Lecture 3: Getting (back) into Java
1 Objective of the lecture
2 Analysis
2.1 Operations
2.2 Assumptions
2.3 Interface design
3 Implementation
3.1 Representation
3.2 Initialization
3.3 Domain-specific methods
3.4 Standard methods
3.5 Full circle
7.1

Lecture 3: Getting (back) into Java

1 Objective of the lecture

This and the next lecture designs a simple example from scratch, starting from a description of the problem. It creates an interface and discusses specific ways to represent data. Next it completes one specific implementation, and then adds another implementation after the first one has been tested and used. Finally it shows how abstraction can be used to minimize code replication, and how designing by interface has several advantages.

2 Analysis

We will design a small library for computing with durations1This is a potentially tricky subject, and we will simplify a lot. Before we start writing code, we must carefully consider the information we want to work with and what operations it needs to support.

What do we mean by a duration? Here are some examples:

2.1 Operations

In an object-oriented system data and operations on it are kept together. Therefore the representation of information is often related to the operations that we wish to perform on it.

Here are some possible operations on durations:

For our design in this lecture, we will support conversion to seconds, one simple form of formatting, addition, and comparisons. The human-friendly units that we will decompose durations into include hours, minutes, and seconds.

2.2 Assumptions

Before proceeding, it is necessary to make some assumptions about what kinds of durations we will work with, because we probably cannot represent any conceivable duration—rather, there will be some limits. Which of the above examples do we need to be able to work with? In particular:

To keep things simple, we answer these questions as follows2Although we make some arbitrary assumptions here to simplify the illustration, in reality assumptions about resolution and range of data should be made according to requirements, stated and extrapolated. The smallest unit of time will be the second; we won’t deal with durations that are not an integer number of seconds. We disallow negative durations, since it isn’t clear what those mean. Rather than specify a maximum duration, we leave it unspecified, allowing that there will be some upper bound determined by our choice of representation. And we’d like durations to stand for pure lengths of time, regardless of what unit they are expressed in, so for example 3 minutes should be equal to 180 seconds.

2.3 Interface design

Now that we have a list of operations and have settled on some assumptions, we can design our interface Duration. It includes the following methods:

/**
 * Durations, with a resolution of seconds. All durations are non-negative.
 */
public interface Duration extends Comparable<Duration> {
  /**
   * Gets the total duration in seconds.
   *
   * @return the number of seconds (non-negative)
   */
  long inSeconds();

  /**
   * Formats this duration in the form {@code H:MM:SS} where the hours and
   * minutes are both zero-padded to two digits, but the hours are not.
   *
   * @return this duration formatted in hours, minutes, and seconds
   */
  String asHms();

  /**
   * Returns the sum of two durations.
   *
   * @param other the duration to add to {@code this}
   * @return the sum of the durations
   */
  Duration plus(Duration other);
}

Things to notice:

3 Implementation

3.1 Representation

In order to implement the Duration interface, we need to decide on a representation. Given that durations will have to be formatted as hours, minutes, and seconds, and that we will likely construct a duration from hours, minutes, and seconds, it seems intuitive to represent a duration using them directly4Intuitive is subjective: what if we find out later that our chosen representation wasn’t the best option after all?:

public final class DurationImpl implements Duration {
  private final int hours;
  private final int minutes;
  private final int seconds;
}
Three qualifiers in this code snippet are worth explaining:

3.2 Initialization

We might expect a constructor that takes the decomposition as parameters to be simple, like so:

/**
 * Constructs a duration in terms of its length in hours, minutes, and
 * seconds.
 *
 * @param hours the number of hours
 * @param minutes the number of minutes
 * @param seconds the number of seconds
 * @throws IllegalArgumentException if any argument is negative
 */
public DurationImpl(int hours, int minutes, int seconds) {
  if (hours < 0 || minutes < 0 || seconds < 0) {
    throw new IllegalArgumentException("must be non-negative");
  }

  // Insert omitted code from below //

  this.hours = hours;
  this.minutes = minutes;
  this.seconds = seconds;
}
This constructor checks that each parameter is non-negative (as it makes sense in this context), to ensure that durations are not negative (this is called an invariant). However, this is not enough. Consider the following durations:
Duration d1 = new DurationImpl(0, 0, 90);
Duration d2 = new DurationImpl(0, 1, 30);

According to our criteria above, it should be the case that d1.equals(d2) (once we’ve overridden equals), since they represent the same amount of time. This means that we somehow need to get them in the same form if we want to compare them. How can we make that happen? We have a choice:

If we choose the former, then other methods may duplicate the work. For example, according to asHms’s contract, it formats both durations in the same canonical form, "0:01:30". That suggests that we can save work—and have a single point of control—if we canonicalize in the constructor. (Furthermore, a real class is likely to have additional methods that require the canonicalized components.) Thus, we add code to the constructor to canonicalize the representation:

if (seconds > 59) {
  minutes += seconds / 60;
  seconds %= 60;
}

if (minutes > 59) {
  hours += minutes / 60;
  minutes %= 60;
}
This ensures that the seconds and minutes never exceed 59 (because if they did, we would add to the minutes or hours, respectively, instead).

3.3 Domain-specific methods

The observer methods are simple:

@Override
public long inSeconds() {
  return 3600 * hours + 60 * minutes + seconds;
}

@Override
public String asHms() {
  return String.format("%d:%02d:%02d", hours, minutes, seconds);
}
In a Java format string, the code %d means that the next parameter is an int. Writing 2 between % and d means to pad the number to be at least two characters long; by default padding is done with spaces, but 02 means to zero-pad the integer to make it two characters long.

