Should I use Java's String.format() if performance is important?

Question

I took hhafez code and added a memory test:

private static void test() {
    Runtime runtime = Runtime.getRuntime();
    long memory;
    ...
    memory = runtime.freeMemory();
    // for loop code
    memory = memory-runtime.freeMemory();

I run this separately for each approach, the '+' operator, String.format and StringBuilder (calling toString()), so the memory used will not be affected by other approaches. I added more concatenations, making the string as "Blah" + i + "Blah"+ i +"Blah" + i + "Blah".

The result are as follow (average of 5 runs each):
Approach       Time(ms)  Memory allocated (long)
'+' operator     747           320,504
String.format  16484       373,312
StringBuilder  769           57,344

We can see that String '+' and StringBuilder are practically identical time-wise, but StringBuilder is much more efficient in memory use. This is very important when we have many log calls (or any other statements involving strings) in a time interval short enough so the Garbage Collector won't get to clean the many string instances resulting of the '+' operator.

And a note, BTW, don't forget to check the logging level before constructing the message.

Conclusions:

1. I'll keep on using StringBuilder.
2. I have too much time or too little life.

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.

All the benchmarks presented here have some flaws, thus results are not reliable.

I was surprised that nobody used JMH for benchmarking, so I did.

Results:

Benchmark             Mode  Cnt     Score     Error  Units
MyBenchmark.testOld  thrpt   20  9645.834 ± 238.165  ops/s  // using +
MyBenchmark.testNew  thrpt   20   429.898 ±  10.551  ops/s  // using String.format

Units are operations per second, the more the better. Benchmark source code. OpenJDK IcedTea 2.5.4 Java Virtual Machine was used.

So, old style (using +) is much faster.

Your old ugly style is automatically compiled by JAVAC 1.6 as :

StringBuilder sb = new StringBuilder("What do you get if you multiply ");
sb.append(varSix);
sb.append(" by ");
sb.append(varNine);
sb.append("?");
String s =  sb.toString();

So there is absolutely no difference between this and using a StringBuilder.

String.format is a lot more heavyweight since it creates a new Formatter, parses your input format string, creates a StringBuilder, append everything to it and calls toString().

Java's String.format works like so:

1. it parses the format string, exploding into a list of format chunks
2. it iterates the format chunks, rendering into a StringBuilder, which is basically an array that resizes itself as necessary, by copying into a new array. this is necessary because we don't yet know how large to allocate the final String
3. StringBuilder.toString() copies his internal buffer into a new String

if the final destination for this data is a stream (e.g. rendering a webpage or writing to a file), you can assemble the format chunks directly into your stream:

new PrintStream(outputStream, autoFlush, encoding).format("hello {0}", "world");

I speculate that the optimizer will optimize away the format string processing. If so, you're left with equivalent amortized performance to manually unrolling your String.format into a StringBuilder.