Question

In Java, what’s the most efficient way to return the common elements from two String Arrays? I can do it with a pair of for loops, but that doesn’t seem to be very efficient. The best I could come up with was converting to a List and then applying retainAll, based on my review of a similar SO question:

List<String> compareList = Arrays.asList(strArr1);
List<String> baseList = Arrays.asList(strArr2);
baseList.retainAll(compareList);

Answer 1

EDITED:

This is a one-liner:

compareList.retainAll(new HashSet<String>(baseList));

The retainAll impl (in AbstractCollection) iterates over this, and uses contains() on the argument. Turning the argument into a HashSet will result in fast lookups, so the loop within the retainAll will execute as quickly as possible.

Also, the name baseList hints at it being a constant, so you will get a significant performance improvement if you cache this:

static final Set<String> BASE = Collections.unmodifiableSet(new HashSet<String>(Arrays.asList("one", "two", "three", "etc")));

static void retainCommonWithBase(Collection<String> strings) {
    strings.retainAll(BASE);
}

If you want to preserve the original List, do this:

static List<String> retainCommonWithBase(List<String> strings) {
   List<String> result = new ArrayList<String>(strings);
   result.retainAll(BASE);
   return result;
}