CollectionUtilitiesExplained

Collection Utilities

Any programmer with experience with the JDK Collections Framework knows and loves the utilities available in java.util.Collections. Guava provides many more utilities along these lines: static methods applicable to all collections. These are among the most popular and mature parts of Guava.

Methods corresponding to a particular interface are grouped in a relatively intuitive manner:

Interface	JDK or Guava?	Corresponding Guava utility class
Collection	JDK	Collections2
List	JDK	Lists
Set	JDK	Sets
SortedSet	JDK	Sets
Map	JDK	Maps
SortedMap	JDK	Maps
Queue	JDK	Queues
Multiset	Guava	Multisets
Multimap	Guava	Multimaps
BiMap	Guava	Maps
Table	Guava	Tables

Looking for transform, filter, and the like? That stuff is in our functional programming article, under functional idioms.

Static constructors

Before JDK 7, constructing new generic collections requires unpleasant code duplication:

List<TypeThatsTooLongForItsOwnGood> list = new ArrayList<TypeThatsTooLongForItsOwnGood>();

I think we can all agree that this is unpleasant. Guava provides static methods that use generics to infer the type on the right side:

List<TypeThatsTooLongForItsOwnGood> list = Lists.newArrayList();
Map<KeyType, LongishValueType> map = Maps.newLinkedHashMap();

To be sure, the diamond operator in JDK 7 makes this less of a hassle:

List<TypeThatsTooLongForItsOwnGood> list = new ArrayList<>();

But Guava goes further than this. With the factory method pattern, we can initialize collections with their starting elements very conveniently.

Set<Type> copySet = Sets.newHashSet(elements);
List<String> theseElements = Lists.newArrayList("alpha", "beta", "gamma");

Additionally, with the ability to name factory methods (Effective Java item 1), we can improve the readability of initializing collections to sizes:

List<Type> exactly100 = Lists.newArrayListWithCapacity(100);
List<Type> approx100 = Lists.newArrayListWithExpectedSize(100);
Set<Type> approx100Set = Sets.newHashSetWithExpectedSize(100);

The precise static factory methods provided are listed with their corresponding utility classes below.

Note: New collection types introduced by Guava don't expose raw constructors, or have initializers in the utility classes. Instead, they expose static factory methods directly, for example:

Multiset<String> multiset = HashMultiset.create();

Iterables

Whenever possible, Guava prefers to provide utilities accepting an Iterable rather than a Collection. Here at Google, it's not out of the ordinary to encounter a "collection" that isn't actually stored in main memory, but is being gathered from a database, or from another data center, and can't support operations like size() without actually grabbing all of the elements.

As a result, many of the operations you might expect to see supported for all collections can be found in Iterables. Additionally, most Iterables methods have a corresponding version in Iterators that accepts the raw iterator.

The overwhelming majority of operations in the Iterables class are lazy: they only advance the backing iteration when absolutely necessary. Methods that themselves return Iterables return lazily computed views, rather than explicitly constructing a collection in memory.

As of Guava 12, Iterables is supplemented by the FluentIterable class, which wraps an Iterable and provides a "fluent" syntax for many of these operations.

The following is a selection of the most commonly used utilities, although many of the more "functional" methods in Iterables are discussed in Guava functional idioms.

General

Method	Description	See Also
concat(Iterable<Iterable>)	Returns a lazy view of the concatenation of several iterables.	concat(Iterable...)
frequency(Iterable, Object)	Returns the number of occurrences of the object.	Compare Collections.frequency(Collection, Object); see Multiset
partition(Iterable, int)	Returns an unmodifiable view of the iterable partitioned into chunks of the specified size.	Lists.partition(List, int), paddedPartition(Iterable, int)
getFirst(Iterable, T default)	Returns the first element of the iterable, or the default value if empty.	Compare Iterable.iterator().next(), FluentIterable.first()
getLast(Iterable)	Returns the last element of the iterable, or fails fast with a NoSuchElementException if it's empty.	getLast(Iterable, T default), FluentIterable.last()
elementsEqual(Iterable, Iterable)	Returns true if the iterables have the same elements in the same order.	Compare List.equals(Object)
unmodifiableIterable(Iterable)	Returns an unmodifiable view of the iterable.	Compare Collections.unmodifiableCollection(Collection)
limit(Iterable, int)	Returns an Iterable returning at most the specified number of elements.	FluentIterable.limit(int)
getOnlyElement(Iterable)	Returns the only element in Iterable. Fails fast if the iterable is empty or has multiple elements.	getOnlyElement(Iterable, T default)

