scala vs java, performance and memory?

Scala makes it very easy to use enormous amounts of memory without realizing it. This is usually very powerful, but occasionally can be annoying. For example, suppose you have an array of strings (called array), and a map from those strings to files (called mapping). Suppose you want to get all files that are in the map and come from strings of length greater than two. In Java, you might

int n = 0;
for (String s: array) {
  if (s.length > 2 && mapping.containsKey(s)) n++;
}
String[] bigEnough = new String[n];
n = 0;
for (String s: array) {
  if (s.length <= 2) continue;
  bigEnough[n++] = mapping.get(s);
}

Whew! Hard work. In Scala, the most compact way to do the same thing is:

val bigEnough = array.filter(_.length > 2).flatMap(mapping.get)

Easy! But, unless you're fairly familiar with how the collections work, what you might not realize is that this way of doing this created an extra intermediate array (with filter), and an extra object for every element of the array (with mapping.get, which returns an option). It also creates two function objects (one for the filter and one for the flatMap), though that is rarely a major issue since function objects are small.

So basically, the memory usage is, at a primitive level, the same. But Scala's libraries have many powerful methods that let you create enormous numbers of (usually short-lived) objects very easily. The garbage collector is usually pretty good with that kind of garbage, but if you go in completely oblivious to what memory is being used, you'll probably run into trouble sooner in Scala than Java.

Note that the Computer Languages Benchmark Game Scala code is written in a rather Java-like style in order to get Java-like performance, and thus has Java-like memory usage. You can do this in Scala: if you write your code to look like high-performance Java code, it will be high-performance Scala code. (You may be able to write it in a more idiomatic Scala style and still get good performance, but it depends on the specifics.)

I should add that per amount of time spent programming, my Scala code is usually faster than my Java code since in Scala I can get the tedious not-performance-critical parts done with less effort, and spend more of my attention optimizing the algorithms and code for the performance-critical parts.

Write your Scala like Java, and you can expect almost identical bytecode to be emitted - with almost identical metrics.

Write it more "idiomatically", with immutable objects and higher order functions, and it'll be a bit slower and a bit larger. The one exception to this rule-of-thumb is when using generic objects in which the type params use the @specialised annotation, this'll create even larger bytecode that can outpace Java's performance by avoiding boxing/unboxing.

Also worth mentioning is the fact that more memory / less speed is an inevitable trade-off when writing code that can be run in parallel. Idiomatic Scala code is far more declarative in nature than typical Java code, and is often a mere 4 characters (.par) away from being fully parallel.

So if

• Scala code takes 1.25x longer than Java code in a single thread
• It can be easily split across 4 cores (now common even in laptops)
• for a parallel run time of (1.24 / 4 =) 0.3125x the original Java

Would you then say that the Scala code is now comparatively 25% slower, or 3x faster?

The correct answer depends on exactly how you define "performance" :)

I'm a new user, so I'm not able to add a comment to Rex Kerr's answer above (allowing new users to "answer" but not "comment" is a very odd rule btw).

I signed up simply to respond to the "phew, Java is so verbose and such hard work" insinuation of Rex's popular answer above. While you can of course write more concise Scala code, the Java example given is clearly bloated. Most Java developers would code something like this:

List<String> bigEnough = new ArrayList<String>();
for(String s : array) {
  if(s.length() > 2 && mapping.get(s) != null) {
    bigEnough.add(mapping.get(s));
  }
}

And of course, if we are going to pretend that Eclipse doesn't do most of the actual typing for you and that every character saved really makes you a better programmer, then you could code this:

List b=new ArrayList();
for(String s:array)
  if(s.length()>2 && mapping.get(s) != null) b.add(mapping.get(s));

Now not only did I save the time it took me to type full variable names and curly braces (freeing me to spend 5 more seconds to think deep algorithmic thoughts), but I can also enter my code in obfuscation contests and potentially earn extra cash for the holidays.