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How to use java.String.format in Scala?

Posted by: admin November 2, 2017 Leave a comment


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.


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.


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.


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 );


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 );


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


You can use this;

String.format("%1$s %2$s %2$s %3$s", "a", "b", "c");


a b b c


Also note that Scala extends String with a number of methods (via implicit conversion to a WrappedString brought in by Predef) so you could also do the following:

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


The official reference is the class Formatter.


In Scala 2.10

val name = "Ivan"
val weather = "sunny"

s"Hello $name, it's $weather today!"


Here is a list of formatters used with String.format()



This is a list of what String.format can do. The same goes for printf

int i = 123;
o.printf( "|%d|%d|%n" ,       i, -i );      // |123|-123|
o.printf( "|%5d|%5d|%n" ,     i, -i );      // |  123| –123|
o.printf( "|%-5d|%-5d|%n" ,   i, -i );      // |123  |-123 |
o.printf( "|%+-5d|%+-5d|%n" , i, -i );      // |+123 |-123 |
o.printf( "|%05d|%05d|%n%n",  i, -i );      // |00123|-0123|

o.printf( "|%X|%x|%n", 0xabc, 0xabc );      // |ABC|abc|
o.printf( "|%04x|%#x|%n%n", 0xabc, 0xabc ); // |0abc|0xabc|

double d = 12345.678;
o.printf( "|%f|%f|%n" ,         d, -d );    // |12345,678000|     |-12345,678000|
o.printf( "|%+f|%+f|%n" ,       d, -d );    // |+12345,678000| |-12345,678000|
o.printf( "|% f|% f|%n" ,       d, -d );    // | 12345,678000| |-12345,678000|
o.printf( "|%.2f|%.2f|%n" ,     d, -d );    // |12345,68| |-12345,68|
o.printf( "|%,.2f|%,.2f|%n" ,   d, -d );    // |12.345,68| |-12.345,68|
o.printf( "|%.2f|%(.2f|%n",     d, -d );    // |12345,68| |(12345,68)|
o.printf( "|%10.2f|%10.2f|%n" , d, -d );    // |  12345,68| | –12345,68|
o.printf( "|%010.2f|%010.2f|%n",d, -d );    // |0012345,68| |-012345,68|

String s = "Monsterbacke";
o.printf( "%n|%s|%n", s );                  // |Monsterbacke|
o.printf( "|%S|%n", s );                    // |MONSTERBACKE|
o.printf( "|%20s|%n", s );                  // |        Monsterbacke|
o.printf( "|%-20s|%n", s );                 // |Monsterbacke        |
o.printf( "|%7s|%n", s );                   // |Monsterbacke|
o.printf( "|%.7s|%n", s );                  // |Monster|
o.printf( "|%20.7s|%n", s );                // |             Monster|

Date t = new Date();
o.printf( "%tT%n", t );                     // 11:01:39
o.printf( "%tD%n", t );                     // 04/18/08
o.printf( "%1$te. %1$tb%n", t );            // 18. Apr


Although @Londo mentioned Scala’s “s” string interpolator, I think Scala’s “f” string interpolator is more relevant to the original question. The example used a few time in other responses could also be written (since Scala 2.10) this way:

scala> val name = "Ivan"
name: String = Ivan
scala> val thing = "Scala"
thing: String = Scala
scala> val formatted = f"Hello $name%s, isn't $thing%s cool?"
formatted: String = Hello Ivan, isn't Scala cool?

The connection to the original question is to be aware that:

  • formatted is defined with a string that is prefixed with the letter “f”. This is the “f” (formatting) string interpolator.
  • The “f” string interpolator uses java.util.Formatter
  • java.lang.String.format uses the same java.util.Formatter

The nice thing about string interpolation is that it lets you see which variable is being substituted directly into the string instead of having to match it with the arguments to the String.format method.


In scala , for string Interpolation we have $ that saves the day and make our life much easy:

For Example: You want to define a function that takes input name and age and says Hello With the name and says its age.
That can be written like this:

def funcStringInterpolationDemo(name:String,age:Int)=s"Hey ! my name is $name and my age is $age"

Hence , When you call this function: like this :


Its output would be :

Hey ! my name is Shivansh and my age is 22

You can write the code to change it in the same line, like if you want to add 10 years to the age !

then function could be :

def funcStringInterpolationDemo(name:String,age:Int)=s"Hey ! my name is $name and my age is ${age+10}"

And now the output would be :

Hey ! my name is Shivansh and my age is 32