Iterable<Integer> concatenated = Iterables.concat(
  Ints.asList(1, 2, 3),
  Ints.asList(4, 5, 6));
// concatenated has elements 1, 2, 3, 4, 5, 6

String lastAdded = Iterables.getLast(myLinkedHashSet);

String theElement = Iterables.getOnlyElement(thisSetIsDefinitelyASingleton);
  // if this set isn't a singleton, something is wrong!

Collection-Like

Typically, collections support these operations naturally on other collections, but not on iterables.

Each of these operations delegates to the corresponding Collection interface method when the input is actually a Collection. For example, if Iterables.size is passed a Collection, it will call the Collection.size method instead of walking through the iterator.

Method	Analogous Collection method	FluentIterable equivalent
addAll(Collection addTo, Iterable toAdd)	Collection.addAll(Collection)
contains(Iterable, Object)	Collection.contains(Object)	FluentIterable.contains(Object)
removeAll(Iterable removeFrom, Collection toRemove)	Collection.removeAll(Collection)
retainAll(Iterable removeFrom, Collection toRetain)	Collection.retainAll(Collection)
size(Iterable)	Collection.size()	FluentIterable.size()
toArray(Iterable, Class)	Collection.toArray(T[])	FluentIterable.toArray(Class)
isEmpty(Iterable)	Collection.isEmpty()	FluentIterable.isEmpty()
get(Iterable, int)	List.get(int)	FluentIterable.get(int)
toString(Iterable)	Collection.toString()	FluentIterable.toString()

FluentIterable

Besides the methods covered above and in the functional idioms [article] functional, FluentIterable has a few convenient methods for copying into an immutable collection:

Result Type	Method
ImmutableList	toImmutableList()
ImmutableSet	toImmutableSet()
ImmutableSortedSet	toImmutableSortedSet(Comparator)

Lists

In addition to static constructor methods and functional programming methods, Lists provides a number of valuable utility methods on List objects.

Method	Description
partition(List, int)	Returns a view of the underlying list, partitioned into chunks of the specified size.
reverse(List)	Returns a reversed view of the specified list. Note: if the list is immutable, consider ImmutableList.reverse() instead.

List<Integer> countUp = Ints.asList(1, 2, 3, 4, 5);
List<Integer> countDown = Lists.reverse(theList); // {5, 4, 3, 2, 1}

List<List<Integer>> parts = Lists.partition(countUp, 2); // {{1, 2}, {3, 4}, {5}}

Static Factories

Lists provides the following static factory methods:

Implementation	Factories
ArrayList	basic, with elements, from Iterable, with exact capacity, with expected size, from Iterator
LinkedList	basic, from Iterable

Comparators

Finding the minimum or maximum of some elements

A seemingly simple task (finding the min or max of some elements) is complicated by the desire to minimize allocations, boxing, and APIs living in a variety of locations. The table below summarizes the best practices for this task.

Only the max() solution is shown in the table below, but the same advice applies for finding a min().

What you're comparing	Exactly 2 instances	More than 2 instances
unboxed numeric primitives (e.g., long, int, double, or float)	Math.max(a, b)	Longs.max(a, b, c), Ints.max(a, b, c), etc.
Comparable instances (e.g., Duration, String, Long, etc.)	Comparators.max(a, b)	Collections.max(asList(a, b, c))
using a custom Comparator (e.g., MyType with myComparator)	Comparators.max(a, b, cmp)	Collections.max(asList(a, b, c), cmp)

Note: We recommend static importing all of the methods involved in these solutions to simplify your code (e.g., prefer max(asList(a, b, c)) over Collections.max(Arrays.asList(a, b, c))).

