Suggestions on how to test a BufferedReader and FileReader that takes in strings and puts them into an ArrayList

file-io, java, mocking, mockito, tdd

Solution

As the filename `countriesInEurope.txt` is hardcoded in your implementation, this is not testable. A good way to make this testable would be to refactor the method to take a `Reader` as parameter:

public List<String> showListOfCourses(Reader reader) throws IOException {
    BufferedReader br = new BufferedReader(reader);
    List<String> courseList = new ArrayList<>();

    // ...

    return courseList;
}

Your main implementation could pass a `FileReader` to this. On the other hand when testing, your test method could pass a `StringReader` instance, which is easy to create with a sample content as a simple string, no temporary files needed, for example:

@Test
public void showListOfCourses_should_read_apple_orange_banana() {
    Reader reader = new StringReader("apple\norange\nbanana");
    assertEquals(Arrays.asList("apple", "orange", "banana"), showListOfCourses(reader));
}

Btw the name of the method is not good, as it doesn't "show" anything. `readListOfCourses` would make more sense.

Problem

I have a class which has a method that reads a text file line by line and then puts each line into an `ArrayList` of Strings. Here is my code: ``` public class ReadFile { public List<String> showListOfCourses() throws IOException { String filename = "countriesInEurope.txt"; FileReader fr = new FileReader(filename); BufferedReader br = new BufferedReader(fr); List<String> courseList = new ArrayList<>(); while (true) { String line = br.readLine(); if (line == null) { break; } courseList.add(line); } br.close(); return courseList; } } ``` I was hoping for a few suggestions on how to test this method through `Mockito` involving Arrange/Act/Assert. I have heard readers which involve text files can be tricky to test and that creating a temporary file for is not best practice as it uses up memory? Any suggestions will be really appreciated.

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