Replace Magic Number with Symbolic Constant <IMG SRC = "http://www.refactoring.com/catalog/updated.gif" border=0>

2003. 8. 2. 15:19

Replace Magic Number with Symbolic Constant

You have a literal number with a particular meaning.

Create a constant, name it after the meaning, and replace the number with it.

        double potentialEnergy(double mass, double height) {
                return mass * height * 9.81;
        }

        double potentialEnergy(double mass, double height) {
                return mass * GRAVITATIONAL_CONSTANT * height;
        }
        static final double GRAVITATIONAL_CONSTANT = 9.81;

For more information see page 204 of Refactoring

Additional Comments

Using a constant method

For this refactoring I used a symbolic constant, which is the most common Java idiom.

However an alternative is the constant method, which is a method of this form

public static double gravitationalConstant() {
return 9.81;
}

This idiom is less familiar to C based programmers, but is very familiar to Smalltalkers (who didn't have constants in their language). On the whole I don't tend to use this in Java as it is less idiomatic to the language. However if you need to replace the simple return with a calculated value then it's worth changing the constant field to a constant method. (I guess there should be a refactoring for that....)