---
title: The Humble Enumeration
---

# What in the heck is an enum?
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```java {0|1|1-2|1-3|1-5}
enum MentalState {
  WAIT,
  OH_GOD,
  WHAT
}
```
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## And _why?_
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```java {0|1|1-2|1-3|1-5}
enum UsefulFor {
  EVERYTHING,
  A_FEW_THINGS,
  NOTHING_AT_ALL
}
```

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# What in the heck is an enum?

```java
enum MentalState {
  WAIT,
  OH_GOD,
  WHAT
}
```

## And _why?_

```java
enum UsefulFor {
  EVERYTHING, // ??? 
  A_FEW_THINGS, // ???
  NOTHING_AT_ALL // ???
}
```

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# Enumerated types
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## An enum is essentially a known set of distinct values.
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### A couple of common enums would be things like:
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* Playing card suits (HEART, SPADE, CLUB, DIAMOND)
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* Days of the week (SUNDAY, MONDAY, etc.)
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* Log levels (DEBUG, INFO, WARNING, ERROR, FATAL)
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* Etc.

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# Okay, that's great, but how do I know when to use one?
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## (Or, "Booleans? More like, _Fooleans!_")
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So, picture this. You've got some code that analyzes a computer network.
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There is a lovely Computer _class_:
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```java
public class Computer {
  String hostname;
  double maxPowerConsumption;
  boolean on;
}
```

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And a lovely _function_ that calculates how much power a collection of computers might use:
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```java
public double estimateWattage(List<Computer> computers) {
  double sum = 0;

  for (var computer : computers) {
    if (computer.on) {
      sum += computer.maxPowerConsumption;
    }
  }

  return sum;
}
```

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# This works great.

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## Until someone says...

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"What about computers in standby?"
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# Oh.
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# True.

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# Well, okay, maybe let's add a new _inStandby_ field to Computer?
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```java {5}
public class Computer {
  String hostname;
  double maxPowerConsumption;
  boolean on;
  boolean inStandby;
}
```

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## and update our wattage function:
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```java {7-8}
public double estimateWattage(List<Computer> computers) {
  double sum = 0;

  for (var computer : computers) {
    if (computer.on) {
      sum += computer.maxPowerConsumption;
    } else if (computer.inStandby) {
      sum += computer.maxPowerConsumption * 0.01;
    }
  }

  return sum;
}
```
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That works.
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Probably.
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But it's starting to look a little ugly.
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And, wait. Is a computer in standby... powered on? Or is it off?

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Hmm...
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# No matter how you answer the question, you've got yourself a problem.
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## There is now an _invalid_ state that an instance of Computer could end up in.
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# Let's say we decide that a computer inStandby is _on._
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## That means that having an instance where `.inStandby == true` and `.on == false` is _illegal._
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And every part of your code better understand that, because the compiler does ___not.___
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### The technical term for this is,
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#### "Yucky"
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### or
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##### "Ew"

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Hmmmmmm....
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# Well. We could try a String.
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```java {0|4}
public class Computer {
  String hostname;
  double maxPowerConsumption;
  String powerState;
}
```
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## But then, how do we know if Computer's powerState is valid?
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### Are we going to _enjoy_ writing code like this?
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```java
public static final String IN_STANDBY = "IN_STANDBY";

if (computer.powerState.toUpperCase().equals(IN_STANDBY)) {
  sum += computer.maxPowerConsumption * 0.01;
}
```
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#### My guess is, not so much.

