The good old StringBuffer vs StringBuilder

Everyone (or at leasr almost everyone) knows that StringBuffer is synchronized and therefore slower than StringBuilder. How do they compare? It depends on a lots of things, e.g. if you append character by character, you will find that StringBuilder is much faster because it avoided synchronization. If you call append just a couple of times with relatively big strings, the difference will be very little.

However, there is another factor here. You can specify initial capacity for both StringBuilder and StringBuffer, and if you don't (and you do not have pass over an initial string either) the capacity will be set to 16. Not much, but at least not wasting the memory :) When they run out of space while appending, they both double the capacity by allocating a new memory area and arrayCopy the old content. This seems to be something where you can gain a little speed again. If you have a guess for the length of the produced string, you can avoid at least the first some memory allocation and arrayCopy.

Let's see how much it matters...

As you can see, there is a huge difference between StringBuffer and StringBuilder, since this test calls append very many times with very short strings. The another difference between pre-allocated memory and the one slowly growing from 16. Now this test constructed an 1024 character length string, therefore with a good initial capacity the pre-allocated version saved 6 re-allocation. And there is the difference, with a good guess, you can still save lots of processing time.

Now let's rewrite the code and instead of a creating the long string by appending single characters, we can use bigger strings and the bigger the strings, the smaller the difference between created by synchronization and at one point, pre-allocation of the space will have more benefit than the synchronization question, and this could be important. This below chart was generated using 64-char strings to construct the same size at the end.

JVM: 64-bit server JVM 1.6.0_22-b04