How to use java.String.format in Scala?

Practical string formatting for beginners

String formatting in Scala is effortless with the format method which is adapted from Java's String.format. Here's how:

val name = "Alice"
val greeting = "Hello, %s!".format(name) 
// Outputs: "Hello, Alice!"


When referring to indexed arguments, use the $s notation like %1$s. This allows Scala to faithfully adhere to the syntax of Java’s Formatter class.

val result = "%1$s has %2$d apples and %3$d oranges".format("Alice", 3, 5) 
// Outputs: "Alice has 3 apples and 5 oranges"
// Hang on! How did Alice gather so many fruits? 🤔


How to handle common issues in string formatting

Incorrect specifiers or placeholders in the format string can lead to java.util.UnknownFormatConversionException, but fear not for we have a solution:

Ensure you avoid using simple numbered specifiers (%1, %2) which can cause this exception, and stick to the safer %1$s, %2$s notation:

val robust = "%1$s passed the %2$s exam".format("Alice", "Scala")
// Output: "Alice passed the Scala exam"
// Interesting. Alice seems to be a Scala nerd 🤓💻


Mastering type-specific formatting

Go beyond the basics and get your hands dirty with type-specific formatting.

For floating-point numbers, use %.2f to limit two decimal places:

val pi = "Pi value is %.2f".format(math.Pi) 
// Output: "Pi value is 3.14"
// And you thought Pi was just a Raspberry 💻🍰 


For long integers, use %,d to ease readability with comma separation:

val population = "Global population: %,d".format(7800000000L) 
// Output: "Global population: 7,800,000,000"
// That's a lot of people! How many of them do you think are coders? 💭🌎


Exploit advanced formatting for pro-level code

Now let's raise the bar a notch and try some advanced formatting.

For repeating arguments in your format, references make it a breeze:

val echo = "%1$s... %1$s... %1$s...".format("Echo") 
// Output: "Echo... Echo... Echo..."
// Perfect for lost heroes in a cave 😄


Also, you can combine conditional logic with format for dynamic string construction:

val score = 98
val message = "You %s the test".format(if (score > 50) "passed" else "failed") 
// Output: "You passed the test"
// I knew you could do it! 🎉


And for multilines, triple quotes ease the task:

val letter = """Dear %s,
               |
               |We are delighted to inform you that your submission "%s" has been accepted!
               |""".stripMargin.format("Alice", "A Journey With Scala") 
// Now that's a headline, Alice is on fire! 🚀🎉
