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CollectionHelpersExplained  
Making it easier to extend collections tools yourself, explained.
Updated Jan 29, 2013 by wasserman.louis

Introduction

Sometimes you need to write your own collection extensions. Perhaps you want to add special behavior when elements are added to a list, or you want to write an Iterable that's actually backed by a database query. Guava provides a number of utilities to make these tasks easier for you, and for us. (We are, after all, in the business of extending the collections framework ourselves.)

Forwarding Decorators

For all the various collection interfaces, Guava provides Forwarding abstract classes to simplify using the decorator pattern.

The Forwarding classes define one abstract method, delegate(), which you should override to return the decorated object. Each of the other methods delegate directly to the delegate: so, for example, ForwardingList.get(int) is simply implemented as delegate().get(int).

By subclassing ForwardingXXX and implementing the delegate() method, you can override only selected methods in the targeted class, adding decorated functionality without having to delegate every method yourself.

Additionally, many methods have a standardMethod implementation which you can use to recover expected behavior, providing some of the same benefits as e.g. extending AbstractList or the other skeleton classes in the JDK.

Let's do an example. Suppose you wanted to decorate a List so that it logged all elements added to it. Of course, we want to log elements no matter which method is used to add them -- add(int, E), add(E), or addAll(Collection) -- so we have to override all of these methods.

class AddLoggingList<E> extends ForwardingList<E> {
  final List<E> delegate; // backing list
  @Override protected List<E> delegate() {
    return delegate;
  }
  @Override public void add(int index, E elem) {
    log(index, elem);
    super.add(index, elem);
  }
  @Override public boolean add(E elem) {
    return standardAdd(elem); // implements in terms of add(int, E)
  }
  @Override public boolean addAll(Collection<? extends E> c) {
    return standardAddAll(c); // implements in terms of add
  }
}

Remember, by default, all methods forward directly to the delegate, so overriding ForwardingMap.put will not change the behavior of ForwardingMap.putAll. Be careful to override every method whose behavior must be changed, and make sure that your decorated collection satisfies its contract.

Generally, most methods provided by the abstract collection skeletons like AbstractList are also provided as standard implementations in the Forwarding decorators.

Interfaces that provide special views sometimes provide Standard implementations of those views. For example, ForwardingMap provides StandardKeySet, StandardValues, and StandardEntrySet classes, each of which delegate their methods to the decorated map whenever possible, or otherwise, they leave methods that can't be delegated as abstract.

Interface Forwarding Decorator
Collection ForwardingCollection
List ForwardingList
Set ForwardingSet
SortedSet ForwardingSortedSet
Map ForwardingMap
SortedMap ForwardingSortedMap
ConcurrentMap ForwardingConcurrentMap
Map.Entry ForwardingMapEntry
Queue ForwardingQueue
Iterator ForwardingIterator
ListIterator ForwardingListIterator
Multiset ForwardingMultiset
Multimap ForwardingMultimap
ListMultimap ForwardingListMultimap
SetMultimap ForwardingSetMultimap

PeekingIterator

Sometimes, the normal Iterator interface isn't enough.

Iterators supports the method Iterators.peekingIterator(Iterator), which wraps an Iterator and returns a PeekingIterator, a subtype of Iterator that lets you peek() at the element that will be returned by the next call to next().

Note: the PeekingIterator returned by Iterators.peekingIterator does not support remove() calls after a peek().

Let's do an example: copying a List while eliminating consecutive duplicate elements.

List<E> result = Lists.newArrayList();
PeekingIterator<E> iter = Iterators.peekingIterator(source.iterator());
while (iter.hasNext()) {
  E current = iter.next();
  while (iter.hasNext() && iter.peek().equals(current)) {
    // skip this duplicate element
    iter.next();
  }
  result.add(current);
}

The traditional way to do this involves keeping track of the previous element, and falling back under certain conditions, but that's a tricky and bug-prone business. PeekingIterator is comparatively straightforward to understand and use.

AbstractIterator

Implementing your own Iterator? AbstractIterator can make your life easier.

It's easiest to explain with an example. Let's say we wanted to wrap an iterator so as to skip null values.

public static Iterator<String> skipNulls(final Iterator<String> in) {
  return new AbstractIterator<String>() {
    protected String computeNext() {
      while (in.hasNext()) {
        String s = in.next();
        if (s != null) {
          return s;
        }
      }
      return endOfData();
    }
  };
}

You implement one method, computeNext(), that just computes the next value. When the sequence is done, just return endOfData() to mark the end of the iteration.

Note: AbstractIterator extends UnmodifiableIterator, which forbids the implementation of remove(). If you need an iterator that supports remove(), you should not extend AbstractIterator.

AbstractSequentialIterator

Some iterators are more easily expressed in other ways. AbstractSequentialIterator provides another way of expressing an iteration.

   Iterator<Integer> powersOfTwo = new AbstractSequentialIterator<Integer>(1) { // note the initial value!
     protected Integer computeNext(Integer previous) {
       return (previous == 1 << 30) ? null : previous * 2;
     }
   };

Here, we implement the method computeNext(T), which accepts the previous value as an argument.

Note that you must additionally pass an initial value, or null if the iterator should end immediately. Note that computeNext assumes that a null value implies the end of iteration -- AbstractSequentialIterator cannot be used to implement an iterator which may return null.

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