Best practice for global constants involving magic numbers

Question

To avoid magic numbers, I always use constants in my code. Back in the old days we used to define constant sets in a methodless interface which has now become an antipattern.

I was wondering what are the best practices? I'm talking about global constants. Is an enum the best choice for storing constants in Java?

score 30 · Answer 1 · answered Oct 14 '10 at 08:33

For magic numbers where the number actual has a meaning and is not just a label you obviously should not use enums. Then the old style is still the best.

public static final int PAGE_SIZE = 300;

When you are just labelling something you would use an enum.

enum Drink_Size
{
   TALL,
   GRANDE,
   VENTI;
}

Sometimes it makes sense to put all your global constants in their own class, but I prefer to put them in the class that they are most closely tied to. That is not always easy to determine, but at the end of the day the most important thing is that your code works :)

Nice clean distinction here for when each is best used – Brian Oct 14 '10 at 08:40 — Brian, Oct 14 '10 at 08:40

score 4 · Answer 2 · answered Oct 14 '10 at 08:23

4

Yes it is. Enum is the best choice.

You are getting for free:

constants
immutable objects
singletons

All in one.

But wait, there's some more. Every enum value can have its own fields and methods. It's a rich constant object with behavior that allows transformation into different forms. Not only toString, but toInt, toWhateverDestination you need.

answered Oct 14 '10 at 08:23

Boris Pavlović

63,078
28
122
148

3

And here's a guide on how to use Enums as constants http://download.oracle.com/javase/1.5.0/docs/guide/language/enums.html – Joel Oct 14 '10 at 08:29

nanda · Answer 3 · 2010-10-14T08:32:19.503

Enum is best for most of the case, but not everything. Some might be better put like before, that is in a special class with public static constants.

Example where enum is not the best solution is for mathematical constants, like PI. Creating an enum for that will make the code worse.

enum MathConstants {
    PI(3.14);

    double a;        

    MathConstants(double a) {
       this.a = a;
    }

    double getValueA() {
       return a;
    }
}

Usage:

MathConstants.PI.getValueA();

Ugly isn't it? Compare to:

MathConstants.PI;

score 0 · Answer 4 · answered Sep 12 '12 at 13:35

0

Forget about enums - now, when static import is available in Java, put all your constants in REAL class (instead of interface) and then just import the static members from all the other ones using import static <Package or Class>.

answered Sep 12 '12 at 13:35

Mikhail Berastau

114
7

Can you clarify why we should, "forget about enums" and elaborate why you would want to import the constants from other classes statically? – allingeek Sep 26 '12 at 02:31
1. The question was about magic numbers. Enumerations aren't good for numbers comparing to standard constants - see @nanda 's answer above. 2. Furthermore, enumerations are ugly decision when you have to store a set of different-type constants. 3. Enums were initially introduced as a container for _series_ of elements. Which means enumeration describes one logical set of units: months, days of week, planets, rainbow colors, etc; and it was a workaround to use enums for constants to avoid the 'interface constants' anti-pattern. Now it is not required anymore - we have static imports instead. – Mikhail Berastau Sep 28 '12 at 11:57

score 0 · Accepted Answer · answered Oct 14 '10 at 08:31

Using interfaces for storing constants is some kind of abusing interfaces.

But using Enums is not the best way for each situation. Often a plain int or whatever else constant is sufficient. Defining own class instances ("type-safe enums") are even more flexible, for example:

public abstract class MyConst {
  public static final MyConst FOO = new MyConst("foo") {
    public void doSomething() {
      ...
    }
  };

  public static final MyConst BAR = new MyConst("bar") {
    public void doSomething() {
      ...
    }
  };

  protected abstract void doSomething();

  private final String id;

  private MyConst(String id) {
    this.id = id;
  }

  public String toString() {
    return id;
  }
  ...
}

how is it more flexible than enums? they can have constant-specific methods too (abstract methods that each individual constant implements). — Norill Tempest, Nov 05 '14 at 23:52

Best practice for global constants involving magic numbers

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