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About Jakub Holy

Jakub Holy
Jakub is an experienced Java[EE] developer working for a lean & agile consultancy in Norway. He is interested in code quality, developer productivity, testing, and in how to make projects succeed.

Surfacing Hidden Design: Seeking A Better Alternative To Interrelated Mutable Fields

What is better, a bunch of mutable boolean fields and methods operating on them, or an explicit expression of the individual states and transitions between them? Lets study an example from a simulation of the progression of a multi-stage infection.

1. Design hidden in primitive mutable fields and methods

The following class, with a number of interrelated mutable (public) fields, which I should have completed with methods for transitions between their states, made me really uneasy (a var is a mutable field, val is immutable):

class Person (val id: Int) {
  var infected = false
  var sick = false
  var immune = false
  var dead = false

  // to complete with simulation logic
  // [infection progression]

The fields keep the state of the progress of an infection. They depend on each other – f.ex. when sick, the person must also be infected, while when dead, she must not be immune.

The progression of the infection is: healthy -> infected and infectious with the chance of 40% (but not visibly sick yet) -> on day 6 visibly sick -> on day 14 dies with the chance of 25% -> if not dead, becomes immune on day 16 – not visibly sick but still infectious -> healthy on day 18.

The problem I have with keeping the state in a bunch of fields is that there is no explicit expression of these rules and that it opens for defects such as setting sick to true while forgetting to set also infected. It is also hard to understand. You could learn the rules by studying the methods that alter the fields but it requires a lot of effort, it isn’t easy to distinguish between an incidental implementation detail and intentional design and this code does not prevent mistakes like the one described.


  • Familiar
  • Easy to design


  • The rules of the infection progression are not clear from the code (well, it wouldn’t be even if the methods were actually shown ), i.e. it communicates poorly the domain concepts and rules
  • The values of multiple fields must be synchronized, failing to ensure that leads to defects
  • Leads to spaghetti code

2. Explicit and defect-preventing expression of the states and transitions

What I would prefer is to make the rules the central piece of the design rather then basing it on the implementation details of how we keep the state (i.e. a set of booleans to manage). In other words, I want to surface the design hidden in this. I want to have an explicit notion of the states – Healthy, Incubating, Sick,  Dead, Immune – with clear transitions between them and I want these states to explicitely carry the secondary information about whether the person is infectious or not and whether this is visible or not.

One way to express this design explicitely is this:

def randomBelow = ... 

/** When should the health state change and what to */
class HealthChange(val time: Int, val newHealth: Health) {
  def immediate = (time == 0)
/** The root of the health hierarchy; the health (i.e. infection) evolves in stages */
sealed abstract class Health(val infectious: Boolean, val visiblyInfectious: Boolean) {
  def evolve(): Option[HealthChange] // returns None if no further change possibly/expected
/** In some stages the person is infected but it isn't visible*/
sealed abstract class CovertlyInfected extends Health(infectious = true, visiblyInfectious = false) {}
/** In other stages the person is infected and it is clearly visible*/
sealed abstract class VisiblyInfected extends Health(infectious = true, visiblyInfectious = true) {}

case object HEALTHY extends Health(infectious = false, visiblyInfectious = false) {
  def evolve() = // 40% chance of getting infected, triggered on meeting an infected person
    if (randomBelow(101) <= 40/*%*/)
      Some(new HealthChange(0, INCUBATIOUS))
      None // no change, stays healthy
case object INCUBATIOUS extends CovertlyInfected {
  def evolve() = // After 6 days if infection without visible effects becomes sick
    Some(new HealthChange(6, SICK))
case object SICK extends VisiblyInfected {
  def evolve() = // die with 25% on day 14 or stays sick for 2 more days, then immune
    if (randomBelow(101) <= 25/*%*/)
        Some(new HealthChange(14 - 6, DEAD))
        Some(new HealthChange(16 - 6, IMMUNE))
case object DEAD extends VisiblyInfected {
  def evolve() = None // Dead people stay dead
/** The symptoms have disappeared but the person can still infect others for 2 more days */
case object IMMUNE extends CovertlyInfected {
  def evolve() =
    Some(new HealthChange(18 - 16, HEALTHY))

class Person (val id: Int, var health: Health = HEALTHY) {
  def tryInfect() { // upon meeting an infected person
    if (health != HEALTHY) throw new IllegalStateException("Only healthy can be infected")
      healthChng => setHealth(healthChng.newHealth) // infection happens immediately
  /** Set the new health stage and schedule the next health change, if any */
  def setHealth(h: Health) {
    this.health = h
    h.evolve().foreach(hNext => {
      if (hNext.immediate)
        afterDelay(hNext.time) {setHealth(hNext.newHealth)}


