A scala implementation of the Java trim method

I'm a Scala beginner and looking at the trim() method of the Java API. I noticed side effects, so I attempted to implement a functional version in Scala.

Here is the Java version:

public String trim() {
int len = count;
int st = 0;
int off = offset;      /* avoid getfield opcode */
char[] val = value;    /* avoid getfield opcode */

while ((st  0) || (len < count)) ? substring(st, len) : this;
}

My Scala version iterates over a string, and when it encounters a space char, the tail of the string is returned. The entire string is then reversed, so it can access the trailing whitespace. This new string is iterated over until a space char is encountered and the tail of this string is returned. This new string is then reversed again to maintain the original string order:

def trim(stringToTrim : String) : String = {
    def removePreString[String](list : List[String]) : java.lang.String = list match {

        case head :: tail => {
            if(head.toString.equals(" ")){
                removePreString(tail)
            }
            else {
                head.toString + tail.mkString
            }
        }
    }//end removePreString

    val preString =    removePreString(stringToTrim.toList)
    val reveresedString = preString.toList.reverse
    val reveresedPostString = removePreString(reveresedString)
  reveresedPostString.reverse

}//end trim

How can this code be improved? Does it seem wasteful reversing the string twice?

No good idea about the concern of reversing the list twice. But you may

-
Let you inner function return a List instead of a string thus having less conversions from list to string and string to list.

-
Change your match to the following pattern (not tested):

case " " :: tail => removePreString(tail)
case _ => _

I just realize you could also use on a list def dropWhile(p: (A) ⇒ Boolean): List[A] which drops longest prefix of elements that satisfy a predicate. You could then chain dropWhile not a space | reverse | dropwhile not a space | reverse...

StackExchange Code Review Q#20452, answer score: 3