Sets

The Sets utility class includes a number of spicy methods.

Set-Theoretic Operations

We provide a number of standard set-theoretic operations, implemented as views over the argument sets. These return a SetView, which can be used:

• as a Set directly, since it implements the Set interface
• by copying it into another mutable collection with copyInto(Set)
• by making an immutable copy with immutableCopy()

Method
union(Set, Set)
intersection(Set, Set)
difference(Set, Set)
symmetricDifference(Set, Set)

For example:

Set<String> wordsWithPrimeLength = ImmutableSet.of("one", "two", "three", "six", "seven", "eight");
Set<String> primes = ImmutableSet.of("two", "three", "five", "seven");

SetView<String> intersection = Sets.intersection(primes, wordsWithPrimeLength); // contains "two", "three", "seven"
// I can use intersection as a Set directly, but copying it can be more efficient if I use it a lot.
return intersection.immutableCopy();

Other Set Utilities

Method	Description	See Also
cartesianProduct(List<Set>)	Returns every possible list that can be obtained by choosing one element from each set.	cartesianProduct(Set...)
powerSet(Set)	Returns the set of subsets of the specified set.

Set<String> animals = ImmutableSet.of("gerbil", "hamster");
Set<String> fruits = ImmutableSet.of("apple", "orange", "banana");

Set<List<String>> product = Sets.cartesianProduct(animals, fruits);
// {{"gerbil", "apple"}, {"gerbil", "orange"}, {"gerbil", "banana"},
//  {"hamster", "apple"}, {"hamster", "orange"}, {"hamster", "banana"}}

Set<Set<String>> animalSets = Sets.powerSet(animals);
// {{}, {"gerbil"}, {"hamster"}, {"gerbil", "hamster"}}

Static Factories

Sets provides the following static factory methods:

Implementation	Factories
HashSet	basic, with elements, from Iterable, with expected size, from Iterator
LinkedHashSet	basic, from Iterable, with expected size
TreeSet	basic, with Comparator, from Iterable

Maps

Maps has a number of cool utilities that deserve individual explanation.

uniqueIndex

Maps.uniqueIndex(Iterable, Function) addresses the common case of having a bunch of objects that each have some unique attribute, and wanting to be able to look up those objects based on that attribute.

Let's say we have a bunch of strings that we know have unique lengths, and we want to be able to look up the string with some particular length.

ImmutableMap<Integer, String> stringsByIndex = Maps.uniqueIndex(strings, new Function<String, Integer> () {
    public Integer apply(String string) {
      return string.length();
    }
  });

If indices are not unique, see Multimaps.index below.

difference

Maps.difference(Map, Map) allows you to compare all the differences between two maps. It returns a MapDifference object, which breaks down the Venn diagram into:

Method	Description
entriesInCommon()	The entries which are in both maps, with both matching keys and values.
entriesDiffering()	The entries with the same keys, but differing values. The values in this map are of type MapDifference.ValueDifference, which lets you look at the left and right values.
entriesOnlyOnLeft()	Returns the entries whose keys are in the left but not in the right map.
entriesOnlyOnRight()	Returns the entries whose keys are in the right but not in the left map.

Map<String, Integer> left = ImmutableMap.of("a", 1, "b", 2, "c", 3);
Map<String, Integer> right = ImmutableMap.of("b", 2, "c", 4, "d", 5);
MapDifference<String, Integer> diff = Maps.difference(left, right);

diff.entriesInCommon(); // {"b" => 2}
diff.entriesDiffering(); // {"c" => (3, 4)}
diff.entriesOnlyOnLeft(); // {"a" => 1}
diff.entriesOnlyOnRight(); // {"d" => 5}

BiMap utilities

The Guava utilities on BiMap live in the Maps class, since a BiMap is also a Map.

BiMap utility	Corresponding Map utility
synchronizedBiMap(BiMap)	Collections.synchronizedMap(Map)
unmodifiableBiMap(BiMap)	Collections.unmodifiableMap(Map)

Static Factories

Maps provides the following static factory methods.

