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Scala Comments

A comment is a note inside code that the computer ignores when the program runs. Comments explain code to the people who read it.

A single-line comment does not print

The // note sits above the printing line. Only the println line runs.

@main def main(): Unit =
  // This note explains the line below
  println("Comments do not print")

A block comment can cover several lines

The /* marker starts the note and */ ends it. Lines inside the note are skipped.

@main def main(): Unit =
  /* This note covers
     two lines of text */
  println("The block comment is skipped")

Commented-out code does not run

Putting // in front of a println turns it into a note. That line prints nothing.

@main def main(): Unit =
  println("This line runs")
  // println("This line does not run")

Example

@main def main(): Unit =
  // Greeting program
  println("Hello")
  /* The next line
     prints the closing word */
  println("Done") // End of the program

Key points

  • Single-line comment: two slashes // start a note that ends at the end of the line.
  • Block comment: /* starts a note and */ ends it, and the note can cover several lines.
  • Ignored: the computer skips comments, so comments never print.
  • Placement: a comment can sit on its own line or after code on the same line.
  • Purpose: comments explain why code exists in plain words for readers.

In this example, the // line and the /* */ block are notes that never print. The two println lines run in order and print Hello and Done.

Note

turning a line of code into a comment stops that code from running.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Scastie

Exercise

Which line is a single-line comment in Scala?