com.google.common.collect
Class Maps

java.lang.Object
  com.google.common.collect.Maps

@GwtCompatible(emulated=true)
public final class Maps
extends Object

Static utility methods pertaining to Map instances. Also see this class's counterparts Lists and Sets.

Since:

2 (imported from Google Collections Library)

Author:

Kevin Bourrillion, Mike Bostock, Isaac Shum, Louis Wasserman

newHashMap

public static <K,V> HashMap<K,V> newHashMap()

Creates a mutable, empty HashMap instance.

Note: if mutability is not required, use ImmutableMap.of() instead.

Note: if K is an enum type, use newEnumMap(java.lang.Class) instead.

Returns:

a new, empty HashMap

newHashMapWithExpectedSize

public static <K,V> HashMap<K,V> newHashMapWithExpectedSize(int expectedSize)

Creates a HashMap instance with enough capacity to hold the specified number of elements without rehashing.

Parameters:

expectedSize - the expected size

Returns:

a new, empty HashMap with enough capacity to hold expectedSize elements without rehashing

Throws:

IllegalArgumentException - if expectedSize is negative

newHashMap

public static <K,V> HashMap<K,V> newHashMap(Map<? extends K,? extends V> map)

Creates a mutable HashMap instance with the same mappings as the specified map.

Note: if mutability is not required, use ImmutableMap.copyOf(Map) instead.

Note: if K is an Enum type, use newEnumMap(java.lang.Class) instead.

Parameters:

map - the mappings to be placed in the new map

Returns:

a new HashMap initialized with the mappings from map

newLinkedHashMap

public static <K,V> LinkedHashMap<K,V> newLinkedHashMap()

Creates a mutable, empty, insertion-ordered LinkedHashMap instance.

Note: if mutability is not required, use ImmutableMap.of() instead.

Returns:

a new, empty LinkedHashMap

newLinkedHashMap

public static <K,V> LinkedHashMap<K,V> newLinkedHashMap(Map<? extends K,? extends V> map)

Creates a mutable, insertion-ordered LinkedHashMap instance with the same mappings as the specified map.

Note: if mutability is not required, use ImmutableMap.copyOf(Map) instead.

Parameters:

map - the mappings to be placed in the new map

Returns:

a new, LinkedHashMap initialized with the mappings from map

newConcurrentMap

public static <K,V> ConcurrentMap<K,V> newConcurrentMap()

Returns a general-purpose instance of ConcurrentMap, which supports all optional operations of the ConcurrentMap interface. It does not permit null keys or values. It is serializable.

This is currently accomplished by calling MapMaker.makeMap().

It is preferable to use MapMaker directly (rather than through this method), as it presents numerous useful configuration options, such as the concurrency level, load factor, key/value reference types, and value computation.

Returns:

a new, empty ConcurrentMap

Since:

3

newTreeMap

public static <K extends Comparable,V> TreeMap<K,V> newTreeMap()

Creates a mutable, empty TreeMap instance using the natural ordering of its elements.

Note: if mutability is not required, use ImmutableSortedMap.of() instead.

Returns:

a new, empty TreeMap

newTreeMap

public static <K,V> TreeMap<K,V> newTreeMap(SortedMap<K,? extends V> map)

Creates a mutable TreeMap instance with the same mappings as the specified map and using the same ordering as the specified map.

Note: if mutability is not required, use ImmutableSortedMap.copyOfSorted(SortedMap) instead.

Parameters:

map - the sorted map whose mappings are to be placed in the new map and whose comparator is to be used to sort the new map

Returns:

a new TreeMap initialized with the mappings from map and using the comparator of map

newTreeMap

public static <C,K extends C,V> TreeMap<K,V> newTreeMap(@Nullable
                                                        Comparator<C> comparator)

Creates a mutable, empty TreeMap instance using the given comparator.

Note: if mutability is not required, use ImmutableSortedMap.orderedBy(comparator).build() instead.

Parameters:

comparator - the comparator to sort the keys with

Returns:

a new, empty TreeMap

newEnumMap

public static <K extends Enum<K>,V> EnumMap<K,V> newEnumMap(Class<K> type)

Creates an EnumMap instance.

Parameters:

type - the key type for this map

Returns:

a new, empty EnumMap

newEnumMap

public static <K extends Enum<K>,V> EnumMap<K,V> newEnumMap(Map<K,? extends V> map)

Creates an EnumMap with the same mappings as the specified map.

Parameters:

map - the map from which to initialize this EnumMap

Returns:

a new EnumMap initialized with the mappings from map

Throws:

IllegalArgumentException - if m is not an EnumMap instance and contains no mappings

newIdentityHashMap

public static <K,V> IdentityHashMap<K,V> newIdentityHashMap()

Creates an IdentityHashMap instance.

Returns:

a new, empty IdentityHashMap

synchronizedBiMap

public static <K,V> BiMap<K,V> synchronizedBiMap(BiMap<K,V> bimap)

Returns a synchronized (thread-safe) bimap backed by the specified bimap. In order to guarantee serial access, it is critical that all access to the backing bimap is accomplished through the returned bimap.