Implementation	Factories
HashMap	basic, from Map, with expected size
LinkedHashMap	basic, from Map
TreeMap	basic, from Comparator, from SortedMap
EnumMap	from Class, from Map
ConcurrentMap	basic
IdentityHashMap	basic

Multisets

Standard Collection operations, such as containsAll, ignore the count of elements in the multiset, and only care about whether elements are in the multiset at all, or not. Multisets provides a number of operations that take into account element multiplicities in multisets.

Method	Explanation	Difference from Collection method
containsOccurrences(Multiset sup, Multiset sub)	Returns true if sub.count(o) <= super.count(o) for all o.	Collection.containsAll ignores counts, and only tests whether elements are contained at all.
removeOccurrences(Multiset removeFrom, Multiset toRemove)	Removes one occurrence in removeFrom for each occurrence of an element in toRemove.	Collection.removeAll removes all occurrences of any element that occurs even once in toRemove.
retainOccurrences(Multiset removeFrom, Multiset toRetain)	Guarantees that removeFrom.count(o) <= toRetain.count(o) for all o.	Collection.retainAll keeps all occurrences of elements that occur even once in toRetain.
intersection(Multiset, Multiset)	Returns a view of the intersection of two multisets; a nondestructive alternative to retainOccurrences.	Has no analogue.

Multiset<String> multiset1 = HashMultiset.create();
multiset1.add("a", 2);

Multiset<String> multiset2 = HashMultiset.create();
multiset2.add("a", 5);

multiset1.containsAll(multiset2); // returns true: all unique elements are contained,
  // even though multiset1.count("a") == 2 < multiset2.count("a") == 5
Multisets.containsOccurrences(multiset1, multiset2); // returns false

Multisets.removeOccurrences(multiset2, multiset1); // multiset2 now contains 3 occurrences of "a"

multiset2.removeAll(multiset1); // removes all occurrences of "a" from multiset2, even though multiset1.count("a") == 2
multiset2.isEmpty(); // returns true

Other utilities in Multisets include:

Method	Description
copyHighestCountFirst(Multiset)	Returns an immutable copy of the multiset that iterates over elements in descending frequency order.
unmodifiableMultiset(Multiset)	Returns an unmodifiable view of the multiset.
unmodifiableSortedMultiset(SortedMultiset)	Returns an unmodifiable view of the sorted multiset.

Multiset<String> multiset = HashMultiset.create();
multiset.add("a", 3);
multiset.add("b", 5);
multiset.add("c", 1);

ImmutableMultiset<String> highestCountFirst = Multisets.copyHighestCountFirst(multiset);

// highestCountFirst, like its entrySet and elementSet, iterates over the elements in order {"b", "a", "c"}

Multimaps

Multimaps provides a number of general utility operations that deserve individual explanation.

index

The cousin to Maps.uniqueIndex, Multimaps.index(Iterable, Function) answers the case when you want to be able to look up all objects with some particular attribute in common, which is not necessarily unique.

Let's say we want to group strings based on their length.

ImmutableSet<String> digits = ImmutableSet.of(
    "zero", "one", "two", "three", "four",
    "five", "six", "seven", "eight", "nine");
Function<String, Integer> lengthFunction = new Function<String, Integer>() {
  public Integer apply(String string) {
    return string.length();
  }
};
ImmutableListMultimap<Integer, String> digitsByLength = Multimaps.index(digits, lengthFunction);
/*
 * digitsByLength maps:
 *  3 => {"one", "two", "six"}
 *  4 => {"zero", "four", "five", "nine"}
 *  5 => {"three", "seven", "eight"}
 */

invertFrom

Since Multimap can map many keys to one value, and one key to many values, it can be useful to invert a Multimap. Guava provides invertFrom(Multimap toInvert, Multimap dest) to let you do this, without choosing an implementation for you.

NOTE: If you are using an ImmutableMultimap, consider ImmutableMultimap.inverse() instead.

