CITS5501 lab 4 (week 5) – ISP and graphs – solutions

0. Recommended reading

Before attempting the exercises in this worksheet, it’s recommended you complete the recommended reading for week 5, and ensure you’ve reviewed the lecture slides on Input Space Partitioning and graph-based testing.

It should be possible to attempt the exercises even without having attended the lectures on graph-based testing, but you might want to revisit your answers after you have attended those lectures.

1. ISP, graphs and control flow

Consider the following Java method for collapsing sequences of blanks, taken from the StringUtils class of Apache Velocity (http://velocity.apache.org/), version 1.3.1:

/**
* Remove/collapse multiple spaces.
*
* @param String string to remove multiple spaces from.
* @return String
*/

public static String collapseSpaces(String argStr) {
  char last = argStr.charAt(0);
  StringBuffer argBuf = new StringBuffer();

  for (int cIdx = 0 ; cIdx < argStr.length(); cIdx++) {
    char ch = argStr.charAt(cIdx);
    if (ch != ' ' || last != ' ') {
      argBuf.append(ch);
      last = ch;
    }
  }
  return argBuf.toString();
}

1. Using the ISP principles we have covered in class, suggest some characteristics we could use to partition the argStr parameter.

   Once you have several characteristics, consider how you might choose combinations of partitions from them. A recommended approach is to aim for “Base Choice” coverage:

   1. For each characteristic, pick a base choice (and explain the reasoning behind that choice). Usually, the base choice will be one that is simpler, smaller, or more likely to occur than the other possibilities.
   2. Select test values for a “base choice” test.
   3. Go through and derive test values for the “non-base” partitions.

   Try writing JUnit tests for some of your test values.

   After you have finished the lab exercises, you might like to drop into one of the timetabled lab sessions and compare the characteristics and test values you derived with those of another student. Are your solutions the same? If not, how do they differ? Do either of your solutions have an advantage over the other?

2. Using the techniques outlined in lecture slides and readings on graph-based testing, try to construct a control flow graph of the method.

   How many nodes do you end up with?
   How many edges?

For the purposes of this exercise, we’ll take a simplified approach: you may ignore calls to other methods, such as .charAt(), and need only model the control flow within the method. (What about possible exceptions? Should they be modelled, or not? Why?)

A typical way of “labelling” your graph nodes needs is to use letters (“A”, “B”, “C” and so on), and to provide a legend, showing a reader which nodes correspond to which lines (or fragments of lines) of code.

Sometimes there may be multiple nodes representing fragments of code all within the same line (e.g. line 11). As long as you have a clear explanation of what each node represents (e.g. “node D: the line 11 loop condition”) then that’s fine.

3. Given your test cases from part (a), try mentally or on paper “executing” several tests, and see what paths of the graph get exercised by each of your tests.

   How would you subjectively rate the “coverage” of the graph by your tests – good? reasonable? poor?

4. Work out whether your tests give the following sorts of coverage:

   1. node coverage
   2. edge coverage

5. What are the prime paths in your graph? What proportion of the prime paths are exercised by the tests you’ve given?

   Can you construct some tests which exercise prime paths you haven’t already covered?

2. Test fixtures

Review the material from the textbook on test automation (ch 6), and the JUnit 4 “Text fixtures” documentation (at https://github.com/junit-team/junit4/wiki/Test-fixtures).

Consider the following code we wish to test:

class MyClass {
   private int x;
   public MyClass(int x) { this.x = x; }

   @Override
   public boolean equals(Object obj) {
      if (!(obj instanceof MyClass)) return false;
      return ((MyClass) obj).x == this.x;
   }
}

Find the Java library documentation for the equals() method (it’s in the Object class), and read what its requirements are.

In Java, all other classes automatically inherit from the Object class, and may also override methods provided by the Object class – this is what the “@Override” annotation on the equals() method means.