It is imperative that the user manually synchronize on the returned map when accessing any of its collection views:

   BiMap<Long, String> map = Maps.synchronizedBiMap(
       HashBiMap.<Long, String>create());
   ...
   Set<Long> set = map.keySet();  // Needn't be in synchronized block
   ...
   synchronized (map) {  // Synchronizing on map, not set!
     Iterator<Long> it = set.iterator(); // Must be in synchronized block
     while (it.hasNext()) {
       foo(it.next());
     }
   }

Failure to follow this advice may result in non-deterministic behavior.

The returned bimap will be serializable if the specified bimap is serializable.

Parameters:

bimap - the bimap to be wrapped in a synchronized view

Returns:

a sychronized view of the specified bimap

difference

public static <K,V> MapDifference<K,V> difference(Map<? extends K,? extends V> left,
                                                  Map<? extends K,? extends V> right)

Computes the difference between two maps. This difference is an immutable snapshot of the state of the maps at the time this method is called. It will never change, even if the maps change at a later time.

Since this method uses HashMap instances internally, the keys of the supplied maps must be well-behaved with respect to Object.equals(java.lang.Object) and Object.hashCode().

Note:If you only need to know whether two maps have the same mappings, call left.equals(right) instead of this method.

Parameters:

left - the map to treat as the "left" map for purposes of comparison

right - the map to treat as the "right" map for purposes of comparison

Returns:

the difference between the two maps

uniqueIndex

public static <K,V> ImmutableMap<K,V> uniqueIndex(Iterable<V> values,
                                                  Function<? super V,K> keyFunction)

Returns an immutable map for which the Map.values() are the given elements in the given order, and each key is the product of invoking a supplied function on its corresponding value.

Parameters:

values - the values to use when constructing the Map

keyFunction - the function used to produce the key for each value

Returns:

a map mapping the result of evaluating the function keyFunction on each value in the input collection to that value

Throws:

IllegalArgumentException - if keyFunction produces the same key for more than one value in the input collection

NullPointerException - if any elements of values is null, or if keyFunction produces null for any value

fromProperties

@GwtIncompatible(value="java.util.Properties")
public static ImmutableMap<String,String> fromProperties(Properties properties)

Creates an ImmutableMap<String, String> from a Properties instance. Properties normally derive from Map<Object, Object>, but they typically contain strings, which is awkward. This method lets you get a plain-old-Map out of a Properties.

Parameters:

properties - a Properties object to be converted

Returns:

an immutable map containing all the entries in properties

Throws:

ClassCastException - if any key in Properties is not a String

NullPointerException - if any key or value in Properties is null

immutableEntry

@GwtCompatible(serializable=true)
public static <K,V> Map.Entry<K,V> immutableEntry(@Nullable
                                                                            K key,
                                                                            @Nullable
                                                                            V value)

Returns an immutable map entry with the specified key and value. The Map.Entry.setValue(V) operation throws an UnsupportedOperationException.

The returned entry is serializable.

Parameters:

key - the key to be associated with the returned entry

value - the value to be associated with the returned entry

unmodifiableBiMap

public static <K,V> BiMap<K,V> unmodifiableBiMap(BiMap<? extends K,? extends V> bimap)

Returns an unmodifiable view of the specified bimap. This method allows modules to provide users with "read-only" access to internal bimaps. Query operations on the returned bimap "read through" to the specified bimap, and attemps to modify the returned map, whether direct or via its collection views, result in an UnsupportedOperationException.

The returned bimap will be serializable if the specified bimap is serializable.

Parameters:

bimap - the bimap for which an unmodifiable view is to be returned

Returns:

an unmodifiable view of the specified bimap

transformValues

public static <K,V1,V2> Map<K,V2> transformValues(Map<K,V1> fromMap,
                                                  Function<? super V1,V2> function)

Returns a view of a map where each value is transformed by a function. All other properties of the map, such as iteration order, are left intact. For example, the code:

   Map<String, Integer> map = ImmutableMap.of("a", 4, "b", 9);
   Function<Integer, Double> sqrt =
       new Function<Integer, Double>() {
         public Double apply(Integer in) {
           return Math.sqrt((int) in);
         }
       };
   Map<String, Double> transformed = Maps.transformValues(map, sqrt);
   System.out.println(transformed);

... prints {a=2.0, b=3.0}.

Changes in the underlying map are reflected in this view. Conversely, this view supports removal operations, and these are reflected in the underlying map.

It's acceptable for the underlying map to contain null keys, and even null values provided that the function is capable of accepting null input. The transformed map might contain null values, if the function sometimes gives a null result.

The returned map is not thread-safe or serializable, even if the underlying map is.

The function is applied lazily, invoked when needed. This is necessary for the returned map to be a view, but it means that the function will be applied many times for bulk operations like Map.containsValue(java.lang.Object) and Map.toString(). For this to perform well, function should be fast. To avoid lazy evaluation when the returned map doesn't need to be a view, copy the returned map into a new map of your choosing.

transformEntries

@Beta
public static <K,V1,V2> Map<K,V2> transformEntries(Map<K,V1> fromMap,
                                                        Maps.EntryTransformer<? super K,? super V1,V2> transformer)

Returns a view of a map whose values are derived from the original map's entries. In contrast to transformValues(java.util.Map, com.google.common.base.Function), this method's entry-transformation logic may depend on the key as well as the value.

