Question

I recently encountered an idiom I haven’t seen before: string assembly by StringWriter and PrintWriter. I mean, I know how to use them, but I’ve always used StringBuilder. Is there a concrete reason for preferring one over the other? The StringBuilder method seems much more natural to me, but is it just style?

I’ve looked at several questions here (including this one which comes closest: StringWriter or StringBuilder ), but none in which the answers actually address the question of whether there’s a reason to prefer one over the other for simple string assembly.

This is the idiom I’ve seen and used many many times: string assembly by StringBuilder:

public static String newline = System.getProperty("line.separator");
public String viaStringBuilder () {
   StringBuilder builder = new StringBuilder();
   builder.append("first thing").append(newline);  // NOTE: avoid doing builder.append("first thing" + newline);
   builder.append("second thing").append(newline);
   // ... several things
   builder.append("last thing").append(newline);
   return builder.toString();
}

And this is the new idiom: string assembly by StringWriter and PrintWriter:

public String viaWriters() {
   StringWriter stringWriter = new StringWriter();
   PrintWriter printWriter = new PrintWriter(stringWriter);
   printWriter.println("first thing");
   printWriter.println("second thing");
   // ... several things
   printWriter.println("last thing");
   printWriter.flush();
   printWriter.close();
   return stringWriter.toString();
}

Edit It appears that there is no concrete reason to prefer one over the other, so I’ve accepted the answer which best matches my understanding, and +1ed all the other answers. In addition, I posted an answer of my own, giving the results of the benchmarking I ran, in response to one of the answers. Thanks to all.

Edit again It turns out that there is a concrete reason to prefer one (specifically the StringBuilder) over the other. What I missed the first time was the addition of the newline. When you add a newline (as above, as a separate append), it’s slightly faster – not hugely, but coupled with the clarity of intent, it’s definitely better. See my answer below for the improved timings.

Answer 1

Stylistically, the StringBuilder approach is cleaner. It is fewer lines of code and is using a class that was specifically designed for the purpose of building strings.

The other consideration is which is more efficient. The correct way to answer that would be to benchmark the alternatives, but (based on the source code) I would expect StringBuilder to be faster.

A StringWriter uses a StringBuffer (rather than a StringBuilder) under the hood to hold the characters written to the "stream". Thus using a StringWriter for string assembly is going to incur StringBuffer‘s synchronization overhead, whether your application needs it or not. But if your application does need the synchronization, then you should consider using a StringBuffer directly.

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CPerkins has done some benchmarking (see his answer) and his results align with my intuition. They say that the difference between optimal StringBuilder vs StringWriter is ~5% which is likely to be insignificant for a typical application¹. However, it is nice to know that the "style" and "performance" criteria are giving the same answer!

^{1 – While a typical application doesn’t spend most of its time assembling strings, there are exceptions. However, one should not spend time optimizing this kind of thing based purely on guesswork. Profile your code first.}