know that the byte array passed to readString actually comes from the test where ByteArrayCharReader also comes from so I do have the parameters ready “out there”. Maybe I have missed something?

I should mention that I am working on a legacy code base and thus I cannot change the interface of StringReader.readString easily.

I’m kind of frustrated I can’t see how to work this out because it seems like a really simple scenario; A is passsed an array and wants to parse the array with an ArrayParser – how to test this behaviorally (not state based)?

Answer 1

You can use factory method pattern. Instead of injecting/instantiating ByteArrayCharReader you simply inject factory, which pure job is to provide instances of requested readers:

public class ReadersFactoryImpl implements ReadersFactory {
    public CharReader CreateByteReader(byte[] content) {
        return new ByteArrayCharReader(content);
    }
}

That’s all. You can easily mock this and set it up so it returns yet another mock. It should be enough to test whether StringReader calls the other reader as it is expected to.