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Class com.sun.java.util.collections.Collections
java.lang.Object
|
+----com.sun.java.util.collections.Collections
- public class Collections
- extends Object
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.)
- See Also:
- Collection, Set, List, Map
-
EMPTY_LIST
- The empty list (immutable).
-
EMPTY_SET
- The empty set (immutable).
-
binarySearch(List, Object)
- Searches the specified list for the specified object using the binary
search algorithm.
-
binarySearch(List, Object, Comparator)
- Searches the specified list for the specified object using the binary
search algorithm.
-
copy(List, List)
- Copies all of the elements from one list into another.
-
enumeration(Collection)
- Returns an enumeration over the specified collection.
-
fill(List, Object)
- Replaces all of the elements of the specified list with the specified
element.
-
max(Collection)
- Returns the maximum element of the given collection, according to the
natural ordering of its elements.
-
max(Collection, Comparator)
- Returns the maximum element of the given collection, according to the
order induced by the specified comparator.
-
min(Collection)
- Returns the minimum element of the given collection, according to the
natural ordering of its elements.
-
min(Collection, Comparator)
- Returns the minimum element of the given collection, according to the
order induced by the specified comparator.
-
nCopies(int, Object)
- Returns an immutable list consisting of n copies of the
specified object.
-
reverse(List)
- Reverses the order of the elements in the specified list.
This method runs in linear time.
-
reverseOrder()
- Returns a comparator that imposes the reverse of the natural
ordering on a collection of objects that implement the
Comparable interface.
-
shuffle(List)
- Randomly permutes the specified list using a default source of
randomness.
-
shuffle(List, Random)
- Randomly permute the specified list using the specified source of
randomness.
-
singleton(Object)
- Returns an immutable set containing only the specified object.
-
sort(List)
- Sorts the specified list into ascending order, according to the
natural ordering of its elements.
-
sort(List, Comparator)
- Sorts the specified list according to the order induced by the
specified comparator.
-
synchronizedCollection(Collection)
- Returns a synchronized (thread-safe) collection backed by the specified
collection.
-
synchronizedCollection(Collection, Object)
-
-
synchronizedList(List)
- Returns a synchronized (thread-safe) list backed by the specified
list.
-
synchronizedList(List, Object)
-
-
synchronizedMap(Map)
- Returns a synchronized (thread-safe) map backed by the specified
map.
-
synchronizedSet(Set)
- Returns a synchronized (thread-safe) set backed by the specified
set.
-
synchronizedSet(Set, Object)
-
-
synchronizedSortedMap(SortedMap)
- Returns a synchronized (thread-safe) sorted map backed by the specified
sorted map.
-
synchronizedSortedSet(SortedSet)
- Returns a synchronized (thread-safe) sorted set backed by the specified
sorted set.
-
unmodifiableCollection(Collection)
- Returns an unmodifiable view of the specified collection.
-
unmodifiableList(List)
- Returns an unmodifiable view of the specified list.
-
unmodifiableMap(Map)
- Returns an unmodifiable view of the specified map.
-
unmodifiableSet(Set)
- Returns an unmodifiable view of the specified set.
-
unmodifiableSortedMap(SortedMap)
- Returns an unmodifiable view of the specified sorted map.
-
unmodifiableSortedSet(SortedSet)
- Returns an unmodifiable view of the specified sorted set.
EMPTY_SET
public static final Set EMPTY_SET
- The empty set (immutable). This set is serializable.
EMPTY_LIST
public static final List EMPTY_LIST
- The empty list (immutable). This list is serializable.
sort
public static void sort(List list)
- Sorts the specified list into ascending order, according to the
natural ordering of its elements. All elements in the list must
implement the Comparable interface. Furthermore, all elements
in the list must be mutually comparable (that is,
e1.compareTo(e2) must not throw a ClassCastException
for any elements e1 and e2 in the 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.
- Parameters:
- list - the list to be sorted.
- See Also:
- Comparable
sort
public static void sort(List list,
Comparator c)
- Sorts the specified list according to the order induced by the
specified comparator. All elements in the list must be mutually
comparable using the specified comparator (that is,
c.compare(e1, e2) must not throw a ClassCastException
for any elements e1 and e2 in the list).
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.
- Parameters:
- list - the list to be sorted.
