This document discusses enumerations (enums) in coding guidelines. It defines an enum as a set of constants that rarely changes, if at all. Enums avoid "magic numbers" and improve code safety and readability. Syntax examples are provided for enums in C, C#, Python, and Java. Java enums can include additional members and behaviors beyond constants. Enums should be used for small, related, and stable sets of constants; other uses may indicate a design issue.

1. Tech Talk: Coding
Guidelines - enum

2. Agenda

Definition / Motivation

Syntax in:
–
C
–
C#
–
Python
–
Java

Examples: good and not-so-good

Summary and guidelines

3. Define Enum

So.. what's an enum?

4. Define Enum

A set of constants

Rarely changes, if at all

5. Motivation

Avoid "magic numbers":
int color = 3;
Color color = Color.GREEN;

6. Motivation

Code Safety:
Compilation passes:
Color color = Color.GREEN; // 3
Compilation fails:
Color color = Shape.SQUARE; // 3

7. Motivation

Readability:
openFile1(true, true)
openFile2(
Permission.WRITE,
SeekMode.FORWARD_ONLY)

8. Examples: Good? Bad?

Spade, Heart, Diamond, Club

January, February, … , December

Mercury, Venus, … , Pluto

Supported formats:
JPG, GIF, … , PNG

Sunday, Monday, … , Saturday

[web]

9. Syntax - C
// without typedef
enum strategy { RANDOM, IMMEDIATE, SEARCH };
enum strategy my_strategy = IMMEDIATE;

10. Syntax - C#
public enum Importance {
None,
Trivial,
Regular,
Important,
Critical
};

11. Syntax - Python
class Importance(Enum):
none = 0
trivial = 1
regular = 2
important = 3
critical = 4

12. Syntax - Java
public enum Direction {
NORTH,
EAST,
SOUTH,
WEST
}

13. Enum in Java

Enum in Java is not only a constant

Enum in Java is a single instance of
a class, shared by all consumers

The class, as any class, can have:
–
Additional members
–
Functions → behavior (!)

14. Enum in Java – ex. 1
public enum Direction {
NORTH(0),
EAST(90),
SOUTH(180),
WEST(270);
private int angle;
public int getAngle() {
return this.angle;
}
Direction(int angle) {
this.angle = angle;
}
}

15. Enum in Java – ex. 1

Additional data is bound to each enum
member

Reverse lookup can be implemented
within the class itself:
public static Direction getByDeg(int d)
–
But.. consider the outcome of an invalid
input (e.g. getByDeg(30)):
–
Throw an exception?
–
Add an artificial enum member such as
UNKNOWN?

16. Enum in Java – ex. 2
public enum ArithmeticOp {
ADD {
@Override
public Double operate(Double a, Double b) {
return a + b;
}
},
MULTIPLY {
@Override
public Double operate(Double a, Double b) {
return a * b;
}
};
public abstract Double operate(Double x, Double y);
}

17. Enum in Java – ex. 2
// in main...
ArithmeticOp.ADD.operate(2.0, 3.0);
// 5.0
ArithmeticOp.MULTIPLY.operate(2.0, 3.0);
// 6.0

18. Enum in Java – ex. 2

Polymorphism is implemented in
enum

Enum class is now responsible to:
–
Data
–
Implementation

Polymorphism is better
implemented using a factory
and/or an IoC container

19. Special Uses of Enum

Singleton
–
Specifically in Java
–
Nice for PoC
–
In real code, use an IoC

Bitwise flags

20. Enum – When to use

A set of constants,

Related to each other

Small set

Rarely changes, if at all

Selecting one of the enum
members means not selecting any
other member

21. Enum – When not to use

Not related to each other
–
Ask:
●
Should I choose one, and only one value,
in every code scenario?
●
Do I use the enum just for convenience of
code completion (IDE)?

22. Enum – When not to use

Large set, say > 10 members

23. Enum – When not to use

Occasionally changes:
–
Ask: will I add new members to this
enum in the next 1-2 years?
–
This means that we enum data.
–
DON'T enum data

24. Enum – When not to use

Flags. Don't use bitwise flags

25. Summary

From now on...

Enum should be used properly:
–
A small, (almost) never-changing
set of constants, related to each
other

Other uses might be wrong:
–
Consider a design meeting

26. Thank you :)