Is it better practice to use String.format over string Concatenation in Java?

One problem with .format is that you lose static type safety. You can have too few arguments for your format, and you can have the wrong types for the format specifiers - both leading to an IllegalFormatException at runtime, so you might end up with logging code that breaks production.

In contrast, the arguments to + can be tested by the compiler.

The security history of printf (on which the format function is modeled) is long and frightening.

About performance:

public static void main(String[] args) throws Exception {      
  long start = System.currentTimeMillis();
  for(int i = 0; i < 1000000; i++){
    String s = "Hi " + i + "; Hi to you " + i*2;
  }
  long end = System.currentTimeMillis();
  System.out.println("Concatenation = " + ((end - start)) + " millisecond") ;

  start = System.currentTimeMillis();
  for(int i = 0; i < 1000000; i++){
    String s = String.format("Hi %s; Hi to you %s",i, + i*2);
  }
  end = System.currentTimeMillis();
  System.out.println("Format = " + ((end - start)) + " millisecond");
}

The timing results are as follows:

• Concatenation = 265 millisecond
• Format = 4141 millisecond

Therefore, concatenation is much faster than String.format.

Since there is discussion about performance I figured I'd add in a comparison that included StringBuilder. It is in fact faster than the concat and, naturally the String.format option.

To make this a sort of apples to apples comparison I instantiate a new StringBuilder in the loop rather than outside (this is actually faster than doing just one instantiation most likely due to the overhead of re-allocating space for the looping append at the end of one builder).

    String formatString = "Hi %s; Hi to you %s";

    long start = System.currentTimeMillis();
    for (int i = 0; i < 1000000; i++) {
        String s = String.format(formatString, i, +i * 2);
    }

    long end = System.currentTimeMillis();
    log.info("Format = " + ((end - start)) + " millisecond");

    start = System.currentTimeMillis();

    for (int i = 0; i < 1000000; i++) {
        String s = "Hi " + i + "; Hi to you " + i * 2;
    }

    end = System.currentTimeMillis();

    log.info("Concatenation = " + ((end - start)) + " millisecond");

    start = System.currentTimeMillis();

    for (int i = 0; i < 1000000; i++) {
        StringBuilder bldString = new StringBuilder("Hi ");
        bldString.append(i).append("; Hi to you ").append(i * 2);
    }

    end = System.currentTimeMillis();

    log.info("String Builder = " + ((end - start)) + " millisecond");

• 2012-01-11 16:30:46,058 INFO [TestMain] - Format = 1416 millisecond
• 2012-01-11 16:30:46,190 INFO [TestMain] - Concatenation = 134 millisecond
• 2012-01-11 16:30:46,313 INFO [TestMain] - String Builder = 117 millisecond

I'd suggest that it is better practice to use String.format(). The main reason is that String.format() can be more easily localised with text loaded from resource files whereas concatenation can't be localised without producing a new executable with different code for each language.

If you plan on your app being localisable you should also get into the habit of specifying argument positions for your format tokens as well:

"Hello %1$s the time is %2$t"

This can then be localised and have the name and time tokens swapped without requiring a recompile of the executable to account for the different ordering. With argument positions you can also re-use the same argument without passing it into the function twice:

String.format("Hello %1$s, your name is %1$s and the time is %2$t", name, time)