That's a very good point:
> String myString = new StringBuffer(256)
Java's default of 16 is WAY too small for almost anything. Java should in my opinion do all this higher-performance plumbing for you automatically, like Perl does. But of course it can't since Strings are immutable objects...
Greg
-----Original Message-----
From: Burnett, David [mailto:[EMAIL PROTECTED]]
Sent: Monday, January 20, 2003 5:42 PM
To: JDJList
Subject: [jdjlist] RE: Slow: string += string
Just to add a litle more to this:
If you give StringBuffer a String argument, it will create a character buffer of that strings length + 16. That still gives you the expanding churn problem when appending more than 16 characters to it.
In the mode of the example using:
String myString = new StringBuffer(256).append("SELECT ").append(columnlist)
.append(" FROM ).append(tablename).append(" WHERE ")
.append(keyname + " = " + key).toString();
is more efficient. It leaves less char arrays to be gc'ed and less arraycopy to be performed.
As with all optimisations, it's only worth the bother if the code is being used many times (in a loop or similar). If so, it may be better to reuse a large StringBuffer by keeping the handle and using reset() after you have finished with it (or providing a query buffer factory that manages buffers for use in a multithreaded app).
db
> -----Original Message-----
> From: David Gallardo [mailto:[EMAIL PROTECTED]]
> Sent: Tuesday, January 21, 2003 9:39 AM
> To: JDJList
> Subject: [jdjlist] RE: Slow: string += string
>
>
> Actually given your example, using string literals, it's more
> efficient to
> write:
>
> String myString = "Something" + "Something Else";
>
> because the compiler doesn't need to concatenate these at runtime.
>
> If you aren't using string literals, but variables instead,
> which do need
> to be concatenated at runtime, this is actually what the
> compiler does
> behind the scenes when you concatenate strings in a single expression
> anyway, even if you don't explicitly use StringBuffer--see
> the StringBuffer
> Javadoc.
>
> In other words, this:
>
> String myString = "SELECT " + columnlist + " FROM " +
> tablename + "
> WHERE " + keyname " = " + key;
>
> and this:
>
> StringBuffer sb = new StringBuffer("SELECT ");
> sb.append(columnlist);
> sb.append(" FROM ")'
> sb.append(tablename);
> sb.append(" WHERE ");
> sb.append(keyname + " = " + key);
> String myString = sb.toString();
>
> are both equivalent to:
>
> String myString = new StringBuffer("SELECT
> ").append(columnlist).append(" FROM ")append(tablename).append(
> " WHERE ").append(keyname + " = " + key).toString();
>
> Choosing between them is mostly a matter of style, unless the
> logic demands
> that you use multiple expressions.
>
> - David Gallardo
>
>
> At 01:07 PM 1/20/2003 -0800, Greg Nudelman wrote:
> >Just to add 2 cents more: here is another shortcut when you
> have to just
> >use the StringBuffer in-place:
> >
> >String myString = new
> StringBuffer().append("Something").append("Something
> >Else").toString();
> >
> >This one is best used for things like long dynamic SQL
> statements, etc.
> >where you don't append in the loop, but still have a fair
> amount of the
> >String objects to add.
> >
> >____________________________________________________
>
> David Gallardo | Software consultant | Author
> Java, C/C++ software development | Database development |
> Internationalization
> Author of "Java Oracle Database Development"
>
>
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