Unit tests for following code with Mockito

Question

List<String> lineArray = new ArrayList<String>();
Resource resource = resourceLoader.getResource("classpath:abc.txt");
InputStream in = resource.getInputStream();
BufferedReader reader = new BufferedReader(new InputStreamReader(in));
String line;
while ((line = reader.readLine()) != null) {
    if(line.startsWith("#")) {
        lineArray.add(reader.readLine());           }
}
reader.close();

The above code is part of a function returning void, I'm able to mock Resource and ResourceLoader but not able to find a way to mock BufferedReader. I also want to mock the List and call Mockito.verify() on List.add().

Answer 1

If the list is local to the method, there is no side effect for you to test. Then again, there is no obvious purpose to using this method in non-test code either, because you'd read the data into the list, and then discard it.

You would need to inject the list as a parameter to the method:

void yourMethod(List<String> lineArray) {
  Resource resource = resourceLoader.getResource("classpath:abc.txt");
  // ... etc.
}

And you can now test this by invoking yourMethod in your test, with a List parameter, that you can inspect afterwards.

I also want to mock the List and call Mockito.verify() on List.add().

There is essentially no need to mock a List , especially for this purpose: inject a list, a regular ArrayList , and just check that the list has grown by 1 after the method call.