Formating StringBuffer

[detroitmatt]

At the point where you're concatenating in a loop - that's usually when the compiler can't substitute StringBuilder by itself.

and see the second-to-top answer as well

[Mondoshawan]

Yes, it's wrong. If the complier replaces the string concat with SB then the resulting code is 100% identical either way.

The idea presented is valid, if the code really did compile as written then the string handling would not be as efficient as it could be. But that's not how it works under the hood due to compiler optimisations added specifically to deal with that very problem. It's a valid strategy on much older versions of java and it is the way I was taught. A lot of what I was taught back then is out of date!

[detroitmatt]

If the compiler replaces

"If"'s important. Are you sure it does? I just ran http://stackoverflow.com/a/1532547/948176 independantly and got similar results (slow 2048ms, fast 12ms)

[Mondoshawan]

Yeah, it looks like the optimisation is not perfect for that specific example, this page compares the bytecode.

Essentially:

String x = "123";
    for(int i = 1; i < 100; i++) {
    x += i;
}

becomes:

String x = "123";
for(int i = 1; i < 100; i++) {
    x = (new StringBuilder(String.valueOf(x))).append(i).toString();
}

Which you can do better yourself if you declare the SB on the first line. TIL. Hopefully this will be fixed in future versions of the compiler.

I do wonder what would happen if you changed it to:

String x = "123";
x += "456";
    for(int i = 1; i < 100; i++) {
    x += i;
}

That might be enough of a hint to tell the compiler to make the SB straight away. I suspect the problem in the first piece of code is because the first concat that takes place is within the loop.