Profiling of String concatenation and the StringBuilder pattern in Apex (Salesforce)

StringBuilding.cls

/**
 * Compares String concatenation performance vs StringBuilder.
 * At around 21000 iterations, actual time spent is nearly identical. At higher iterations,
 * StringBuilder comes out ahead in less actual time spent and with lower CPU usage. However in
 * lower iterations, concatenation wins both measurements.
 *
 * Another consideration is that the StringBuilder pattern does not seem to affect the heap
 * size limit at all, which could be a bug with Apex profiling.
 */

final Integer iterations = 21000;
Long start;

/**
 * String concatenation
 */
start = System.currentTimeMillis();

String s = '';
for (Integer i = 0; i < iterations; i++) {
	s += String.valueOf(i);
}

Long concatTime = System.currentTimeMillis() - start;

/**
 * StringBuilder
 */
start = System.currentTimeMillis();

final List<String> strs = new List<String>();

public void append(String str) {
	strs.add(str);
}

public String build() {
	return String.join(strs, '');
}

for (Integer i = 0; i < iterations; i++) {
	append(String.valueOf(i));
}
build();

Long sbTime = System.currentTimeMillis() - start;

/**
 * Log time spent
 */
System.debug('');
System.debug('String concat ms: ' + concatTime);
System.debug('StringBuilder ms: ' + sbTime);
System.debug('');
System.debug('concat vs builder: ' + (Decimal.valueOf(concatTime) / Decimal.valueOf(sbTime)).setScale(2) + '%');
System.debug('builder vs concat: ' + (Decimal.valueOf(sbTime) / Decimal.valueOf(concatTime)).setScale(2) + '%');
System.debug('');