- c - the comparator to determine the order of the array.
- See Also:
- Comparator
binarySearch
public static int binarySearch(List list,
Object key)
- Searches the specified list for the specified object using the binary
search algorithm. The list must be sorted into ascending order
according to the natural ordering of its elements (as by the
sort(List) method, above) prior to making this call. If it is
not sorted, the results are undefined. If the list contains multiple
elements equal to the specified object, there is no guarantee which one
will be found.
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.
- Parameters:
- list - the list to be searched.
- key - the key to be searched for.
- Returns:
- index of the search key, if it is contained in the list;
otherwise, (-(insertion point) - 1). The
insertion point is defined as the point at which the
key would be inserted into the list: the index of the first
element greater than the key, or list.size(), if all
elements in the list are less than the specified key. Note
that this guarantees that the return value will be >= 0 if
and only if the key is found.
- See Also:
- Comparable, sort
binarySearch
public static int binarySearch(List list,
Object key,
Comparator c)
- Searches the specified list for the specified object using the binary
search algorithm. The list must be sorted into ascending order
according to the specified comparator (as by the Sort(List,
Comparator) method, above), prior to making this call. If it is
not sorted, the results are undefined. If the list contains multiple
elements equal to the specified object, there is no guarantee which one
will be found.
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.
- Parameters:
- list - the list to be searched.
- key - the key to be searched for.
- c - the comparator by which the list is ordered.
- Returns:
- index of the search key, if it is contained in the list;
otherwise, (-(insertion point) - 1). The
insertion point is defined as the point at which the
key would be inserted into the list: the index of the first
element greater than the key, or list.size(), if all
elements in the list are less than the specified key. Note
that this guarantees that the return value will be >= 0 if
and only if the key is found.
- See Also:
- Comparable, sort
reverse
public static void reverse(List l)
- Reverses the order of the elements in the specified list.
This method runs in linear time.
- Parameters:
- list - the list whose elements are to be reversed.
shuffle
public static void shuffle(List list)
- Randomly permutes the specified list using a default source of
randomness. All permutations occur with approximately equal
likelihood.
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.
- Parameters:
- list - the list to be shuffled.
shuffle
public static void shuffle(List list,
Random rnd)
- Randomly permute the specified list using the specified source of
randomness. All permutations occur with equal likelihood
assuming that the source of randomness is fair.
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.
- Parameters:
- list - the list to be shuffled.
- r - the source of randomness to use to shuffle the list.
fill
public static void fill(List list,
Object o)
- Replaces all of the elements of the specified list with the specified
element.
This method runs in linear time.
- Parameters:
- list - the list to be filled with the specified element.
- o - The element with which to fill the specified list.
copy
public static void copy(List dest,
List src)
- Copies all of the elements from one list into another. After the
operation, the index of each copied element in the destination list
will be identical to its index in the source list. The destination
list must be at least as long as the source list. If it is longer, the
remaining elements in the destination list are unaffected.
This method runs in linear time.
- Parameters:
- dest - The destination list.
- src - The source list.
min
public static Object min(Collection coll)
- Returns the minimum element of the given collection, according to the
natural ordering of its elements. All elements in the
collection must implement the Comparable interface.
Furthermore, all elements in the collection must be mutually
comparable (that is, e1.compareTo(e2) must not throw a
ClassCastException for any elements e1 and
e2 in the collection).
This method iterates over the entire collection, hence it requires
time proportional to the size of the collection.
- Parameters:
- coll - the collection whose minimum element is to be determined.
- Returns:
- the minimum element of the given collection, according
to the natural ordering of its elements.
- See Also:
- Comparable
min
public static Object min(Collection coll,
Comparator comp)
- Returns the minimum element of the given collection, according to the
order induced by the specified comparator. All elements in the
collection must be mutually comparable by the specified
comparator (that is, comp.compare(e1, e2) must not throw a
ClassCastException for any elements e1 and
e2 in the collection).
This method iterates over the entire collection, hence it requires
time proportional to the size of the collection.
- Parameters:
- coll - the collection whose minimum element is to be determined.
- Returns:
- the minimum element of the given collection, according
to the specified comparator.
- See Also:
- Comparable
max
public static Object max(Collection coll)
- Returns the maximum element of the given collection, according to the
natural ordering of its elements. All elements in the
collection must implement the Comparable interface.
