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JavaTM 2 Platform Std. Ed. v1.3 |
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SUMMARY: INNER | FIELD | CONSTR | METHOD | DETAIL: FIELD | CONSTR | METHOD |
java.lang.Object | +--java.util.Collections
This class consists exclusively of static methods that operate on or return collections. It contains polymorphic algorithms that operate on collections, "wrappers", which return a new collection backed by a specified collection, and a few other odds and ends.
The documentation for the polymorphic algorithms contained in this class generally includes a brief description of the implementation. Such descriptions should be regarded as implementation notes, rather than parts of the specification. Implementors should feel free to substitute other algorithms, so long as the specification itself is adhered to. (For example, the algorithm used by sort does not have to be a mergesort, but it does have to be stable.)
Collection
,
Set
,
List
,
Map
Field Summary | |
static List |
EMPTY_LIST
The empty list (immutable). |
static Map |
EMPTY_MAP
The empty map (immutable). |
static Set |
EMPTY_SET
The empty set (immutable). |
Method Summary | |
static int |
binarySearch(List list,
Object key)
Searches the specified list for the specified object using the binary search algorithm. |
static int |
binarySearch(List list,
Object key,
Comparator c)
Searches the specified list for the specified object using the binary search algorithm. |
static void |
copy(List dest,
List src)
Copies all of the elements from one list into another. |
static Enumeration |
enumeration(Collection c)
Returns an enumeration over the specified collection. |
static void |
fill(List list,
Object o)
Replaces all of the elements of the specified list with the specified element. |
static Object |
max(Collection coll)
Returns the maximum element of the given collection, according to the natural ordering of its elements. |
static Object |
max(Collection coll,
Comparator comp)
Returns the maximum element of the given collection, according to the order induced by the specified comparator. |
static Object |
min(Collection coll)
Returns the minimum element of the given collection, according to the natural ordering of its elements. |
static Object |
min(Collection coll,
Comparator comp)
Returns the minimum element of the given collection, according to the order induced by the specified comparator. |
static List |
nCopies(int n,
Object o)
Returns an immutable list consisting of n copies of the specified object. |
static void |
reverse(List l)
Reverses the order of the elements in the specified list. |
static Comparator |
reverseOrder()
Returns a comparator that imposes the reverse of the natural ordering on a collection of objects that implement the Comparable interface. |
static void |
shuffle(List list)
Randomly permutes the specified list using a default source of randomness. |
static void |
shuffle(List list,
Random rnd)
Randomly permute the specified list using the specified source of randomness. |
static Set |
singleton(Object o)
Returns an immutable set containing only the specified object. |
static List |
singletonList(Object o)
Returns an immutable list containing only the specified object. |
static Map |
singletonMap(Object key,
Object value)
Returns an immutable map, mapping only the specified key to the specified value. |
static void |
sort(List list)
Sorts the specified list into ascending order, according to the natural ordering of its elements. |
static void |
sort(List list,
Comparator c)
Sorts the specified list according to the order induced by the specified comparator. |
static Collection |
synchronizedCollection(Collection c)
Returns a synchronized (thread-safe) collection backed by the specified collection. |
static List |
synchronizedList(List list)
Returns a synchronized (thread-safe) list backed by the specified list. |
static Map |
synchronizedMap(Map m)
Returns a synchronized (thread-safe) map backed by the specified map. |
static Set |
synchronizedSet(Set s)
Returns a synchronized (thread-safe) set backed by the specified set. |
static SortedMap |
synchronizedSortedMap(SortedMap m)
Returns a synchronized (thread-safe) sorted map backed by the specified sorted map. |
static SortedSet |
synchronizedSortedSet(SortedSet s)
Returns a synchronized (thread-safe) sorted set backed by the specified sorted set. |
static Collection |
unmodifiableCollection(Collection c)
Returns an unmodifiable view of the specified collection. |
static List |
unmodifiableList(List list)
Returns an unmodifiable view of the specified list. |
static Map |
unmodifiableMap(Map m)
Returns an unmodifiable view of the specified map. |
static Set |
unmodifiableSet(Set s)
Returns an unmodifiable view of the specified set. |
static SortedMap |
unmodifiableSortedMap(SortedMap m)
Returns an unmodifiable view of the specified sorted map. |
static SortedSet |
unmodifiableSortedSet(SortedSet s)
Returns an unmodifiable view of the specified sorted set. |
Methods inherited from class java.lang.Object |
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
Field Detail |
public static final Set EMPTY_SET
public static final List EMPTY_LIST
public static final Map EMPTY_MAP
Method Detail |
public static void sort(List list)
This sort is guaranteed to be stable: equal elements will not be reordered as a result of the sort.