The equals() method should test whether the Object “obj” is “equal to” the receiver object, this, where what “equal to” means is decided on by the implementer of the class. (The equals() method in Java serves the same purpose as the __eq__ special method in Python.) The implementer is free to decide for themselves what “equal to” means for their class.

In order to avoid suprising behaviour for callers of the method, in general equality should be an equivalence relation; for instance, an object should always be equal to itself, and if a.equals(b) is true and b.equals(c) is true, then a.equals(c) should also be true.

The instanceof keyword in Java allows us to check whether an object is an instace of some class (or some class that inherits from that class, directly or indirectly). Normally, implementers of equals will want to return “false” whenever we try to compare with objects not of the same class.

Create a new Java project, create a MyClass.java file containing the code above, and check that it compiles.

A test class

Suppose we use the following test code for our MyClass class:

import static org.junit.jupiter.api.Assertions.*;
import org.junit.jupiter.api.AfterEach;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;

public class MyClassTest {
   private MyClass mc1;
   private MyClass mc2;
   private MyClass mc3;

   @BeforeEach
   public void setUp() {
     mc1 = new MyClass(3);
     mc2 = new MyClass(5);
     mc3 = new MyClass(3);
   }

   @Test
   /* Test the case when, for two objects, the second is null */
   public void equalsWhenNullRef() { fail("incomplete"); }

   @Test
   /* Test the case when, for two objects, they are not equal */
   public void equalsWhenNotEq() {/*...*/}

   @Test
   /* Test the case when, for two objects, they are equall */
   public void equalsWhenEq() {/*...*/}

}

In this case, the instance variables mc1, mc2 and mc3 are potential fixtures for any test.

1. Given the test code above, how many times will the setUp() method execute?

   Compile and run the tests and check whether this is the case.

2. It is good practice, when writing new tests, to ensure that at first they fail. This is useful as a warning, so that you know the test is not yet complete. (We don’t want to accidentally give our code to other developers when it contains tests that are incomplete, or do the wrong thing.)

   Insert code into the test methods that will always fail. What JUnit method have you used? Are there any other ways you can think of (or spot in the JUnit documentation) for writing a test that always fails?

3. Fill in code for the test methods in this class.

4. Are there any other tests you think we should add in order to thoroughly test our class? What are they?

5. Using the material from lectures, and the JUnit user guide, write a “teardown” method. What code should go in it? Based on your understanding of the Java language, is a “teardown” method necessary in this case? Why or why not?

  @AfterEach
  public void tearDown() {
    mc1 = null;
    mc2 = null;
    mc3 = null;
  }

3. Class design, invariants, and testability

Suppose we are writing a DigitalTimer class representing 24-hour times – it could be used, for example, in simulation programs. Rather than measuring “real” time, it measures simulated time, where every call to the tick() method causes one second to “elapse”. Other objects in the simulation could then query a DigitalTimer object to find out what the current time is.¹

A colleague has suggested the following method signatures and documentation for the class:

/** Represents a simulated time, where the "tick" method causes the time to be incremented
  * by one second.
  */
public class DigitalTimer {
  /** Construct a new DigitalTimer, initialized to time zero.
   */
  public DigitalTimer();

  /** Get the "hours" component of the time elapsed since
   * the timer was started.
   */
  public int getHours();

  /** Get the "minutes" component of the time elapsed since
   * the timer was started.
   */
  public int getMinutes();

  /** Get the "seconds" component of the time elapsed since
   * the timer was started.
   */
  public int getSeconds();

  /** Cause one simulated second to elapse. Once the timer reaches
   * 23 hours, 59 minutes, and 59 seconds, further calls to tick will
   * have no effect.
   */
  public void tick();
}

The method signatures your colleague has written represent the public API for the class; they don’t show the implementation.

Question 3.1:

Before going on – what might be useful attributes (fields) for the class? These will be part of the implementation. Can you think of multiple different alternatives? Should they be added to the API? Why, or why not?

