Question

Taking a class in OOP Java and since I am completely new to the language and as of yet unaware of the many tools it has to offer I find myself fumbling in the dark for solutions to simple things, I can hardcode these problems but I feel there is far simpler ways to do this with java.util.

Assume I have a list of strings

String[] stringOne = {"a","b","c", "potato"};
String[] stringTwo = {"potato", "13"};

How do I check how many times any given item in stringTwo occurs in stringOne? Or vice versa. I would rather not double loop everytime I have this problem (or make a double-loop method).

Right now I get away with using Collections.frequency() and only looping once but is there a simpler way? Thanks.

Answer 1

Assume I have a list of strings

String[] stringOne = {"a","b","c", "potato"};
String[] stringTwo = {"potato", "13"};

These are “arrays”, not “lists”. Arrays.asList(stringOne) is a list wrapper for the array referenced by stringOne.

How do I check how many times any given item in stringTwo occurs in stringOne?

new HashSet(Arrays.asList(stringOne)).retainAll(Arrays.asList(stringTwo))

will give you the set of elements in both.

A similar approach with a Bag instead of a set will get you counts.

retainAll(Collection coll)

Remove any members of the bag that are not in the given collection, respecting cardinality. That is, if the given collection coll contains n copies of a given object and the bag has m > n copies, then delete m – n copies from the bag. In addition, if e is an object in the bag but !coll.contains(e), then remove e and any of its copies.

The notion of equivalence used in the collection classes is that defined by Object.equals(object) which java.lang.String overrides to compare lexicographically.

If you need the result back in an array, try myBag.toArray(new String[0]) which dumps the contents to a string. The argument is explained in the documentation, but in short, its a type-hint that works around Java’s broken array variance.