The specified list must be modifiable, but need not be resizable.
The sorting algorithm is a modified mergesort (in which the merge is omitted if the highest element in the low sublist is less than the lowest element in the high sublist). This algorithm offers guaranteed n log(n) performance, and can approach linear performance on nearly sorted lists.
This implementation dumps the specified list into an array, sorts the array, and iterates over the list resetting each element from the corresponding position in the array. This avoids the n2 log(n) performance that would result from attempting to sort a linked list in place.
list
- the list to be sorted.ClassCastException
- if the list contains elements that are not
mutually comparable (for example, strings and integers).UnsupportedOperationException
- if the specified list's
list-iterator does not support the set operation.Comparable
public static void sort(List list, Comparator c)
This sort is guaranteed to be stable: equal elements will not be reordered as a result of the sort.
The sorting algorithm is a modified mergesort (in which the merge is omitted if the highest element in the low sublist is less than the lowest element in the high sublist). This algorithm offers guaranteed n log(n) performance, and can approach linear performance on nearly sorted lists.
The specified list must be modifiable, but need not be resizable. This implementation dumps the specified list into an array, sorts the array, and iterates over the list resetting each element from the corresponding position in the array. This avoids the n2 log(n) performance that would result from attempting to sort a linked list in place.
list
- the list to be sorted.c
- the comparator to determine the order of the list. A
null value indicates that the elements' natural
ordering should be used.ClassCastException
- if the list contains elements that are not
mutually comparable using the specified comparator.UnsupportedOperationException
- if the specified list's
list-iterator does not support the set operation.Comparator
public static int binarySearch(List list, Object key)
This method runs in log(n) time for a "random access" list (which provides near-constant-time positional access). It may run in n log(n) time if it is called on a "sequential access" list (which provides linear-time positional access).
If the specified list implements the AbstracSequentialList interface, this method will do a sequential search instead of a binary search; this offers linear performance instead of n log(n) performance if this method is called on a LinkedList object.list
- the list to be searched.key
- the key to be searched for.ClassCastException
- if the list contains elements that are not
mutually comparable (for example, strings and
integers), or the search key in not mutually comparable
with the elements of the list.Comparable
,
sort(List)
public static int binarySearch(List list, Object key, Comparator c)
This method runs in log(n) time for a "random access" list (which provides near-constant-time positional access). It may run in n log(n) time if it is called on a "sequential access" list (which provides linear-time positional access).
If the specified list implements the AbstracSequentialList interface, this method will do a sequential search instead of a binary search; this offers linear performance instead of n log(n) performance if this method is called on a LinkedList object.list
- the list to be searched.key
- the key to be searched for.c
- the comparator by which the list is ordered. A
null value indicates that the elements' natural
ordering should be used.ClassCastException
- if the list contains elements that are not
mutually comparable using the specified comparator,
or the search key in not mutually comparable with the
elements of the list using this comparator.Comparable
,
sort(List, Comparator)
public static void reverse(List l)
This method runs in linear time.
l
- the list whose elements are to be reversed.UnsupportedOperationException
- if the specified list's
list-iterator does not support the set operation.public static void shuffle(List list)
The hedge "approximately" is used in the foregoing description because default source of randomenss is only approximately an unbiased source of independently chosen bits. If it were a perfect source of randomly chosen bits, then the algorithm would choose permutations with perfect uniformity.
This implementation traverses the list backwards, from the last element up to the second, repeatedly swapping a randomly selected element into the "current position". Elements are randomly selected from the portion of the list that runs from the first element to the current position, inclusive.
This method runs in linear time for a "random access" list (which provides near-constant-time positional access). It may require quadratic time for a "sequential access" list.
list
- the list to be shuffled.UnsupportedOperationException
- if the specified list's
list-iterator does not support the set operation.public static void shuffle(List list, Random rnd)
This implementation traverses the list backwards, from the last element up to the second, repeatedly swapping a randomly selected element into the "current position". Elements are randomly selected from the portion of the list that runs from the first element to the current position, inclusive.
