String interpolation vs concatenation

Question

Is string interpolation more performant than concatenation to build a string in scala? For example:

val myVal = "def"
val test = s"abs${myVal}ghi"

vs

val test = "abc" + myVal + "ghi"

I am asking because I need to write the toString method for a class which has a lot of parameters and the interpolated string therefore gets really long and confusing and I would like to break it up into multiple lines but fear that it will perform badly if I concatenate.

Answer 1

I made a simple example:

class Employee(name: String, age: Int) {
  private var s = 0.0
  def salary = s
  def salary_=(s: Double) = this.s = s
  def toStringConcat(): String = {
    "Name: " + name + ", age: " + age + ", salary: " + salary
  }
  def toStringInterpol(): String = {
    s"Name: $name, age: $age, salary: $salary"
  }
}

object Program {
  val empl = new Employee("John", 30)
  empl.salary = 10.50
  val times = 10000000;

  def main(args: Array[String]): Unit = {
    // warming-up
    val resultConcat = empl.toStringConcat
    val resultInterpol = empl.toStringInterpol
    println("Concat -> " + resultConcat)
    println("Interpol -> " + resultInterpol)

    val secondsConcat0 = run(empl.toStringConcat)
    val secondsInterpol0 = run(empl.toStringInterpol)
    val secondsConcat1 = run(empl.toStringConcat)
    val secondsInterpol1 = run(empl.toStringInterpol)

    println("Concat-0: " + secondsConcat0 + "s")
    println("Concat-1: " + secondsConcat1 + "s")
    println("Interpol-0: " + secondsInterpol0 + "s")
    println("Interpol-1: " + secondsInterpol1 + "s")
  }

  def run(call: () => String): Double = {
    val time0 = System.nanoTime()
    var result = ""
    for (i <- 0 until times) {
      result = call()
    }
    val time1 = System.nanoTime()

    val elapsedTime = time1 - time0;
    val seconds = elapsedTime / 1000000000.0;
    seconds
  }
}

Results were the following:

Concat -> Name: John, age: 30, salary: 10.5
Interpol -> Name: John, age: 30, salary: 10.5
Concat-0: 2.831298161s
Concat-1: 2.725815448s
Interpol-0: 3.846891864s
Interpol-1: 3.753401004s

Interpolation case was slower. The reason can be found in generated code:

  public String toStringConcat()
  {
    return new StringBuilder().append("Name: ").append(this.name).append(", age: ").append(BoxesRunTime.boxToInteger(this.age)).append(", salary: ").append(BoxesRunTime.boxToDouble(salary())).toString();
  }

  public String toStringInterpol()
  {
    return new StringContext(Predef..MODULE$.wrapRefArray((Object[])new String[] { "Name: ", ", age: ", ", salary: ", "" })).s(Predef..MODULE$.genericWrapArray(new Object[] { this.name, BoxesRunTime.boxToInteger(this.age), BoxesRunTime.boxToDouble(salary()) }));
  }

I used Scala 2.10.3 and IntelliJ IDEA 13.