String.replace benchmark

String.replace JVM implementations comparison based on benchmarks

4 implementations of String.replace were compared:

• 'manual': a manual implementation using indexOf and StringBuilder.append
• 'platform': an implementation delegating to java.lang.String.replace
• 'regex': an implementation based on a literal regex pattern and an escaped literal replacement
• 'regex1': same as above, but using a manual replacement loop
• 'stdlib': the current implementation in the kotlin-stdlib 1.4.10, basically split(oldValue) + join(newValue)

Varying parameters:

• JVM: 1.8.0_191, 11.0.1
• ignoreCase: false for case-sensitive search, true for case-insensitive
• needle: the string being searched in a test string
  • "unique", a string that can be easily found just by its first letter
  • ">>back", a string that shares its first two chars with a lot of other substrings in the test string
• occurrences: the number of searched substrings actually found in a test string
• totalLength: the length of a test string

Results

• fixed totalLength=5000, occurrences=10; varying ignoreCase and needle

  • JDK 8
  • JDK 11

• fixed ignoreCase=false, needle=>>back, varying totalLength (100, 100K) and occurrences (0, 1, 10)

  log scale recommended

  • JDK 8
  • JDK 11

• fixed ignoreCase=true, needle=>>back, varying totalLength (100, 100K) and occurrences (0, 1, 10)

  log scale recommended

  • JDK 8
  • JDK 11

Conclusions

case-sensitive replacement (ignoreCase=false)

With JDK 8:

• 'platform' performs the same as 'regex'
• 'regex' outperforms 'manual' (~x3) on long strings when the string being searched requires a lot of backtracking
• 'regex' takes time to compile the regular expression, thus is slower than 'manual' on short strings

With JDK 11:

• String operations are generally faster than with JDK 8
• 'platform' is roughly the same as 'manual'
• 'regex' is generally slower than 'manual'
• 'stdlib' (split+join) is faster! (~x1.14) than 'platform' on long strings

case-insensitive replacement (ignoreCase=true)

• no platform method available for case-insensitive replacement

With JDK 8:

• 'regex' outperforms 'manual' (~x1.5-2) even on short strings (except one case: many occurrences in a short string, x2.5 slower)

With JDK 11:

• 'regex' outperforms 'manual' (~x1.1-2) except in the case of many occurrences in a long string (x1.2 slower) and in a short string (x2.5 slower)