JetBrains/intellij-community · error · FormatDecode.IllegalFormatException

previous flag ''<'' used but no previous format specifier fo

Error message

previous flag ''<'' used but no previous format specifier found for ''{0}''

What it means

IllegalFormatException from FormatDecode when the '<' (previous argument) flag is used in a specifier but no previous argument reference exists — either it is the first specifier or the previous specifier could not establish a position. previousAllowed tracks whether a usable 'last position' exists; '<' requires one.

Source

Thrown at java/java-analysis-impl/src/com/siyeh/ig/format/FormatDecode.java:293

      }

      if (posSpec != null) {
        if (isAllBitsSet(flagBits, PREVIOUS)) {
          throw new IllegalFormatException(
            InspectionGadgetsBundle.message("format.string.error.unnecessary.position.specifier", posSpec, specifier));
        }
        final String num = posSpec.substring(0, posSpec.length() - 1);
        pos = Integer.parseInt(num) - 1;
        if (pos < 0) {
          throw new IllegalFormatException(
            InspectionGadgetsBundle.message("format.string.error.illegal.position.specifier", posSpec, specifier));
        }
        previousAllowed = true;
      }
      else if (isAllBitsSet(flagBits, PREVIOUS)) {
        // reuse last pos
        if (!previousAllowed) {
          throw new IllegalFormatException(InspectionGadgetsBundle.message("format.string.error.previous.element.not.found", specifier));
        }
      }
      else {
        previousAllowed = true;
        pos = implicit++;
      }

      final Validator allowed;
      if (isAllVerifier) {
        allowed = new AllValidatorWithRange(TextRange.create(matcher.start(), matcher.end()),
                                            new Spec(posSpec, flags, width, precision, dateSpec, conversion));
      }
      else if (dateSpec != null) {  // a t or T
        checkFlags(flagBits, LEFT_JUSTIFY | PREVIOUS, specifier);
        DateTimeConversionType dateTimeConversionType = getDateTimeConversionType(conversion.charAt(0));
        if (dateTimeConversionType == DateTimeConversionType.UNKNOWN) {
          throw new IllegalFormatException(
            InspectionGadgetsBundle.message("format.string.error.unknown.conversion", dateSpec + conversion));

View on GitHub (pinned to be881553f2)

Solutions

1. Remove the '<' flag, or ensure an earlier specifier consumes an argument before it
2. Replace '<' with an explicit 'N$' index for robustness

Example fix

// before
String.format("%<s", "a")
// after
String.format("%1$s", "a")

static boolean previousFlagLegal(String fmt) {
  boolean prevAllowed = false;
  for (String spec : fmt.split("(?<!%)%")) {
    boolean hasPrev = spec.contains("<");
    boolean indexed = spec.matches("\\d+\\$.*");
    if (hasPrev && !prevAllowed && !indexed) return false;
    prevAllowed = true; // any consuming specifier establishes a position
  }
  return true;
}

try { FormatDecode.decode(fmt, argc); } catch (IllegalFormatException e) { /* replace leading '<' with explicit 'N$' */ }

When it happens

Trigger: A format string starting with '%<s' (the '<' flag on the first specifier), or following a specifier type that did not set previousAllowed; decode hits PREVIOUS bit with previousAllowed == false.

Common situations: Deleting the first specifier from a format string and forgetting to strip '<' from the next; reorder/copy of specifiers leaving a leading relative-previous flag.

Related errors

• {2, choice, 1#flag|1<flags} ''{0}'' not allowed in ''{1}''
• unexpected character ''{0}'' in ''{1}''
• duplicate flag ''{0}'' in ''{1}''
• width (''{0}'') not allowed in ''{1}''
• unnecessary argument position specifier ''{0}'' in ''{1}''

AI-assisted analysis of JetBrains/intellij-community@be881553f2 (2026-08-14). Data as JSON: /api/errors/cd460d88bcb9c16f. Report an issue: GitHub.