Similar to the way in which most strings are more than strings, most constant values are more than just constants. A constant value can usually be assigned a name that denotes the meaning of that constant. Avoid using the explicit value in a specification or executable code.* Declare the value as a constant and use the name of that constant in the document or the code. 

If Sam mentions that the late fee for a rental is $3, I create a constant:

  Dollar RENTAL_LATE_FEE = 3.00;

When reading the relevant documents later on, I need not concentrate on the actual value, only on the assigned name. Suppose this value was not transformed into a constant and the value of 3.00 was used frequently in the documents for other purposes. If I went searching for it, I would have to examine each appearance carefully to see if it was a reference to the rental late fee or to some other value.

You might not get rid of every constant value. The value 0 often appears in initializing variables or setting the initial index for an array. There is little to be gained by creating a named constant for zero.

If the value that a name represents is subject to change, the value should be kept with a configuration mechanism. In that case, the code would use the symbolic name to look up the configured value. The configuration mechanism could use an XML configuration file, a database table, or another form of persistence to store the values. For example, RENTAL_ LATE_FEE is probably something that should exist in a configuration file rather than a con-

NEVER LET A CONSTANT SLIP INTO CODE

Use a symbolic name for all values.