Question

Strings are immutable, meaning, once they have been created they cannot be changed.

So, does this mean that it would take more memory if you append things with += than if you created a StringBuffer and appended text to that?

If you use +=, you would create a new ‘object’ each time that has to be saved in the memory, wouldn’t you?

Answer 1

Yes, you will create a new object each time with +=. That doesn’t mean it’s always the wrong thing to do, however. It depends whether you want that value as a string, or whether you’re just going to use it to build the string up further.

If you actually want the result of x + y as a string, then you might as well just use string concatenation. However, if you’re really going to (say) loop round and append another string, and another, etc – only needing the result as a string at the very end, then StringBuffer/StringBuilder are the way to go. Indeed, looping is really where StringBuilder pays off over string concatenation – the performance difference for 5 or even 10 direct concatenations is going to be quite small, but for thousands it becomes a lot worse – basically because you get O(N²) complexity with concatenation vs O(N) complexity with StringBuilder.

In Java 5 and above, you should basically use StringBuilder – it’s unsynchronized, but that’s almost always okay; it’s very rare to want to share one between threads.

I have an article on all of this which you might find useful.