This is a quite embarrassing bug because it affects probably the most used class in Java. However I think using += for the Strings is not that popular among experienced developers because they want to be sure that no unnecessary Strings are created in the pool. I assume the execution times would be similar with the optimizations you mentioned, but according to Effective Java(3rd edition, published this year) using += for Strings in the loop is still 6,5 times slower than using StringBuilder. I need to check it myself.