  • The rules and stages of the infection are now explicit and first-class members of the code; Domain-Driven Design in practice
  • The transitions between the states are clear, explicit, and we cannot get the person into an invalid state (provided we defined the transitions correctly)
  • We don’t need anymore to synchronize the state of multiple variables


  • The code is likely longer than a bunch of bool fields and methods for transitioning between their states
  • It may seem complicated because, instead of one class and few methods, we have suddenly a class hierarchy; but it actually prevents the complexity of the original spaghetti code so, though not “easy” to comprehend, it is “simple” as per R. Hickey


I often encounter code like this, especially in older legacy applications that have been evolved according to changing business needs with primary focus on “features” without updating/refactoring the underlaying design accordingly. Frankly, it is hell. The low-level code working on a number of interdependant fields (in a class that has likely also a number of unrelated fields or fields that are depending on these only in certain use cases) is a  heaven for defects to hide in and multiply in. And it is hard to understand since the design – the rules, concepts, and intentions – have not been made explicit (yet). And hard to understand means hard to change.

Therefore it is important to survey your code regularly and surface the design hidden in it, hiding away low-level implementation details and making the key concepts, states, transitions, rules etc. first-class members of the code base so that reading the code feels as communicating and learning rather than as an archeology.


Update 1

Jerrinot’s Enum and Java based code, included here for easier readability:

public enum Health {
    HEALTHY(false, false, false, false),
    INCUBATIOUS(true, false, false, false),

    private boolean infected;
    private boolean sick;
    private boolean immune;
    private boolean dead;

    private Health(boolean infected, boolean sick, boolean immune, boolean dead) {
        this.infected  = infected;
        this.sick = sick;
        this.immune = immune;
        this.dead = dead;

    public boolean isInfected() {
        return infected;

    public boolean isSick() {
        return sick;

    public Health evolve() {
        switch (this) {
           case HEALTHY:
               return computeProbabability..() ? INCUBATIOUS : HEALTHY;

Update 2

Jerrinot’s version in Scala:

sealed class Health(val infectious: Boolean, val visiblyInfectious: Boolean) {

  def evolve(implicit randomGenerator: RandomIntGenerator, config: MySimConfig): Option[HealthChange] =
    this match {
      case Healthy =>
        if (randomGenerator.randomBelow(101) <= config.transmissibilityPct)
          Some(new HealthChange(0, Incubatious))
        else None
      case Incubatious => 
        Some(new HealthChange(6, Sick))
      case Sick =>
        if (randomGenerator.randomBelow(101) <= config.mortalityPct)
          Some(new HealthChange(14 - 6, Dead))
        else None
      case Dead => 
      case Immune => 
        Some(new HealthChange(18 - 16, Healthy))
      case Vaccinated => 
//                                 infected?  visibly?
object Healthy extends      Health(false,     false)
object Incubatious extends  Health(true,      false)
object Sick extends         Health(true,      true)
object Dead extends         Health(true,      true)
object Immune extends       Health(true,      false)
object Vaccinated extends   Health(false,     false)

The “primitive” version with nearly full code, excluding the information what happens when:

  // P.S.: Excuse the underscores ...
  class Person (val id: Int) {

    // Simplified primitive version, assuming the information when should the evolution
    // happen is handled somewhere else
    def evolveHealth(): Unit = {
      if (!_infected)
        if (randomGenerator.randomBelow(101) <= config.transmissibilityPct)
          this._infected = true
      if (_infected && !_sick && !_immune)
        this._sick = true
      if (_sick)
        if (randomGenerator.randomBelow(101) <= config.mortalityPct)
          this._dead = true
          this._unDead = true // did not die but still sick
      if (_unDead)
        this._immune = true
        this._sick = true
      if (_immune)
        this._immune = false
        this._infected = false
    private var _unDead = false
    private var _infected = false
    private var _sick = false
    private var _immune = false
    private var _dead = false


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