There are several places where in JDK where the same character is appended multiple times (usually it is padding).

For each individuall append a few manipulations are performed, which may be done only once.

Such method may be added as a default method into Appendable interface:
default Appendable appendN(char c, int n) {
    if (n < 0) throw new IndexOutOfBoundsException();
    for (int i = 0; i < n; ++i)
        append(c);
    return this;
}

AbstractStringBuilder may then provide an optimized overridden implementation.


Examples of where such a method could be used in JDK:
macosx/classes/sun/util/locale/provider/HostLocaleProviderAdapterImpl.java- private static void appendN(char c, int n, StringBuilder sb) {
macosx/classes/sun/util/locale/provider/HostLocaleProviderAdapterImpl.java- for (int i = 0; i < n; i++) {
macosx/classes/sun/util/locale/provider/HostLocaleProviderAdapterImpl.java: sb.append(c);
macosx/classes/sun/util/locale/provider/HostLocaleProviderAdapterImpl.java- }
macosx/classes/sun/util/locale/provider/HostLocaleProviderAdapterImpl.java- }

share/classes/java/util/Formatter.java- for (int i = 0; i < sp; i++) {
share/classes/java/util/Formatter.java: a.append(' ');
share/classes/java/util/Formatter.java- }

share/classes/java/util/Formatter.java- private void trailingZeros(StringBuilder sb, int nzeros) {
share/classes/java/util/Formatter.java- for (int i = 0; i < nzeros; i++) {
share/classes/java/util/Formatter.java: sb.append('0');
share/classes/java/util/Formatter.java- }
share/classes/java/util/Formatter.java- }

share/classes/java/util/regex/PatternSyntaxException.java: for (int i = 0; i < index; i++) sb.append(' ');

share/classes/java/math/BigInteger.java- int numLeadingZeros = digitsPerLong[radix]-digitGroup[i].length();
share/classes/java/math/BigInteger.java- if (numLeadingZeros != 0) {
share/classes/java/math/BigInteger.java: buf.append(zeros[numLeadingZeros]);
share/classes/java/math/BigInteger.java- }

share/classes/java/math/BigInteger.java- for (int i=s.length(); i < digits; i++) {
share/classes/java/math/BigInteger.java: sb.append('0');
share/classes/java/math/BigInteger.java- }

share/classes/java/math/BigDecimal.java- for (int i = 0; i < trailingZeros; i++) {
share/classes/java/math/BigDecimal.java: buf.append('0');
share/classes/java/math/BigDecimal.java- }

share/classes/java/math/BigDecimal.java- for (int i=0; i<-insertionPoint; i++) {
share/classes/java/math/BigDecimal.java: buf.append('0');
share/classes/java/math/BigDecimal.java- }

share/classes/java/math/BigDecimal.java- for (; pad>0; pad--) {
share/classes/java/math/BigDecimal.java: buf.append('0');
share/classes/java/math/BigDecimal.java- }

share/classes/java/text/DecimalFormat.java- for (int i=digitList.decimalAt; i<minExponentDigits; ++i) {
share/classes/java/text/DecimalFormat.java: result.append(zero);
share/classes/java/text/DecimalFormat.java- }

share/classes/java/text/DecimalFormat.java- for (i=0; i<minExponentDigits; ++i)
share/classes/java/text/DecimalFormat.java: result.append(localized ? symbols.getZeroDigit() :
share/classes/java/text/DecimalFormat.java- PATTERN_ZERO_DIGIT);

share/classes/java/text/DigitList.java- for (int i = count; i < decimalAt; ++i) {
share/classes/java/text/DigitList.java: temp.append('0');
share/classes/java/text/DigitList.java- }

share/classes/java/lang/invoke/LambdaForm.java- for (int i = buf.length() - leadingZero; i < 3; i++)
share/classes/java/lang/invoke/LambdaForm.java- buf.insert(leadingZero, '0');

share/classes/java/time/format/DateTimeFormatterBuilder.java- for (int i = 0; i < minWidth - str.length(); i++) {
share/classes/java/time/format/DateTimeFormatterBuilder.java: buf.append(decimalStyle.getZeroDigit());
share/classes/java/time/format/DateTimeFormatterBuilder.java- }

share/classes/java/time/format/DateTimeFormatterBuilder.java- for (int i = 0; i < minWidth; i++) {
share/classes/java/time/format/DateTimeFormatterBuilder.java: buf.append(decimalStyle.getZeroDigit());
share/classes/java/time/format/DateTimeFormatterBuilder.java- }

share/classes/sun/util/calendar/CalendarUtils.java- for (int i = 1; i < width && d < n; i++) {
share/classes/sun/util/calendar/CalendarUtils.java: sb.append('0');
share/classes/sun/util/calendar/CalendarUtils.java- n /= 10;
share/classes/sun/util/calendar/CalendarUtils.java- }

share/classes/sun/util/calendar/CalendarUtils.java- for (int i = 1; i < width && d < n; i++) {
share/classes/sun/util/calendar/CalendarUtils.java: sb.append('0');
share/classes/sun/util/calendar/CalendarUtils.java- n /= 10;
share/classes/sun/util/calendar/CalendarUtils.java- }

share/classes/sun/security/x509/AVA.java- for (int i = 0; i < spaceCount; i++) {
share/classes/sun/security/x509/AVA.java: temp.append(' ');
share/classes/sun/security/x509/AVA.java- }



Two more places, where we append copies of a string, not a char:

share/classes/java/lang/Class.java- for (int i = 0; i < arrayDepth; i++)
share/classes/java/lang/Class.java: sb.append("[]");

share/classes/java/lang/Class.java- for (int i = 0; i < dimensions; i++) {
share/classes/java/lang/Class.java: sb.append("[]");
share/classes/java/lang/Class.java- }