Question

I found really hard to write unit test for this method, it basically exits the program when user types a quit command.

SytemExit class:

public class SystemExit {
    public void exit(int status) {
        System.exit(status);
    }
}

My static method:

public static void exitWhenQuitDetected() {
final SystemExit systemExit = new SystemExit();
final String QUIT = "quit";
String line = "";
try {
    final InputStreamReader input = new InputStreamReader(System.in);
    final BufferedReader in = new BufferedReader(input);
    while (!(line.equals(QUIT))) {
        line = in.readLine();
        if (line.equals(QUIT)) {
            System.out.println("You are now quiting the program");                  
            systemExit.exit(1);
        }
    }
} catch (Exception e) {
    System.err.println("Error: " + e.getMessage());
}
}   

Something is not quite right here as I am struggling to unit test the method exitWhenQuitDetected (I am using Mockito for mocking). How would I mock the InputStreamReader and verify the SystemExit.exit method gets called when it sees a quit? Can you shed some lights on this please? Thanks.

Added the test I am working on at the moment, it’s not working.

    @Test
@Ignore
public void shouldExitProgramWhenTypeQuit() {
    String quit = "quit";           
    SystemExit systemExit = mock(SystemExit.class);
    try {
        BufferedReader bufferedReader = mock(BufferedReader.class);
        when(bufferedReader.readLine()).thenReturn(quit + "\n");
        SomeClass.exitWhenQuitDetected();
        verify(systemExit, times(1)).exit(1);
    } catch (IOException e) {           
        e.printStackTrace();
    }       
}

Answer 1

You’ve done 90% of the work already, by placing the actual exiting code off in a separate class with no logic of its own. Your difficulty is caused by your use of a static method.

I would advise making the exitWhenQuitDetected not static. Put it in a class that you can instantiate when you need it, and that you can create with a mocked SystemExit. Something like this.

public class SomeClass{
  private final SystemExit exiter;
  private final static String QUIT = "quit";
  public SomeClass(){
    this(new SystemExit());
  }

  SomeClass(SystemExit exiter){
    this.exiter = exiter;
  }

  public static void exitWhenQuitDetected() {    
    String line = "";    
    try {    
      final InputStreamReader input = new InputStreamReader(System.in);    
      final BufferedReader in = new BufferedReader(input);    
      while (!(line.equals(QUIT))) {    
        line = in.readLine();    
        if (line.equals(QUIT)) {    
          System.out.println("You are now quiting the program");                      
          exiter.exit(1);    
        }    
      }    
    } catch (Exception e) {    
      System.err.println("Error: " + e.getMessage());    
    }    
  }       

  // ...
}

Then, in your test, you can make a mock of SystemExit, and use the package-private constructor of SomeClass to create an object that will use your mock as its exiter. You can then run your test, and verify on your mock SystemExit.