Initializing a Set with an Iterable

Question

I want to initialize a Set Implementation (HashSet) in Java with an Iterable. However, the constructor of HashSet doesn't accept Iterables, but only Collections type objects.

Is there a way to convert from Iterable to some subtype of Collections.

Tunisia

I think the question itself is different enough (since one is an ordered collection) or the other question is not general enough to support both ordered and unordered. – Winger

Well, the linkled duplicate answers the literal question asked by providing "a way to convert from Iterable to some subtype of Collection". However, I agree that this (constructing a temporary ArrayList) is not necessarily the best way to intialize a HashSet from an Iterable. – Histone

If you have Spring Data on the class path you may use Streamable.of(iterable).toSet(). If Spring Data happens to be the source of the Iterable you have even more options: https://mcmap.net/q/74121/-easy-way-to-convert-iterable-to-collection – Hautegaronne

Answer 1

HashSet constructor relies on more than what Iterable offers: it wants to know the size of the collection up front in order to optimally construct the underlying HashMap. If you have a true, austere Iterable, which doesn't know its size, then you'll have to realize the Iterable up front by turning it into a regular Collection in any of a number of obvious ways.

If, on the other hand, you have a richer object that already knows its size, then it would pay to create a minimalist adapter class that wraps your Iterable into a collection, implementing just size in addition to forwarding the call to iterator.

public class IterableCollection<T> implements Collection<T>
{
   private final Iterable<T> iterable;

   public IterableCollection(Iterable<T> it) { this.iterable = it; }

   @Override public Iterator<T> iterator() { return iterable.iterator(); }

   @Override public int size() { return ... custom code to determine size ... }

   @Override .... all others ... { throw new UnsupportedOperationException(); }
}

Castanon

Good explanation. I think I get your point and intuition. – Tunisia

Answer 2

You can use Guava.

Set<T> set = Sets.newHashSet(iterable);

or to make it read like a sentence static import,

import static com.google.common.collect.Sets.*;

Set<T> set = newHashSet(iterable);

Fawcette

Answer 3

Sure, it's shown in this answer. Basically, iterate over the iterable and copy its contents in a collection:

public static <T> List<T> copyIterable(Iterable<T> iterable) {
    Iterator<T> iter = iterable.iterator();
    List<T> copy = new ArrayList<T>();
    while (iter.hasNext())
        copy.add(iter.next());
    return copy;
}

Use it as follows, the resulting List object can be passed as a parameter to the HashSet constructor.

Iterable<Integer> list = Arrays.asList(1, 2, 3);
List<Integer> copy = copyIterable(list);
Set<Integer> aSet = new HashSet<Integer>(copy);

EDIT

I've been mistaken all along. Iterable is a superinterface of Collection, so a simple (but unsafe) cast will do the trick, as long as the Iterable was a Collection to begin with.

Iterable<Integer> list = Arrays.asList(1, 2, 3);
Set<Integer> aSet = new HashSet<Integer>((Collection)list); // it works!

Slapbang

I was avoiding to explicitly iterating over Iterable or consuming an Iterator myself. I'm looking for a mechanism so that it is acceptable to the constructor of the HashSet or any Set Implementation. (BTW: I didn't down vote you). But I would definitely upvote you if you can show me a workaround. :-) – Tunisia

I'm getting this on my Netbeans. no suitable constructor found for HashSet(Iterator<Integer>) – Tunisia

@WChargin fixed it, thanks. please reconsider the downvote. – Uralic

@Tunisia pass the list, not the iterator – Uralic

@Tunisia I just realized how simple is the solution to this problem. Please take a look at the update, above – Uralic

A Collection is an Iterable, but an Iterable is not necessarily a Collection. So, your cast is unsafe (as you stated) and could fail at runtime if the concrete Iterable was not also a Collection. – Swedenborgianism

casting an Iterable to a Collection is a bad idea – Soluble

@Swedenborgianism I was just about to say that. I don't have a list. Infact, I have a custom DT, which exposes Iterator() and Iterable () functionality. – Tunisia

@Swedenborgianism I added a clarification stating this: "as long as the Iterable was a Collection to begin with" – Uralic

@Tunisia then, absolutely, you have no option but to copy your custom iterable DT to a proper Collection. You should have mentioned this from the beginning! – Uralic

Answer 4

The Iterable interface allows the "foreach" syntax to work, so the cleanest way is likely:

public <T> Set<T> toSet(Iterable<T> collection) {
    HashSet<T> set = new HashSet<T>();
    for (T item: collection)
        set.add(item);
    return set;
}

Pyrophotometer

Answer 5

Just add each one.

public static <T> Set<T> setFromIterable(Iterable<T> i) {
    HashSet<T> set = new HashSet<T>();
    Iterator<T> it = i.iterator();
    while (it.hasNext()) {
        set.add(it.next());
    }
    return set;
}

Iterable<Integer> someIterable = ...;
Set<Integer> someSet = setFromIterable(someIterable);

Note that you don't use the constructor new HashSet<Integer>(someIterator), because that doesn't exist. Just call the static method.

Hypsometry

I think you are confusing Iterable and Iterator. – Rootless

Yes, you're right. But even iterator() doesn't work. The object I have also gives back an Iterator(), but even that is not acceptable to the constructor. – Tunisia

Kirk: you're right; thanks. @Vaid please see my edit. – Hypsometry

Answer 6

I use this one-liner (with Java 8+), which only relies on java.util.stream:

StreamSupport.stream(myIterable.spliterator(), false).collect(Collectors.toSet());

// or with static imports:
stream(myIterable.spliterator(), false).collect(toSet());

Rebhun

Answer 7

Putting somewhat a repeated answer for conciseness. Below worked for me for converting the Iterable of String type to a Set(Java8).

Iterable<String> stringIterable = Arrays.asList("str1", "str2", "str3");
Set<String> stringHashSet = new HashSet<>((Collection<? extends String>) stringIterable);

Superjacent

This only works for a Collection, and if you already know that you have something more specific then you should type it correctly. – Rebhun