Furthermore, all elements in the collection must be mutually
comparable (that is, e1.compareTo(e2) must not throw a
ClassCastException for any elements e1 and
e2 in the collection).
This method iterates over the entire collection, hence it requires
time proportional to the size of the collection.
- Parameters:
- coll - the collection whose maximum element is to be determined.
- Returns:
- the maximum element of the given collection, according
to the natural ordering of its elements.
- See Also:
- Comparable
max
public static Object max(Collection coll,
Comparator comp)
- Returns the maximum element of the given collection, according to the
order induced by the specified comparator. All elements in the
collection must be mutually comparable by the specified
comparator (that is, comp.compare(e1, e2) must not throw a
ClassCastException for any elements e1 and
e2 in the collection).
This method iterates over the entire collection, hence it requires
time proportional to the size of the collection.
- Parameters:
- coll - the collection whose maximum element is to be determined.
- Returns:
- the maximum element of the given collection, according
to the specified comparator.
- See Also:
- Comparable
unmodifiableCollection
public static Collection unmodifiableCollection(Collection c)
- Returns an unmodifiable view of the specified collection. This method
allows modules to provide users with "read-only" access to internal
collections. Query operations on the returned collection "read through"
to the specified collection, and attempts to modify the returned
collection, whether direct or via its iterator, result in an
UnsupportedOperationException.
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.
- Parameters:
- c - the collection for which an unmodifiable view is to be
returned.
- Returns:
- an unmodifiable view of the specified collection.
unmodifiableSet
public static Set unmodifiableSet(Set s)
- Returns an unmodifiable view of the specified set. This method allows
modules to provide users with "read-only" access to internal sets.
Query operations on the returned set "read through" to the specified
set, and attempts to modify the returned set, whether direct or via its
iterator, result in an UnsupportedOperationException.
The returned set will be serializable if the specified set
is serializable.
- Parameters:
- s - the set for which an unmodifiable view is to be returned.
- Returns:
- an unmodifiable view of the specified set.
unmodifiableSortedSet
public static SortedSet unmodifiableSortedSet(SortedSet s)
- Returns an unmodifiable view of the specified sorted set. This method
allows modules to provide users with "read-only" access to internal
sorted sets. Query operations on the returned sorted set "read
through" to the specified sorted set. Attempts to modify the returned
sorted set, whether direct, via its iterator, or via its
subSet, headSet, or tailSet views, result in
an UnsupportedOperationException.
The returned sorted set will be serializable if the specified sorted set
is serializable.
- Parameters:
- s - the sorted set for which an unmodifiable view is to be
returned.
- Returns:
- an unmodifiable view of the specified sorted set.
unmodifiableList
public static List unmodifiableList(List list)
- Returns an unmodifiable view of the specified list. This method allows
modules to provide users with "read-only" access to internal
lists. Query operations on the returned list "read through" to the
specified list, and attempts to modify the returned list, whether
direct or via its iterator, result in an
UnsupportedOperationException.
The returned list will be serializable if the specified list
is serializable.
- Parameters:
- list - the list for which an unmodifiable view is to be returned.
- Returns:
- an unmodifiable view of the specified list.
unmodifiableMap
public static Map unmodifiableMap(Map m)
- Returns an unmodifiable view of the specified map. This method
allows modules to provide users with "read-only" access to internal
maps. Query operations on the returned map "read through"
to the specified map, and attempts to modify the returned
map, whether direct or via its collection views, result in an
UnsupportedOperationException.
The returned map will be serializable if the specified map
is serializable.
- Parameters:
- m - the map for which an unmodifiable view is to be returned.
- Returns:
- an unmodifiable view of the specified map.
unmodifiableSortedMap
public static SortedMap unmodifiableSortedMap(SortedMap m)
- Returns an unmodifiable view of the specified sorted map. This method
allows modules to provide users with "read-only" access to internal
sorted maps. Query operations on the returned sorted map "read through"
to the specified sorted map. Attempts to modify the returned
sorted map, whether direct, via its collection views, or via its
subMap, headMap, or tailMap views, result in
an UnsupportedOperationException.
The returned sorted map will be serializable if the specified sorted map
is serializable.
- Parameters:
- m - the sorted map for which an unmodifiable view is to be
returned.
