The JVM does rewrite individual concatenations as StringBuilders (so a+b+c+d results in one StringBuilder with three .append() operations), but the real perform

The JVM does rewrite individual concatenations as StringBuilders (so a+b+c+d results in one StringBuilder with three .append() operations), but the real performance problem is concatenating in a loop. String result = ""; for (String str : myStrings) { result += str + " "; // on each execution, a new StringBuilder is created } as opposed to StringBuilder result = new StringBuilder(); for (String str : myStrings) { result.append(str).append(" "); } The JVM can't hoist the generated StringBuilder out of loops to produce the second, faster piece of code. I believe the CLR is the same.