Question 3.2:

Consider whether there are any invariants that govern the values of these attributes. Class invariants are constraints on what values the attributes of the class can take. They do not form part of the public API: they are an implementation detail, for use by the developer of the class (and any other developers maintaining the code).

For this lab, it’s suggested you use three int attributes, hours, minutes and seconds. Using your colleague’s API sketch as a starting point, add in the attributes and add “skeleton” method bodies to the code – e.g., for getSeconds(), you might start with

  public int getSeconds() {
    return -1;
  }

The aim is to write code that will compile (but isn’t yet a correct implementation).

You should be able to come up with a number of class invariants relating these attributes. Write them down as inline comments (not as Javadoc comments) in the body of the class. If you drop in on a timetabled lab session, compare the invariants you have proposed with those of other students (or with the lab facilitator).

Before writing a constructor and filling in the method bodies, write a DigitalTimerTest class:

import static org.junit.jupiter.api.Assertions.*;
import org.junit.jupiter.api.Test;

public class DigitalTimerTest {

  @Test
  public void testConstructor() {

  }


  @Test
  public void testTick() {

  }

}

See if you can suggest some possible test cases for this JUnit class, and try implementing them. Obviously, they will fail at first; but since your tests should rely only on the documented behaviour of the class, it’s quite possible to write them before the class has been implemented. This approach is used in the test-driven development methodology (TDD).

Lastly, try filling in the method bodies for DigitalTimerTest. Some notes to bear in mind:

• The job of a constructor is to initialize a new class object, ensuring (by the time the constructor has finished) that the object is in a sensible state and all the invariants hold.
• The job of any mutator method is to alter the state of an object in some way. Assuming the class invariants hold before the mutator is called, the mutator might break them temporarily (say, temporarily setting “seconds” to 60); but it should ensure the invariants hold again by the time it finishes executing.

You could find it useful, while testing, to write a private “checkInvariants” method, which tests whether the invariants hold, and throws an IllegalStateException if not. Such an exception typically signals a logic error in the program, so generally shouldn’t be caught.

public class DigitalTimer {
  private int hours; // invariant: 0 <= hours <= 23
  private int minutes; // invariant: 0 <= minutes <= 59
  private int seconds; // invariant: 0 <= seconds <= 59

  public DigitalTimer() {
    this.hours = 0;
    this.minutes = 0;
    this.seconds = 0;
  }

  public int getHours() {
    return hours;
  }

  public int getMinutes() {
    return minutes;
  }

  public int getSeconds() {
    return seconds;
  }

  public void tick() {
    if (hours == 23 && minutes == 59 && seconds == 59)
      return;

    seconds++;

    if (seconds == 60) {
      seconds = 0;
      minutes++;
    }

    if (minutes == 60) {
      minutes = 0;
      hours++;
    }
  }

}

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

public class DigitalTimerTest {

  @Test
  public void testConstructor_initialValues() {
    DigitalTimer timer = new DigitalTimer();
    assertEquals(0, timer.getHours(), "Initial hours should be 0");
    assertEquals(0, timer.getMinutes(), "Initial minutes should be 0");
    assertEquals(0, timer.getSeconds(), "Initial seconds should be 0");
  }

  @Test
  public void testTick_incrementSeconds() {
    DigitalTimer timer = new DigitalTimer();
    timer.tick();
    assertEquals(0, timer.getHours(), "Hours should still be 0 after 1 tick");
    assertEquals(0, timer.getMinutes(), "Minutes should still be 0 after 1 tick");
    assertEquals(1, timer.getSeconds(), "Seconds should be incremented by 1");
  }

  @Test
  public void testTick_incrementMinutes() {
    DigitalTimer timer = new DigitalTimer();

    for (int i = 0; i < 60; i++) {
      timer.tick();
    }

    assertEquals(0, timer.getHours(), "Hours should still be 0 after 60 ticks");
    assertEquals(1, timer.getMinutes(), "Minutes should be incremented by 1 after 60 ticks");
    assertEquals(0, timer.getSeconds(), "Seconds should reset to 0 after 60 ticks");
  }

