Combine multiple Collections into a single logical Collection

Question

Assume  I have a constant number of collections  e g  3 ArrayLists  as members of a class  Now  I want to expose all the elements to other classes so they can simply iterate over all elements  ideally  read only   I m using guava collections and I wonder how I could use guava iterables iterators to generate a logical view on the internal collections without making temporary copies

User · Answer

Plain Java 8 solutions using a Stream   Constant number  Assuming private Collection lt T gt  c  c2  c3   One solution   public Stream lt T gt  stream         return Stream concat Stream concat c stream    c2 stream     c3 stream         Another solution   public Stream lt T gt  stream         return Stream of c  c2  c3  flatMap Collection  stream       Variable number  Assuming private Collection lt Collection lt T gt  gt  cs   public Stream lt T gt  stream         return cs stream   flatMap Collection  stream

User · Answer

With Guava  you can use Iterables concat Iterable lt T gt        it creates a live view of all the iterables  concatenated into one  if you change the iterables  the concatenated version also changes   Then wrap the concatenated iterable with Iterables unmodifiableIterable Iterable lt T gt    I hadn t seen the read-only requirement earlier    From the Iterables concat       JavaDocs      Combines multiple iterables into a   single iterable  The returned iterable   has an iterator that traverses the   elements of each iterable in inputs    The input iterators are not polled   until necessary  The returned   iterable s iterator supports remove     when the corresponding input iterator   supports it    While this doesn t explicitly say that this is a live view  the last sentence implies that it is  supporting the Iterator remove   method only if the backing iterator supports it is not possible unless using a live view   Sample Code   final List lt Integer gt  first    Lists newArrayList 1  2  3   final List lt Integer gt  second   Lists newArrayList 4  5  6   final List lt Integer gt  third    Lists newArrayList 7  8  9   final Iterable lt Integer gt  all       Iterables unmodifiableIterable          Iterables concat first  second  third    System out println all   third add 9999999   System out println all     Output       1  2  3  4  5  6  7  8  9     1  2  3  4  5  6  7  8  9  9999999      Edit   By Request from Damian  here s a similar method that returns a live Collection View  public final class CollectionsX        static class JoinedCollectionView lt E gt  implements Collection lt E gt             private final Collection lt   extends E gt    items           public JoinedCollectionView final Collection lt   extends E gt    items                this items   items                      Override         public boolean addAll final Collection lt   extends E gt  c                throw new UnsupportedOperationException                        Override         public void clear                 for  final Collection lt   extends E gt  coll   items                    coll clear                                      Override         public boolean contains final Object o                throw new UnsupportedOperationException                        Override         public boolean containsAll final Collection lt   gt  c                throw new UnsupportedOperationException                        Override         public boolean isEmpty                 return  iterator   hasNext                        Override         public Iterator lt E gt  iterator                 return Iterables concat items  iterator                        Override         public boolean remove final Object o                throw new UnsupportedOperationException                        Override         public boolean removeAll final Collection lt   gt  c                throw new UnsupportedOperationException                        Override         public boolean retainAll final Collection lt   gt  c                throw new UnsupportedOperationException                        Override         public int size                 int ct   0              for  final Collection lt   extends E gt  coll   items                    ct    coll size                              return ct                      Override         public Object   toArray                 throw new UnsupportedOperationException                        Override         public  lt T gt  T   toArray T   a                throw new UnsupportedOperationException                        Override         public boolean add E e                throw new UnsupportedOperationException                                     Returns a live aggregated collection view of the collections passed in          lt p gt         All methods except   link Collection size       link Collection clear              link Collection isEmpty    and   link Iterable iterator            throw   link UnsupportedOperationException  in the returned Collection          lt p gt         None of the above methods is thread safe  nor would there be an easy way        of making them               public static  lt T gt  Collection lt T gt  combine          final Collection lt   extends T gt     items            return new JoinedCollectionView lt T gt  items              private CollectionsX

User · Answer

You could create a new List and addAll   of your other Lists to it  Then return an unmodifiable list with Collections unmodifiableList

User · Answer

If you re using at least Java 8  see my other answer   If you re already using Google Guava  see Sean Patrick Floyd s answer   If you re stuck at Java 7 and don t want to include Google Guava  you can write your own  read-only  Iterables concat   using no more than Iterable and Iterator   Constant number  public static  lt E gt  Iterable lt E gt  concat final Iterable lt   extends E gt  iterable1                                       final Iterable lt   extends E gt  iterable2        return new Iterable lt E gt               Override         public Iterator lt E gt  iterator                 return new Iterator lt E gt                      final Iterator lt   extends E gt  iterator1   iterable1 iterator                    final Iterator lt   extends E gt  iterator2   iterable2 iterator                      Override                 public boolean hasNext                         return iterator1 hasNext      iterator2 hasNext                                        Override                 public E next                         return iterator1 hasNext     iterator1 next     iterator2 next                                                          Variable number   SafeVarargs public static  lt E gt  Iterable lt E gt  concat final Iterable lt   extends E gt     iterables        return concat Arrays asList iterables       public static  lt E gt  Iterable lt E gt  concat final Iterable lt Iterable lt   extends E gt  gt  iterables        return new Iterable lt E gt              final Iterator lt Iterable lt   extends E gt  gt  iterablesIterator   iterables iterator              Override         public Iterator lt E gt  iterator                 return  iterablesIterator hasNext     Collections emptyIterator                                                     new Iterator lt E gt                      Iterator lt   extends E gt  iterableIterator   nextIterator                      Override                 public boolean hasNext                         return iterableIterator hasNext                                        Override                 public E next                         final E next   iterableIterator next                        findNext                        return next                                     Iterator lt   extends E gt  nextIterator                         return iterablesIterator next   iterator                                       Iterator lt E gt  findNext                         while   iterableIterator hasNext                              if   iterablesIterator hasNext                                  break                                                    iterableIterator   nextIterator                                              return this                                  findNext

User · Answer

Here is my solution for that   EDIT - changed code a little bit  public static  lt E gt  Iterable lt E gt  concat final Iterable lt   extends E gt  list1  Iterable lt   extends E gt  list2        return new Iterable lt E gt                  public Iterator lt E gt  iterator                         return new Iterator lt E gt                                  protected Iterator lt   extends E gt  listIterator   list1 iterator                    protected Boolean checkedHasNext                  protected E nextValue                  private boolean startTheSecond                   public void theNext                                         if  listIterator hasNext                                                  checkedHasNext   true                          nextValue   listIterator next                                              else if  startTheSecond                          checkedHasNext   false                      else                                               startTheSecond   true                          listIterator   list2 iterator                            theNext                                                             public boolean hasNext                                         if  checkedHasNext    null                          theNext                        return checkedHasNext                                     public E next                                         if   hasNext                            throw new NoSuchElementException                        checkedHasNext   null                      return nextValue                                      public void remove                                         listIterator remove

[java] Combine multiple Collections into a single logical Collection?

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