Do Not Use Magic Numbers

Summary

Magic numbers should be replaced with well-named constants that describe their meaning.

Detail

Placing numeric or string literals directly into source code causes two problems:

1. It is unlikely that the meaning of the literal will be clear
2. If the value changes updates are required where ever the literal has been duplicated

Literals should therefore be replaced with well-named constants and Enums.

Bad

public void fnord(int i) {
  if (i == 1) {
    performSideEffect();
  }
}

Better

public void fnord(int i) {
  if (i == VALID) {
    performSideEffect();
  }
}

You've missed the point

public void fnord(int i) {
  if (i == ONE) {
    performSideEffect();
  }
}

If the constants you extract relate to an identifiable concept, create an Enum instead:

Good

public void fnord(FnordStatus status) {
  if (status == FnordStatus.VALID) {
    performSideEffect();
  }
}

Some coding standards make statements such as "0 and 1 are exceptions to this rule". This is, however, an oversimplification.

Sometimes 0 and 1 will have a clear local meaning as they are being used as part of low level code e.g.:

  if (list.size() == 0) {...}

But 0 and 1 may also have domain-specific values that should be extracted into constants like any other literal.

Server-side Java can also often be re-written in a cleaner fashion without the use of numeric literals, e.g.:

  if (list.isEmpty()) {...}