Why does EK9 reject repeated literal values?

← Code Quality · Ref: Q800

EK9 raises E11065 when the same literal value appears 3 or more times in a file. Repeated literals are a maintenance liability.

WHY REJECTED

When a literal needs to change, every occurrence must be found and updated. Research (Bettenburg & Shang, 2010) found that ~50% of changes to duplicated code are inconsistent - only some instances get updated.

THRESHOLDS

- 3+ occurrences in a single file: error
- 4+ occurrences across files in the same module: error

FIX: EXTRACT TO CONSTANT OR VARIABLE

  //Before: repeated literal
  stdout.println("processing")
  stdout.println("processing")
  stdout.println("processing")

  //After: named constant
  statusMsg <- "processing"
  stdout.println(statusMsg)
  stdout.println(statusMsg)
  stdout.println(statusMsg)

See Q799 for boolean argument naming. See Q777 for discarded returns.

Example

defines module qa.codequality.repeatedliteral

  defines program

    MagicLiteralDemo()
      stdout <- Stdout()

      statusMsg <- "processing"
      stdout.println(statusMsg)
      stdout.println(statusMsg)
      stdout.println(statusMsg)

Common mistakes

E11065 — The same literal 'processing' appears 3 times in the file. Extract it to a named variable or constant to avoid maintenance errors when the value needs to change. See ek9 -h E11065 for details.

Incorrect:

      stdout.println("processing")
      stdout.println("processing")
      stdout.println("processing")

Correct:

      statusMsg <- "processing"
      stdout.println(statusMsg)
      stdout.println(statusMsg)
      stdout.println(statusMsg)

Coming from another language?

Java: no compiler detection, relies on SonarQube S1192 or PMD. Python: no detection, relies on pylint. Rust: no detection for repeated literals. Kotlin: no compiler detection. Go: no detection. EK9: compile-time error E11065 at threshold of 3 per file or 4 per module.

Keywords: constant, maintenance, extract, literal, duplication, magic, E11065, repeated