Question

We have to build Strings all the time for log output and so on. Over the JDK versions we have learned when to use StringBuffer (many appends, thread safe) and StringBuilder (many appends, non-thread-safe).

What’s the advice on using String.format()? Is it efficient, or are we forced to stick with concatenation for one-liners where performance is important?

e.g. ugly old style,

String s = 'What do you get if you multiply ' + varSix + ' by ' + varNine + '?'; 

vs. tidy new style (String.format, which is possibly slower),

String s = String.format('What do you get if you multiply %d by %d?', varSix, varNine); 

Note: my specific use case is the hundreds of ‘one-liner’ log strings throughout my code. They don’t involve a loop, so StringBuilder is too heavyweight. I’m interested in String.format() specifically.

Answer 1

I wrote a small class to test which has the better performance of the two and + comes ahead of format. by a factor of 5 to 6. Try it your self

import java.io.*; import java.util.Date;  public class StringTest{      public static void main( String[] args ){     int i = 0;     long prev_time = System.currentTimeMillis();     long time;      for( i = 0; i< 100000; i++){         String s = 'Blah' + i + 'Blah';     }     time = System.currentTimeMillis() - prev_time;      System.out.println('Time after for loop ' + time);      prev_time = System.currentTimeMillis();     for( i = 0; i<100000; i++){         String s = String.format('Blah %d Blah', i);     }     time = System.currentTimeMillis() - prev_time;     System.out.println('Time after for loop ' + time);      } } 

Running the above for different N shows that both behave linearly, but String.format is 5-30 times slower.

The reason is that in the current implementation String.format first parses the input with regular expressions and then fills in the parameters. Concatenation with plus, on the other hand, gets optimized by javac (not by the JIT) and uses StringBuilder.append directly.