- Returns:
- an unmodifiable view of the specified sorted map.
synchronizedCollection
public static Collection synchronizedCollection(Collection c)
- Returns a synchronized (thread-safe) collection backed by the specified
collection. In order to guarantee serial access, it is critical that
all access to the backing collection is accomplished
through the returned collection.
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.
- Parameters:
- c - the collection to be "wrapped" in a synchronized collection.
- Returns:
- a synchronized view of the specified collection.
synchronizedCollection
static Collection synchronizedCollection(Collection c,
Object mutex)
synchronizedSet
public static Set synchronizedSet(Set s)
- Returns a synchronized (thread-safe) set backed by the specified
set. In order to guarantee serial access, it is critical that
all access to the backing set is accomplished
through the returned set.
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.
- Parameters:
- s - the set to be "wrapped" in a synchronized set.
- Returns:
- a synchronized view of the specified set.
synchronizedSet
static Set synchronizedSet(Set s,
Object mutex)
synchronizedSortedSet
public static SortedSet synchronizedSortedSet(SortedSet s)
- Returns a synchronized (thread-safe) sorted set backed by the specified
sorted set. In order to guarantee serial access, it is critical that
all access to the backing sorted set is accomplished
through the returned sorted set (or its views).
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.
- Parameters:
- s - the sorted set to be "wrapped" in a synchronized sorted set.
- Returns:
- a synchronized view of the specified sorted set.
synchronizedList
public static List synchronizedList(List list)
- Returns a synchronized (thread-safe) list backed by the specified
list. In order to guarantee serial access, it is critical that
all access to the backing list is accomplished
through the returned 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.
- Parameters:
- list - the list to be "wrapped" in a synchronized list.
- Returns:
- a synchronized view of the specified list.
synchronizedList
static List synchronizedList(List list,
Object mutex)
synchronizedMap
public static Map synchronizedMap(Map m)
- Returns a synchronized (thread-safe) map backed by the specified
map. In order to guarantee serial access, it is critical that
all access to the backing map is accomplished
through the returned map.
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.
- Parameters:
- m - the map to be "wrapped" in a synchronized map.
- Returns:
- a synchronized view of the specified map.
synchronizedSortedMap
public static SortedMap synchronizedSortedMap(SortedMap m)
- Returns a synchronized (thread-safe) sorted map backed by the specified
sorted map. In order to guarantee serial access, it is critical that
all access to the backing sorted map is accomplished
through the returned sorted map (or its views).
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.
- Parameters:
- m - the sorted map to be "wrapped" in a synchronized sorted map.
- Returns:
- a synchronized view of the specified sorted map.
singleton
public static Set singleton(Object o)
- Returns an immutable set containing only the specified object.
The returned set is serializable.
- Returns:
- an immutable set containing only the specified object.
nCopies
public static List nCopies(int n,
Object o)
- Returns an immutable list consisting of n copies of the
specified object. The newly allocated data object is tiny (it contains
a single reference to the data object). This method is useful in
combination with the List.addAll method to grow lists.
The returned list is serializable.
- Parameters:
- n - the number of elements in the returned list.
- o - the element to appear repeatedly in the returned list.
- Returns:
- an immutable list consisting of n copies of the
specified object.
- See Also:
- addAll, addAll
reverseOrder
public static Comparator reverseOrder()
- Returns a comparator that imposes the reverse of the natural
ordering on a collection of objects that implement the
Comparable interface. (The natural ordering is the ordering
imposed by the objects' own compareTo method.) This enables a
simple idiom for sorting (or maintaining) collections (or arrays) of
objects that implement the Comparable interface in
reverse-natural-order. For example, suppose a is an array of
strings. Then:
Arrays.sort(a, Collections.reverseOrder());
sorts the array in reverse-lexicographic (alphabetical) order.
The returned comparator is serializable.
- Returns:
- a comparator that imposes the reverse of the natural
ordering on a collection of objects that implement
the Comparable interface.
- See Also:
- Comparable
enumeration
public static Enumeration enumeration(Collection c)
- Returns an enumeration over the specified collection. This provides
interoperatbility with legacy APIs that require an enumeration
as input.
- Parameters:
- c - the collection for which an enumeration is to be returned.
- Returns:
- an enumeration over the specified collection.
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