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java.lang.Object | +--java.util.Collections
Untamed: 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 methods of this class all throw a NullPointerException if the collections provided to them are null.
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.)
The "destructive" algorithms contained in this class, that is, the algorithms that modify the collection on which they operate, are specified to throw UnsupportedOperationException if the collection does not support the appropriate mutation primitive(s), such as the set method. These algorithms may, but are not required to, throw this exception if an invocation would have no effect on the collection. For example, invoking the sort method on an unmodifiable list that is already sorted may or may not throw UnsupportedOperationException.
UnmodifiableCollection,
Set,
List,
Map| Field Summary | |
private static int |
BINARYSEARCH_THRESHOLD
|
private static int |
COPY_THRESHOLD
|
static List |
EMPTY_LIST
Enabled: The empty list (immutable). |
static Map |
EMPTY_MAP
Enabled: The empty map (immutable). |
static Set |
EMPTY_SET
Enabled: The empty set (immutable). |
private static int |
FILL_THRESHOLD
|
private static int |
INDEXOFSUBLIST_THRESHOLD
|
private static Random |
r
|
private static int |
REPLACEALL_THRESHOLD
|
private static Comparator |
REVERSE_ORDER
|
private static int |
REVERSE_THRESHOLD
|
private static int |
ROTATE_THRESHOLD
|
private static int |
SHUFFLE_THRESHOLD
|
| Constructor Summary | |
private |
Collections()
|
| Method Summary | |
static int |
binarySearch(List list,
Object key)
Enabled: Searches the specified list for the specified object using the binary search algorithm. |
static int |
binarySearch(List list,
Object key,
Comparator c)
Enabled: Searches the specified list for the specified object using the binary search algorithm. |
static void |
copy(List dest,
List src)
Enabled: Copies all of the elements from one list into another. |
static Enumeration |
enumeration(Collection c)
Enabled: Returns an enumeration over the specified collection. |
private static boolean |
eq(Object o1,
Object o2)
Returns true if the specified arguments are equal, or both null. |
static void |
fill(List list,
Object obj)
Enabled: Replaces all of the elements of the specified list with the specified element. |
private static Object |
get(ListIterator i,
int index)
Gets the ith element from the given list by repositioning the specified list listIterator. |
private static int |
indexedBinarySearch(List list,
Object key)
|
private static int |
indexedBinarySearch(List l,
Object key,
Comparator c)
|
static int |
indexOfSubList(List source,
List target)
Enabled: Returns the starting position of the first occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence. |
private static int |
iteratorBinarySearch(List list,
Object key)
|
private static int |
iteratorBinarySearch(List l,
Object key,
Comparator c)
|
static int |
lastIndexOfSubList(List source,
List target)
Enabled: Returns the starting position of the last occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence. |
static ArrayList |
list(Enumeration e)
Enabled: Returns an array list containing the elements returned by the specified enumeration in the order they are returned by the enumeration. |
static Object |
max(Collection coll)
Enabled: Returns the maximum element of the given collection, according to the natural ordering of its elements. |
static Object |
max(Collection coll,
Comparator comp)
Enabled: Returns the maximum element of the given collection, according to the order induced by the specified comparator. |
static Object |
min(Collection coll)
Enabled: Returns the minimum element of the given collection, according to the natural ordering of its elements. |
static Object |
min(Collection coll,
Comparator comp)
Enabled: Returns the minimum element of the given collection, according to the order induced by the specified comparator. |
static List |
nCopies(int n,
Object o)
Enabled: Returns an immutable list consisting of n copies of the specified object. |
static boolean |
replaceAll(List list,
Object oldVal,
Object newVal)
Enabled: Replaces all occurrences of one specified value in a list with another. |
static void |
reverse(List list)
Enabled: Reverses the order of the elements in the specified list. |
static Comparator |
reverseOrder()
Enabled: Returns a comparator that imposes the reverse of the natural ordering on a collection of objects that implement the Comparable interface. |
static void |
rotate(List list,
int distance)
Enabled: Rotates the elements in the specified list by the specified distance. |
private static void |
rotate1(List list,
int distance)
|
private static void |
rotate2(List list,
int distance)
|
static void |
shuffle(List list)
Enabled: Randomly permutes the specified list using a default source of randomness. |
static void |
shuffle(List list,
