Question

What is a magic number?

Why do many programmers advise that they be avoided?

Answer 1

A magic number is a direct usage of a number in the code.

For example, if you have (in Java):

public class Foo {     public void setPassword(String password) {          // don't do this          if (password.length() > 7) {               throw new InvalidArgumentException('password');          }     } } 

This should be refactored to:

public class Foo {     public static final int MAX_PASSWORD_SIZE = 7;      public void setPassword(String password) {          if (password.length() > MAX_PASSWORD_SIZE) {               throw new InvalidArgumentException('password');          }     } } 

It improves readability of the code and it’s easier to maintain. Imagine the case where I set the size of the password field in the GUI. If I use a magic number, whenever the max size changes, I have to change in two code locations. If I forget one, this will lead to inconsistencies.

The JDK is full of examples like in Integer, Character and Math classes.

PS: Static analysis tools like FindBugs and PMD detects the use of magic numbers in your code and suggests the refactoring.