Adding elements to global list object in scala

Question

Allright guys, pardon me if this question has already been asked. Yet another Scala newbie question. So for example if I want to have a global List object which can be used as a place holder container, in Java I can easily do;

//share list object
private List<String> strList = new ArrayList<>();
void add(String el)
{
 strList.add(e);
}

static void main(String[] args) { 
{
  add("36 Chambers");
  out.println(strList.get(0));//assume only element
}

Similarly, if I simulate the same syntax in Scala - I end up with java.lang.IndexOutOfBoundsException: 0. How can one achieve similar with simple Scala?

  private var strList: List[String] = Nil
  def add(el: String) {
    strList :+ el
  }

  def main(args: Array[String]) {   
    add("36 Chambers")
    println(s(0)) //assume only element
  }

Won't bother writing an answer since you've already accepted one, but List appears to be totally the wrong collection for your purposes - you should use a Vector, which has both fast append and prepend, and is the standard implementation of an indexed collection for fast lookup. You can write strVector = strVector :+ el, or the short version strVector :+= el. — Luigi Plinge
– Luigi Plinge, Commented May 15, 2013 at 0:41
@LuigiPlinge please add an answer - this helps others to easily dig and find relevant answers like yours. I will give you a vote up. — Bitmap
– Bitmap, Commented May 15, 2013 at 8:32

flavian · Accepted Answer · 2013-05-14 23:55:07Z

1

To strictly answer your question:

private var strList: List[String] = Nil
def add(el: String) {
       strList = el :: strList;
}

You are using the wrong operator. You need to use ::. And you need to update inline because a List is of immutable length.

I'm switching the el and strList because :: is right associative, which means the method comes from the object to the right of the :: operator.

Also, the :: operator will perform a prepend, which means your elements will be in reverse order.

You can either call reverse at the end(before usage etc) or if you want something semantically similar to a Java ArrayList perhaps consider a Scala ListBuffer, a mutable collection to which you can append.

import scala.collection.mutable.ListBuffer;
private var strList: ListBuffer[String] = Nil
def add(el: String) {
       strList += el;
}

edited May 14, 2013 at 23:55

answered May 14, 2013 at 23:46

flavian

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