Unable to mock BufferedWriter class in junit

Question

I am using a BufferedWriter object in my source code

BufferedWriter outputToErrorFile = new BufferedWriter(new FileWriter(file));
outputToErrorFile.append(\"         


        

Answer 1

I wouldn't mock BufferedWritter at all. Create a writer backed by a ByteArrayOutputStream in your test, pass it to the class you are testing, then inspect it when you are done. This may require refactoring your code to create a seam for testing. Here's one possible implementation:

public void writeToWriter(Writer writer) {
  writer.append("some string");
}

Then your test can look like this:

ByteArrayOutputStream stream = new ByteArrayOutputStream();
OutputStreamWriter writer = new OutputStreamWriter(stream, charset);

objectUnderTest.writeToWriter(writer);

String actualResult = writer.toString(charset);
assertEquals("some string", actualResult);

If the method you are testing needs to open and close the stream, I'm fond of using Guava's CharSink class:

public void writeToSink(CharSink sink) {
  try (Writer writer = sink.openBufferedStream()) {
    writer.append("some string");
  }
}