Tech Talk: Coding Guidelines - enum
Agenda
Definition / Motivation
Syntax in:C
C#
Python
Java
Examples: good and not-so-good
Summary and guidelines
Define Enum
So.. what's an enum?
Define Enum
A set of constants
Rarely changes, if at all
Motivation
Avoid "magic numbers":
int color = 3;
Color color = Color.GREEN;
Motivation
Code Safety:
Compilation passes:
Color color = Color.GREEN; // 3
Compilation fails:
Color color = Shape.SQUARE; // 3
Motivation
Readability:openFile1(true, true)
openFile2(
Permission.WRITE,
SeekMode.FORWARD_ONLY)
Examples: Good? Bad?
Spade, Heart, Diamond, Club
January, February, , December
Mercury, Venus, , Pluto
Supported formats:
JPG, GIF, , PNG
Sunday, Monday, , Saturday
[web]
Syntax - C
// without typedefenum strategy { RANDOM, IMMEDIATE, SEARCH
};enum strategy my_strategy = IMMEDIATE;
Syntax - C#
public enum Importance { None, Trivial, Regular, Important,
Critical};
Syntax - Python
class Importance(Enum): none = 0 trivial = 1 regular = 2
important = 3 critical = 4
Syntax - Java
public enum Direction { NORTH, EAST, SOUTH, WEST}
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 (!)
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; }}
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?
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);}
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
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
Special Uses of Enum
SingletonSpecifically in Java
Nice for PoC
In real code, use an IoC
Bitwise flags
Bitwise Enum
What:
Enum as a set of Boolean flags
Usually up to 32/64 different flags
Bitwise Enum
How, in C#:
public enum MyColors{ Yellow = 1, Green = 2, Red = 4, Blue =
8}
if (myProps.AllowedColors.HasFlag(MyColor.Yellow)){ // Yellow
has been set...}
Bitwise Enum
Why:Reduce memory usage
Trade-off:Increase code complexity
Is it justified?Rarely
Bitwise Enum
Should we use it?
No
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
Enum When not to use
Not related to each otherAsk:Should I choose one, and only one
value, in every code scenario?
Do I use the enum just for convenience of code completion
(IDE)?
Enum When not to use
Large set, say > 10 members
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
Enum When not to use
Flags. Don't use bitwise flags
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
Thank you :)