All other properties of the transformed map, such as iteration order, are left intact. For example, the code:

   Map<String, Boolean> options =
       ImmutableMap.of("verbose", true, "sort", false);
   EntryTransformer<String, Boolean, String> flagPrefixer =
       new EntryTransformer<String, Boolean, String>() {
         public String transformEntry(String key, Boolean value) {
           return value ? key : "no" + key;
         }
       };
   Map<String, String> transformed =
       Maps.transformEntries(options, flagPrefixer);
   System.out.println(transformed);

... prints {verbose=verbose, sort=nosort}.

Changes in the underlying map are reflected in this view. Conversely, this view supports removal operations, and these are reflected in the underlying map.

It's acceptable for the underlying map to contain null keys and null values provided that the transformer is capable of accepting null inputs. The transformed map might contain null values if the transformer sometimes gives a null result.

The returned map is not thread-safe or serializable, even if the underlying map is.

The transformer is applied lazily, invoked when needed. This is necessary for the returned map to be a view, but it means that the transformer will be applied many times for bulk operations like Map.containsValue(java.lang.Object) and Object.toString(). For this to perform well, transformer should be fast. To avoid lazy evaluation when the returned map doesn't need to be a view, copy the returned map into a new map of your choosing.

Warning: This method assumes that for any instance k of EntryTransformer key type K, k.equals(k2) implies that k2 is also of type K. Using an EntryTransformer key type for which this may not hold, such as ArrayList, may risk a ClassCastException when calling methods on the transformed map.

Since:

7

filterKeys

public static <K,V> Map<K,V> filterKeys(Map<K,V> unfiltered,
                                        Predicate<? super K> keyPredicate)

Returns a map containing the mappings in unfiltered whose keys satisfy a predicate. The returned map is a live view of unfiltered; changes to one affect the other.

The resulting map's keySet(), entrySet(), and values() views have iterators that don't support remove(), but all other methods are supported by the map and its views. When given a key that doesn't satisfy the predicate, the map's put() and putAll() methods throw an IllegalArgumentException.

When methods such as removeAll() and clear() are called on the filtered map or its views, only mappings whose keys satisfy the filter will be removed from the underlying map.

The returned map isn't threadsafe or serializable, even if unfiltered is.

Many of the filtered map's methods, such as size(), iterate across every key/value mapping in the underlying map and determine which satisfy the filter. When a live view is not needed, it may be faster to copy the filtered map and use the copy.

Warning: keyPredicate must be consistent with equals, as documented at Predicate.apply(T). Do not provide a predicate such as Predicates.instanceOf(ArrayList.class), which is inconsistent with equals.

filterValues

public static <K,V> Map<K,V> filterValues(Map<K,V> unfiltered,
                                          Predicate<? super V> valuePredicate)

Returns a map containing the mappings in unfiltered whose values satisfy a predicate. The returned map is a live view of unfiltered; changes to one affect the other.

The resulting map's keySet(), entrySet(), and values() views have iterators that don't support remove(), but all other methods are supported by the map and its views. When given a value that doesn't satisfy the predicate, the map's put(), putAll(), and Map.Entry.setValue(V) methods throw an IllegalArgumentException.

When methods such as removeAll() and clear() are called on the filtered map or its views, only mappings whose values satisfy the filter will be removed from the underlying map.

The returned map isn't threadsafe or serializable, even if unfiltered is.

Many of the filtered map's methods, such as size(), iterate across every key/value mapping in the underlying map and determine which satisfy the filter. When a live view is not needed, it may be faster to copy the filtered map and use the copy.

Warning: valuePredicate must be consistent with equals, as documented at Predicate.apply(T). Do not provide a predicate such as Predicates.instanceOf(ArrayList.class), which is inconsistent with equals.

filterEntries

public static <K,V> Map<K,V> filterEntries(Map<K,V> unfiltered,
                                           Predicate<? super Map.Entry<K,V>> entryPredicate)

Returns a map containing the mappings in unfiltered that satisfy a predicate. The returned map is a live view of unfiltered; changes to one affect the other.

The resulting map's keySet(), entrySet(), and values() views have iterators that don't support remove(), but all other methods are supported by the map and its views. When given a key/value pair that doesn't satisfy the predicate, the map's put() and putAll() methods throw an IllegalArgumentException. Similarly, the map's entries have a Map.Entry.setValue(V) method that throws an IllegalArgumentException when the existing key and the provided value don't satisfy the predicate.

When methods such as removeAll() and clear() are called on the filtered map or its views, only mappings that satisfy the filter will be removed from the underlying map.

The returned map isn't threadsafe or serializable, even if unfiltered is.

Many of the filtered map's methods, such as size(), iterate across every key/value mapping in the underlying map and determine which satisfy the filter. When a live view is not needed, it may be faster to copy the filtered map and use the copy.

Warning: entryPredicate must be consistent with equals, as documented at Predicate.apply(T).