The operation plus is slightly more difficult. The easiest way to add two durations is to convert them both to raw seconds, add, and then convert back to hours, minutes, and seconds. The first step we can do with inSeconds(), but to convert back from seconds, we need to do a little more work. Anticipating that its use could be more general, we define a second constructor:

/**
 * Constructs a duration in terms of its length in seconds.
 *
 * @param inSeconds the number of seconds (non-negative)
 * @throws IllegalArgumentException {@code inSeconds} is negative
 */
public DurationImpl(long inSeconds) {
  if (inSeconds < 0) {
    throw new IllegalArgumentException("must be non-negative");
  }

  this.seconds = (int) (inSeconds % 60);
  this.minutes = (int) (inSeconds / 60 % 60);
  this.hours = (int) (inSeconds / 3600);  // overflow...
}

In each assignment above, (int) is a numeric cast that converts from type long, in this case, to type int. Because the range of long is larger, not all longs can be represented correctly as ints, and instead will overflow, producing the wrong answer. The first two casts are guaranteed not to overflow, since x % 60 is between 0 and 59, plenty small to fit in an int, for any x. However, if inSeconds is large enough6In particular, if inSeconds / 3600 > (long) Integer.MAX_VALUE. then the resulting object won’t be right.

With the new constructor, plus is easy:

@Override
public Duration plus(Duration that) {
  return new DurationImpl(this.inSeconds() + that.inSeconds());
}

3.4 Standard methods

Because Duration extends Comparable<Duration>, we need to implement the compareTo(Duration) method. We could do this by comparing the components, but it’s simpler to compare the total seconds:

@Override
public int compareTo(Duration that) {
  return Long.compare(this.inSeconds(), that.inSeconds());
}
Note that the Long class has a static method for comparing primitive, lowercase-ell long values, because primitive values cannot themselves have methods.

All classes should override Object.toString() with something reasonable. In this case, asHms is a fine implementation of toString:

@Override
public String toString() {
  return asHms();
}

Finally, we need to decide whether to override Object.equals(Object) and Object.hashCode(). (Never override one without the other—we’ll talk about this soon.) The default implementations inherited from Object use pointer equality and a hash function compatible with pointer equality. This most often makes sense for distinguishing stateful objects, since two objects that currently have the same state can change in the future. However, for immutable value objects like DurationImpl, if we construct two objects representing the same length of time, those values are essentially equal. Thus, it makes sense to override equals to define extensional equality, and hashCode to compute hash codes using the same values that are compared by equals. Here’s a first attempt:

@Override
public boolean equals(Object o) {
  // Fast path for pointer equality:
  if (this == o) {
    return true;
  }

  // If o isn't the right class then it can't be equal:
  if (! (o instanceof DurationImpl)) {
    return false;
  }

  // The successful instanceof check means our cast will succeed:
  DurationImpl that = (DurationImpl) o;

  return this.hours == that.hours
      && this.minutes == that.minutes
      && this.seconds == that.seconds;
}

@Override
public int hashCode() {
  return Objects.hash(hours, minutes, seconds);
}
The static method Objects.hash(Object...) takes any number of arguments, each of which it hashes using that argument’s hashCode() method, and then combining the results in a reasonable way.

However, if we choose to, we can do better. Because Duration is an interface, we should expect that it will be implemented more times in the future. Perhaps it would be useful if the current and all future implementations of Duration could work together seamlessly, including being considered equal if they represent the same length of time. To do this, we need to rewrite the equals method to compare not only to another DurationImpl but to any Duration via its interface. We can do this in this case because inSeconds() is part of the interface and returns sufficient information to test for equality. So we write a more general equals that compares seconds:

@Override
public boolean equals(Object that) {
  if (this == that) {
    return true;
  }

  if (! (that instanceof Duration)) {
    return false;
  }

  return ((Duration) that).inSeconds() == this.inSeconds();
}

We’re nearly done, but not yet. Above I wrote that equals and hashCode should always be overridden together (or not at all), and similarly, changing one of them often requires changing the other. A proper hashCode method will return the same hash code for any two Durations that are equal according to the equals method. That means that hashCode should only use as inputs values that are part of the equals comparison, which in this case means we should hash the total seconds:

@Override
public int hashCode() {
  return Long.hashCode(inSeconds());
}

3.5 Full circle

Finally, building the extra flexibility into DurationImpl doesn’t do us any good if future implementations of Duration don’t follow the same rules. So we document how equals and hashCode must work for all implementations of the Duration interface in the interface itself:

/**
 * Durations, with a resolution of seconds. All durations are non-negative.
 *
 * <p>Different {@code Duration} implementations should work together,
 * meaning that:
 *
 * <ul>
 *   <li>Two durations must be equal if they have the same number of seconds.</li>
 *   <li>The hash code of a duration is the result of calling
 *        {@link Long#hashCode(long)} on its length in seconds.</li>
 * </ul>
 * </p>
 */
public interface Duration extends Comparable<Duration> {
  ...
}

1This is a potentially tricky subject, and we will simplify a lot

2Although we make some arbitrary assumptions here to simplify the illustration, in reality assumptions about resolution and range of data should be made according to requirements, stated and extrapolated

3In IntelliJ, if you press enter after the second *, it generates a Javadoc comment template for you.

4Intuitive is subjective: what if we find out later that our chosen representation wasn’t the best option after all?

5Why is immutability desirable? Methods that take or return immutable types, written here and elsewhere are implicitly guaranteed not to change these types, deliberately or by accident. This makes them easier to document and use as well.

6In particular, if inSeconds / 3600 > (long) Integer.MAX_VALUE.