This method runs in linear time for a "random access" list (which provides near-constant-time positional access). It may require quadratic time for a "sequential access" list.
list
- the list to be shuffled.rnd
- the source of randomness to use to shuffle the list.UnsupportedOperationException
- if the specified list's
list-iterator does not support the set operation.public static void fill(List list, Object o)
This method runs in linear time.
list
- the list to be filled with the specified element.o
- The element with which to fill the specified list.UnsupportedOperationException
- if the specified list's
list-iterator does not support the set operation.public static void copy(List dest, List src)
This method runs in linear time.
dest
- The destination list.src
- The source list.IndexOutOfBoundsException
- if the destination list is too small
to contain the entire source List.UnsupportedOperationException
- if the destination list's
list-iterator does not support the set operation.public static Object min(Collection coll)
This method iterates over the entire collection, hence it requires time proportional to the size of the collection.
coll
- the collection whose minimum element is to be determined.ClassCastException
- if the collection contains elements that are
not mutually comparable (for example, strings and
integers).NoSuchElementException
- if the collection is empty.Comparable
public static Object min(Collection coll, Comparator comp)
This method iterates over the entire collection, hence it requires time proportional to the size of the collection.
coll
- the collection whose minimum element is to be determined.comp
- the comparator with which to determine the minimum element.
A null value indicates that the elements' natural
ordering should be used.ClassCastException
- if the collection contains elements that are
not mutually comparable using the specified comparator.NoSuchElementException
- if the collection is empty.Comparable
public static Object max(Collection coll)
This method iterates over the entire collection, hence it requires time proportional to the size of the collection.
coll
- the collection whose maximum element is to be determined.ClassCastException
- if the collection contains elements that are
not mutually comparable (for example, strings and
integers).NoSuchElementException
- if the collection is empty.Comparable
public static Object max(Collection coll, Comparator comp)
This method iterates over the entire collection, hence it requires time proportional to the size of the collection.
coll
- the collection whose maximum element is to be determined.comp
- the comparator with which to determine the maximum element.
A null value indicates that the elements' natural
ordering should be used.ClassCastException
- if the collection contains elements that are
not mutually comparable using the specified comparator.NoSuchElementException
- if the collection is empty.Comparable
public static Collection unmodifiableCollection(Collection c)
The returned collection does not pass the hashCode and equals operations through to the backing collection, but relies on Object's equals and hashCode methods. This is necessary to preserve the contracts of these operations in the case that the backing collection is a set or a list.
The returned collection will be serializable if the specified collection is serializable.
c
- the collection for which an unmodifiable view is to be
returned.public static Set unmodifiableSet(Set s)
The returned set will be serializable if the specified set is serializable.
s
- the set for which an unmodifiable view is to be returned.public static SortedSet unmodifiableSortedSet(SortedSet s)
The returned sorted set will be serializable if the specified sorted set is serializable.
s
- the sorted set for which an unmodifiable view is to be
returned.public static List unmodifiableList(List list)
The returned list will be serializable if the specified list is serializable.
list
- the list for which an unmodifiable view is to be returned.public static Map unmodifiableMap(Map m)
The returned map will be serializable if the specified map is serializable.
m
- the map for which an unmodifiable view is to be returned.public static SortedMap unmodifiableSortedMap(SortedMap m)
The returned sorted map will be serializable if the specified sorted map is serializable.
m
- the sorted map for which an unmodifiable view is to be
returned.public static Collection synchronizedCollection(Collection c)
It is imperative that the user manually synchronize on the returned collection when iterating over it:
Collection c = Collections.synchronizedCollection(myCollection); ... synchronized(c) { Iterator i = c.iterator(); // Must be in the synchronized block while (i.hasNext()) foo(i.next()); }Failure to follow this advice may result in non-deterministic behavior.
The returned collection does not pass the hashCode and equals operations through to the backing collection, but relies on Object's equals and hashCode methods. This is necessary to preserve the contracts of these operations in the case that the backing collection is a set or a list.
