I've heard that the compiler (or was it the JVM?) will automatically use a StringBuilder for some string concatenation. When is the right time to explicitly declare one? I don't need a StringBuffer for being thread-safe.
Thanks.
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1possible duplicate of What happens when Java Compiler sees many String concatenations in one line?Paul Creasey– Paul Creasey2010-11-23 23:37:26 +00:00Commented Nov 23, 2010 at 23:37
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1Note: one situation where its won't use a StringBuilder is when the string can be concatenated at Compile time. In which case it just produces one lone string (no StringBuilder)Peter Lawrey– Peter Lawrey2010-11-24 09:28:08 +00:00Commented Nov 24, 2010 at 9:28
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BTW: If you ever need to use StringBuffer for thread safety, you have a design issue IMHO. :PPeter Lawrey– Peter Lawrey2010-11-24 09:28:45 +00:00Commented Nov 24, 2010 at 9:28
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1 Answer
The compiler will use it automatically for any string concatenation using "+".
You'd usually use it explicitly if you wanted to concatenate in a loop. For example:
StringBuilder builder = new StringBuilder();
for (String name : names)
{
builder.append(name);
builder.append(", ");
}
if (builder.length() > 0)
{
builder.setLength(builder.length() - 2);
}
System.out.println("Names: " + builder);
Another situation would be where you wanted to build up a string over multiple methods, or possibly conditionalise some bits of the building. Basically, if you're not building the string in a single statement (where the compiler can help you) you should at least consider using StringBuilder.
9 Comments
Matt Ball
I think it's worth mentioning that
StringBuilder will not be used in cases like this: String str = ""; str += <something that isn't a compile-time constant>.
Jon Onstott
Ah that makes sense, thanks! BTW I thought you were a C# guy ;o)
Jon Skeet
@Jon: C# is certainly my preferred language - but I've written more professional code in Java than C# :)
Jon Skeet
@Matt: Please give a fuller example. I've just tried it, and it does use
StringBuilder with the compiler I'm using (JDK 7). In theory it could use String.concat instead, but I haven't seen any evidence of that happening. Of course, the exact implementation isn't specified, but I can't remember seeing any compiler use String.concat.Matt Ball
@Jon: sorry, I realized that comment was less-than-clear. This answer illustrates what I meant (read: failed) to say.
StringBuilder will be used, but in the concatenation in my first comment, it would produce bytecode equivalent to new StringBuilder().append("foo").append(<something that...>).toString();. Bah, I should have initialized str to a non-empty string in that comment, too. |
