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format java scala string

How to use java.String.format in Scala?

327

I am trying to use a .format method of a string. But if I place %1, %2, etc. in the string, java.util.UnknownFormatConversionException is thrown pointing to a confusing Java source code piece:

private void checkText(String s) {

    int idx;

    // If there are any '%' in the given string, we got a bad format
    // specifier.
    if ((idx = s.indexOf('%')) != -1) {
        char c = (idx > s.length() - 2 ? '%' : s.charAt(idx + 1));
        throw new UnknownFormatConversionException(String.valueOf(c));
    }
}

From this I understand that % char is forbidden. If so, then what should I use for argument placeholders?

I use Scala 2.8.

    303

    While all the previous responses are correct, they’re all in Java. Here’s a Scala example:

    val placeholder = "Hello %s, isn't %s cool?"
    val formatted = placeholder.format("Ivan", "Scala")
    

    I also have a blog post about making format like Python’s % operator that might be useful.

    2

    298

    You don’t need to use numbers to indicate positioning. By default, the position of the argument is simply the order in which it appears in the string.

    Here’s an example of the proper way to use this:

    String result = String.format("The format method is %s!", "great");
    // result now equals  "The format method is great!".
    

    You will always use a % followed by some other characters to let the method know how it should display the string. %s is probably the most common, and it just means that the argument should be treated as a string.

    I won’t list every option, but I’ll give a few examples just to give you an idea:

    // we can specify the # of decimals we want to show for a floating point:
    String result = String.format("10 / 3 = %.2f", 10.0 / 3.0);
    // result now equals  "10 / 3 = 3.33"
    
    // we can add commas to long numbers:
    result = String.format("Today we processed %,d transactions.", 1000000);
    // result now equals  "Today we processed 1,000,000 transactions."
    

    String.format just uses a java.util.Formatter, so for a full description of the options you can see the Formatter javadocs.

    And, as BalusC mentions, you will see in the documentation that is possible to change the default argument ordering if you need to. However, probably the only time you’d need / want to do this is if you are using the same argument more than once.

    0

      127

      Instead of looking at the source code, you should read the javadoc String.format() and Formatter syntax.

      You specify the format of the value after the %. For instance for decimal integer it is d, and for String it is s:

      String aString = "world";
      int aInt = 20;
      String.format("Hello, %s on line %d",  aString, aInt );
      

      Output:

      Hello, world on line 20
      

      To do what you tried (use an argument index), you use: *n*$,

      String.format("Line:%2$d. Value:%1$s. Result: Hello %1$s at line %2$d", aString, aInt );
      

      Output:

      Line:20. Value:world. Result: Hello world at line 20