  @Test
  public void testTick_incrementHours() {
    DigitalTimer timer = new DigitalTimer();

    for (int i = 0; i < 3600; i++) {
      timer.tick();
    }

    assertEquals(1, timer.getHours(), "Hours should be incremented by 1 after 3600 ticks");
    assertEquals(0, timer.getMinutes(), "Minutes should reset to 0 after 3600 ticks");
    assertEquals(0, timer.getSeconds(), "Seconds should reset to 0 after 3600 ticks");
  }

  @Test
  public void testTick_maxTimeReached() {
    DigitalTimer timer = new DigitalTimer();

    // Set the timer to 23:59:59
    for (int i = 0; i < (23 * 3600 + 59 * 60 + 59); i++) {
      timer.tick();
    }

    assertEquals(23, timer.getHours(), "Hours should be 23 after 86399 ticks");
    assertEquals(59, timer.getMinutes(), "Minutes should be 59 after 86399 ticks");
    assertEquals(59, timer.getSeconds(), "Seconds should be 59 after 86399 ticks");

    // Tick once more, timer should remain at 23:59:59
    timer.tick();

    assertEquals(23, timer.getHours(), "Hours should remain 23 after max time is reached");
    assertEquals(59, timer.getMinutes(), "Minutes should remain 59 after max time is reached");
    assertEquals(59, timer.getSeconds(), "Seconds should remain 59 after max time is reached");
  }
}

How will you test that tick operates correctly after, say, 59 seconds, or once the time reaches 23:59:59?

There are a few different options here:

Test through public API

If possible, it’s usually preferable to test a class solely through its public methods and constructors, treating the implementation of the class as a “black box”. The reason this is preferred is because it ensures the class behaves correctly when its behaviour is invoked via the API – which is, after all, how we expect it to be used.

However, it might prove challenging to test the class thoroughly when some behaviours are difficult to trigger. If this is the case, we might consider using another method (see below), or we might consider redesigning the API to make the class more testable.

Use reflection

Ideally, our classes should be designed with testability in mind. But if necessary, it’s possible to use Java’s reflection API. This allows us to obtain information about the methods and fields of an object at runtime, and to alter their visibility.

You can see an example of how to access private field information here, and read more about the reflection API in Oracle’s technical article on the topic here.

This allows us to directly set particular internal states, which can be convenient for testing. But a disadvantage is that the tests are now tightly coupled to the class’s implementation, and will likely break if that implementation is altered. By setting the state directly, we’re also bypassing the publicly documented API – we need to make sure that the state we set is one that could still be achieved directly through the API (otherwise, we’re testing something that couldn’t actually happen). Finally, use of reflection tends to make tests more complex and harder to maintain.

Use protected, “package-private” setter methods, and/or inheritance

An alternative to using the reflection API is to write protected or “package-private” methods that set the state directly.

Protected methods of some class can be invoked by other classes in the same package, or by subclasses; they’re indicated by writing “protected” as a method modifier. “Package-private” methods can be accessed by other classes in the same package, and are indicated by not writing any method modifier – for instance:

  public setMinutes(int minutes) {
    this.minutes = minutes;
  }

Our test classes can then use these methods to directly set the state of the class under test. This is simpler than using the reflection API, but other than that, has much the same drawbacks: we’re subverting the public API, and our tests are now more tightly coupled to the implementation.

One variant of this is to subclass our original class-under-test, and add mutators to the subclass. Technically, if we haven’t made our class under test final, this isn’t subverting the API (as long as our subclass maintains all the invariants of the original class), since inheritance is an allowable way of using a class.

But in general, it’s better to only allow classes to be inherited from if they’ve been designed with that in mind – it’s better practice to make classes final, preventing inheritance.

In the present case, it should be possible to test the class entirely through its public API (though it might seem a little awkward).