Random rnd)
Enabled: Randomly permute the specified list using the specified source of randomness. |
static Set |
singleton(Object o)
Enabled: Returns an immutable set containing only the specified object. |
static List |
singletonList(Object o)
Enabled: Returns an immutable list containing only the specified object. |
static Map |
singletonMap(Object key,
Object value)
Enabled: Returns an immutable map, mapping only the specified key to the specified value. |
static void |
sort(List list)
Enabled: Sorts the specified list into ascending order, according to the natural ordering of its elements. |
static void |
sort(List list,
Comparator c)
Enabled: Sorts the specified list according to the order induced by the specified comparator. |
static void |
swap(List list,
int i,
int j)
Enabled: Swaps the elements at the specified positions in the specified list. |
private static void |
swap(Object[] arr,
int i,
int j)
Swaps the two specified elements in the specified array. |
static Collection |
synchronizedCollection(Collection c)
Enabled: Returns a synchronized (thread-safe) collection backed by the specified collection. |
(package private) static Collection |
synchronizedCollection(Collection c,
Object mutex)
|
static List |
synchronizedList(List list)
Enabled: Returns a synchronized (thread-safe) list backed by the specified list. |
(package private) static List |
synchronizedList(List list,
Object mutex)
|
static Map |
synchronizedMap(Map m)
Enabled: Returns a synchronized (thread-safe) map backed by the specified map. |
static Set |
synchronizedSet(Set s)
Enabled: Returns a synchronized (thread-safe) set backed by the specified set. |
(package private) static Set |
synchronizedSet(Set s,
Object mutex)
|
static SortedMap |
synchronizedSortedMap(SortedMap m)
Enabled: Returns a synchronized (thread-safe) sorted map backed by the specified sorted map. |
static SortedSet |
synchronizedSortedSet(SortedSet s)
Enabled: Returns a synchronized (thread-safe) sorted set backed by the specified sorted set. |
static Collection |
unmodifiableCollection(Collection c)
Enabled: Returns an unmodifiable view of the specified collection. |
static List |
unmodifiableList(List list)
Enabled: Returns an unmodifiable view of the specified list. |
static Map |
unmodifiableMap(Map m)
Enabled: Returns an unmodifiable view of the specified map. |
static Set |
unmodifiableSet(Set s)
Enabled: Returns an unmodifiable view of the specified set. |
static SortedMap |
unmodifiableSortedMap(SortedMap m)
Enabled: Returns an unmodifiable view of the specified sorted map. |
static SortedSet |
unmodifiableSortedSet(SortedSet s)
Enabled: 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 |
private static final int BINARYSEARCH_THRESHOLD
private static final int REVERSE_THRESHOLD
private static final int SHUFFLE_THRESHOLD
private static final int FILL_THRESHOLD
private static final int ROTATE_THRESHOLD
private static final int COPY_THRESHOLD
private static final int REPLACEALL_THRESHOLD
private static final int INDEXOFSUBLIST_THRESHOLD
private static Random r
public static final Set EMPTY_SET
public static final List EMPTY_LIST
public static final Map EMPTY_MAP
private static final Comparator REVERSE_ORDER
| Constructor Detail |
private Collections()
| 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. 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. 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). If the specified list
does not implement the RandomAccess and is large, this method
will do an iterator-based binary search that performs O(n) link
traversals and O(log n) element comparisons.
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)
private static int indexedBinarySearch(List list,
Object key)
private static int iteratorBinarySearch(List list,
Object key)
private static Object get(ListIterator i,
int index)
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). If the specified list
does not implement the RandomAccess and is large, this
this method will do an iterator-based binary search that performs
O(n) link traversals and O(log n) element comparisons.
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)
private static int indexedBinarySearch(List l,
Object key,
Comparator c)
private static int iteratorBinarySearch(List l,
Object key,
Comparator c)
public static void reverse(List list)
This method runs in linear time.
list - the list whose elements are to be reversed.
UnsupportedOperationException - if the specified list or
its list-iterator does not support the set method.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. If the specified list does not
implement the RandomAccess interface and is large, this
implementation dumps the specified list into an array before shuffling
it, and dumps the shuffled array back into the list. This avoids the
quadratic behavior that would result from shuffling a "sequential
access" list in place.
list - the list to be shuffled.