ArrayListMultimap<String, Integer> multimap = ArrayListMultimap.create();
multimap.putAll("b", Ints.asList(2, 4, 6));
multimap.putAll("a", Ints.asList(4, 2, 1));
multimap.putAll("c", Ints.asList(2, 5, 3));

TreeMultimap<Integer, String> inverse = Multimaps.invertFrom(multimap, TreeMultimap.<String, Integer> create());
// note that we choose the implementation, so if we use a TreeMultimap, we get results in order
/*
 * inverse maps:
 *  1 => {"a"}
 *  2 => {"a", "b", "c"}
 *  3 => {"c"}
 *  4 => {"a", "b"}
 *  5 => {"c"}
 *  6 => {"b"}
 */

forMap

Need to use a Multimap method on a Map? forMap(Map) views a Map as a SetMultimap. This is particularly useful, for example, in combination with Multimaps.invertFrom.

Map<String, Integer> map = ImmutableMap.of("a", 1, "b", 1, "c", 2);
SetMultimap<String, Integer> multimap = Multimaps.forMap(map);
// multimap maps ["a" => {1}, "b" => {1}, "c" => {2}]
Multimap<Integer, String> inverse = Multimaps.invertFrom(multimap, HashMultimap.<Integer, String> create());
// inverse maps [1 => {"a", "b"}, 2 => {"c"}]

Wrappers

Multimaps provides the traditional wrapper methods, as well as tools to get custom Multimap implementations based on Map and Collection implementations of your choice.

Multimap type	Unmodifiable	Synchronized	Custom
Multimap	unmodifiableMultimap	synchronizedMultimap	newMultimap
ListMultimap	unmodifiableListMultimap	synchronizedListMultimap	newListMultimap
SetMultimap	unmodifiableSetMultimap	synchronizedSetMultimap	newSetMultimap
SortedSetMultimap	unmodifiableSortedSetMultimap	synchronizedSortedSetMultimap	newSortedSetMultimap

The custom Multimap implementations let you specify a particular implementation that should be used in the returned Multimap. Caveats include:

• The multimap assumes complete ownership over of map and the lists returned by factory. Those objects should not be manually updated, they should be empty when provided, and they should not use soft, weak, or phantom references.
• No guarantees are made on what the contents of the Map will look like after you modify the Multimap.
• The multimap is not threadsafe when any concurrent operations update the multimap, even if map and the instances generated by factory are. Concurrent read operations will work correctly, though. Work around this with the synchronized wrappers if necessary.
• The multimap is serializable if map, factory, the lists generated by factory, and the multimap contents are all serializable.
• The collections returned by Multimap.get(key) are not of the same type as the collections returned by your Supplier, though if you supplier returns RandomAccess lists, the lists returned by Multimap.get(key) will also be random access.

Note that the custom Multimap methods expect a Supplier argument to generate fresh new collections. Here is an example of writing a ListMultimap backed by a TreeMap mapping to LinkedList.

ListMultimap<String, Integer> myMultimap = Multimaps.newListMultimap(
  Maps.<String, Collection<Integer>>newTreeMap(),
  new Supplier<LinkedList<Integer>>() {
    public LinkedList<Integer> get() {
      return Lists.newLinkedList();
    }
  });

Tables

The Tables class provides a few handy utilities.

customTable

Comparable to the Multimaps.newXXXMultimap(Map, Supplier) utilities, Tables.newCustomTable(Map, Supplier<Map>) allows you to specify a Table implementation using whatever row or column map you like.

// use LinkedHashMaps instead of HashMaps
Table<String, Character, Integer> table = Tables.newCustomTable(
  Maps.<String, Map<Character, Integer>>newLinkedHashMap(),
  new Supplier<Map<Character, Integer>> () {
    public Map<Character, Integer> get() {
      return Maps.newLinkedHashMap();
    }
  });

transpose

The transpose(Table<R, C, V>) method allows you to view a Table<R, C, V> as a Table<C, R, V>.

Wrappers

These are the familiar unmodifiability wrappers you know and love. Consider, however, using ImmutableTable instead in most cases.

• unmodifiableTable
• unmodifiableRowSortedTable