The returned collection will be serializable if the specified collection is serializable.
c
- the collection to be "wrapped" in a synchronized collection.public static Set synchronizedSet(Set s)
It is imperative that the user manually synchronize on the returned set when iterating over it:
Set s = Collections.synchronizedSet(new HashSet()); ... synchronized(s) { Iterator i = s.iterator(); // Must be in the synchronized block while (i.hasNext()) foo(i.next()); }Failure to follow this advice may result in non-deterministic behavior.
The returned set will be serializable if the specified set is serializable.
s
- the set to be "wrapped" in a synchronized set.public static SortedSet synchronizedSortedSet(SortedSet s)
It is imperative that the user manually synchronize on the returned sorted set when iterating over it or any of its subSet, headSet, or tailSet views.
SortedSet s = Collections.synchronizedSortedSet(new HashSortedSet()); ... synchronized(s) { Iterator i = s.iterator(); // Must be in the synchronized block while (i.hasNext()) foo(i.next()); }or:
SortedSet s = Collections.synchronizedSortedSet(new HashSortedSet()); SortedSet s2 = s.headSet(foo); ... synchronized(s) { // Note: s, not s2!!! Iterator i = s2.iterator(); // Must be in the synchronized block while (i.hasNext()) foo(i.next()); }Failure to follow this advice may result in non-deterministic behavior.
The returned sorted set will be serializable if the specified sorted set is serializable.
s
- the sorted set to be "wrapped" in a synchronized sorted set.public static List synchronizedList(List list)
It is imperative that the user manually synchronize on the returned list when iterating over it:
List list = Collections.synchronizedList(new ArrayList()); ... synchronized(list) { Iterator i = list.iterator(); // Must be in synchronized block while (i.hasNext()) foo(i.next()); }Failure to follow this advice may result in non-deterministic behavior.
The returned list will be serializable if the specified list is serializable.
list
- the list to be "wrapped" in a synchronized list.public static Map synchronizedMap(Map m)
It is imperative that the user manually synchronize on the returned map when iterating over any of its collection views:
Map m = Collections.synchronizedMap(new HashMap()); ... Set s = m.keySet(); // Needn't be in synchronized block ... synchronized(m) { // Synchronizing on m, not s! Iterator i = s.iterator(); // Must be in synchronized block while (i.hasNext()) foo(i.next()); }Failure to follow this advice may result in non-deterministic behavior.
The returned map will be serializable if the specified map is serializable.
m
- the map to be "wrapped" in a synchronized map.public static SortedMap synchronizedSortedMap(SortedMap m)
It is imperative that the user manually synchronize on the returned sorted map when iterating over any of its collection views, or the collections views of any of its subMap, headMap or tailMap views.
SortedMap m = Collections.synchronizedSortedMap(new HashSortedMap()); ... Set s = m.keySet(); // Needn't be in synchronized block ... synchronized(m) { // Synchronizing on m, not s! Iterator i = s.iterator(); // Must be in synchronized block while (i.hasNext()) foo(i.next()); }or:
SortedMap m = Collections.synchronizedSortedMap(new HashSortedMap()); SortedMap m2 = m.subMap(foo, bar); ... Set s2 = m2.keySet(); // Needn't be in synchronized block ... synchronized(m) { // Synchronizing on m, not m2 or s2! Iterator i = s.iterator(); // Must be in synchronized block while (i.hasNext()) foo(i.next()); }Failure to follow this advice may result in non-deterministic behavior.
The returned sorted map will be serializable if the specified sorted map is serializable.
m
- the sorted map to be "wrapped" in a synchronized sorted map.public static Set singleton(Object o)
o
- the sole object to be stored in the returned set.public static List singletonList(Object o)
o
- the sole object to be stored in the returned list.public static Map singletonMap(Object key, Object value)
key
- the sole key to be stored in the returned map.value
- the value to which the returned map maps key.public static List nCopies(int n, Object o)
n
- the number of elements in the returned list.o
- the element to appear repeatedly in the returned list.IllegalArgumentException
- if n < 0.List.addAll(Collection)
,
List.addAll(int, Collection)
public static Comparator reverseOrder()
Arrays.sort(a, Collections.reverseOrder());sorts the array in reverse-lexicographic (alphabetical) order.
The returned comparator is serializable.
Comparable
public static Enumeration enumeration(Collection c)
c
- the collection for which an enumeration is to be returned.
|
JavaTM 2 Platform Std. Ed. v1.3 |
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