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# Instead,
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# What if we try an enum?
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```java
enum PowerState {
  POWERED_ON,
  STANDBY,
  POWERED_OFF
}
```
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## Enums can describe more than two distinct states.
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### But they're a lot more strict than a String.
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#### And we can use _that_ inside our Computer class.
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```java {0|4}
public class Computer {
  String hostname;
  double maxPowerConsumption;
  PowerState powerState;
}
```
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# Then estimateWattage() cleans up pretty nicely!
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```java {6-14}
public double estimateWattage(List<Computer> computers) {
  double sum = 0;

  for (var computer : computers) {
    switch (computer.powerState) {
      case POWERED_ON:
        sum += computer.maxPowerConsumption;
        break;
      case STANDBY:
        sum += computer.maxPowerConsumption * 0.01;
        break;
      case POWERED_OFF:
        // Do nothing
        break;
    }
  }

  return sum;
}
```
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# And adding a new power state has become a _lot_ easier:
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```java {5-7}
switch (computer.powerState) {
  case POWERED_ON:
    sum += computer.maxPowerConsumption;
    break;
  case IDLE:
    sum += computer.maxPowerConsumption * 0.12;
    break;
  case STANDBY:
    sum += computer.maxPowerConsumption * 0.01;
    break;
  case POWERED_OFF:
    // Do nothing
    break;
}
```
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## This is pretty much the extent of enum capabilites in C, C++, and C#
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### But now _this_ is starting to look repetitive.

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# So, what if we do some Java?
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## Could we store that multiplier on the enum itself?
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```java {2-99}
enum PowerState {
  POWERED_ON(1.0),
  IDLE(0.12),
  STANDBY(0.01),
  POWERED_OFF(0);

  public final double wattageMultiplier;

  PowerState(double mult) {
    this.wattageMultiplier = mult;
  }
}
```
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# Hey, we can!
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## Java lets us associate any arbitrary data with an enum.
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### Though it's good practice to stick with something immutable.
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(Also: note the semicolon after the last value. It was optional before!)

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# Now, estimateWattage() doesn't need to know what the multipliers are!
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```java
public double estimateWattage(List<Computer> computers) {
  double sum = 0;

  for (var computer : computers) {
    var power = computer.powerState;
    sum += computer.maxPowerConsumption * power.wattageMultiplier;
  }

  return sum;
}
```
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## It just needs to know that a multiplier _exists._

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# And, what the heck, let's use streams.
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```java
public double estimateWattage(List<Computer> computers) {
  return computers.stream()
    .mapToDouble(computer ->
        computer.maxPowerConsumption *
        computer.powerState.wattageMultiplier)
    .sum();
}
```
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## _Functional!_
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### If you've futzed with streams a lot, you may notice an opportunity here.
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## If Computer had some sort of method for:
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```java
computer.maxPowerConsumption *
computer.powerState.wattageMultiplier
```
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# We could use that directly in mapToDouble(), via a _method reference_

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# So, let's define that method.
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```java {2-99}
public class Computer {
  // snip

  public double currentPowerConsumption() {
    return maxPowerConsumption * powerState.wattageMultiplier;
  }
}
```
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## Simple enough.
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# Cool.
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# And that leaves us with this fairly-neat implementation:
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```java
public double estimateWattage(List<Computer> computers) {
  return computers.stream()
    .mapToDouble(Computer::currentPowerConsumption)
    .sum();
}
```
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## It _almost_ doesn't even need a dedicated method anymore.
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### But I'll leave that as an exercise for the reader.
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# Side note
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## In Java, you can also define methods directly on an enum.
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### For example, say we still wanted a way to check if a computer is on or off.
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#### We could add a method directly to PowerState:
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```java {7-99}
enum PowerState {
  POWERED_ON(1.0),
  IDLE(0.12),
  STANDBY(0.01),
  POWERED_OFF(0);

  // snip

  public boolean isOn() {
      return this != POWERED_OFF;
  }
}
```
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#### And that keeps the logic all in one place.

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# In summary,
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## Wait, when should you use enums?
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# Basically any time you have a _known_ set of _distinct_ states.
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## When should you definitely _not_ use an enum?
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### Things like:
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* Names
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* Numbers
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* IPs
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* Prices
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* Etc.
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Because they aren't known beforehand, and there are too many to enumerate yourself.
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## So, use an enum when you have at least two states.
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# But probably less than a hundred.
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