UnsupportedOperationException - if the specified list or
its list-iterator does not support the set method.
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. If the specified list does not
implement the RandomAccess interface and is large, this
implementation dumps the specified list into an array before shuffling
it, and dumps the shuffled array back into the list. This avoids the
quadratic behavior that would result from shuffling a "sequential
access" list in place.
list - the list to be shuffled.rnd - the source of randomness to use to shuffle the list.
UnsupportedOperationException - if the specified list or its
list-iterator does not support the set operation.
public static void swap(List list,
int i,
int j)
list - The list in which to swap elements.i - the index of one element to be swapped.j - the index of the other element to be swapped.
IndexOutOfBoundsException - if either i or j
is out of range (i < 0 || i >= list.size()
|| j < 0 || j >= list.size()).
private static void swap(Object[] arr,
int i,
int j)
public static void fill(List list,
Object obj)
This method runs in linear time.
list - the list to be filled with the specified element.obj - The element with which to fill the specified list.
UnsupportedOperationException - if the specified list or its
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.Comparablepublic 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 void rotate(List list,
int distance)
For example, suppose list comprises [t, a, n, k, s]. After invoking Collections.rotate(list, 1) (or Collections.rotate(list, -4)), list will comprise [s, t, a, n, k].
Note that this method can usefully be applied to sublists to move one or more elements within a list while preserving the order of the remaining elements. For example, the following idiom moves the element at index j forward to position k (which must be greater than or equal to j):
Collections.rotate(list.subList(j, k+1), -1);
To make this concrete, suppose list comprises
[a, b, c, d, e]. To move the element at index 1
(b) forward two positions, perform the following invocation:
Collections.rotate(l.subList(1, 4), -1);
The resulting list is [a, c, d, b, e].
To move more than one element forward, increase the absolute value of the rotation distance. To move elements backward, use a positive shift distance.
If the specified list is small or implements the RandomAccess interface, this implementation exchanges the first
element into the location it should go, and then repeatedly exchanges
the displaced element into the location it should go until a displaced
element is swapped into the first element. If necessary, the process
is repeated on the second and successive elements, until the rotation
is complete. If the specified list is large and doesn't implement the
RandomAccess interface, this implementation breaks the
list into two sublist views around index -distance mod size.
Then the reverse(List) method is invoked on each sublist view,
and finally it is invoked on the entire list. For a more complete
description of both algorithms, see Section 2.3 of Jon Bentley's
Programming Pearls (Addison-Wesley, 1986).
list - the list to be rotated.distance - the distance to rotate the list. There are no
constraints on this value; it may be zero, negative, or
greater than list.size().
UnsupportedOperationException - if the specified list or
its list-iterator does not support the set method.
private static void rotate1(List list,
int distance)
private static void rotate2(List list,
int distance)
public static boolean replaceAll(List list,
Object oldVal,
Object newVal)
list - the list in which replacement is to occur.oldVal - the old value to be replaced.newVal - the new value with which oldVal is to be
replaced.
UnsupportedOperationException - if the specified list or
its list-iterator does not support the set method.
public static int indexOfSubList(List source,
List target)
This implementation uses the "brute force" technique of scanning over the source list, looking for a match with the target at each location in turn.
source - the list in which to search for the first occurrence
of target.target - the list to search for as a subList of source.
public static int lastIndexOfSubList(List source,
List target)
This implementation uses the "brute force" technique of iterating over the source list, looking for a match with the target at each location in turn.
source - the list in which to search for the last occurrence
of target.target - the list to search for as a subList of source.
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. Similarly, the returned list will implement
RandomAccess if the specified list does.
the
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.
static Collection synchronizedCollection(Collection c,
Object mutex)
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.
static Set synchronizedSet(Set s,
Object mutex)
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.
static List synchronizedList(List list,
Object mutex)
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.
Comparablepublic static Enumeration enumeration(Collection c)
c - the collection for which an enumeration is to be returned.
Enumerationpublic static ArrayList list(Enumeration e)
e - enumeration providing elements for the returned
array list
Enumeration,
ArrayList
private static boolean eq(Object o1,
Object o2)
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