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Class java.util.Collections
java.lang.Object
|
+----java.util.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, "views" and "wrappers", which return a new collection
backed by a specified collection, and a few other odds and ends.
- Since:
- JDK1.2
- See Also:
- Collection, Set, List, Map
Collections()
-
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.
enumeration(Collection)
- Returns an Enumeration over the specified Collection.
max(Collection)
- Returns the maximum element of the given Collection, according
to the natural comparison method 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 comparison method 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.
sort(List)
- Sorts the specified List into ascending order, according
to the natural comparison method of its elements.
sort(List, Comparator)
- Sorts the specified List according to the order induced by the
specified Comparator.
subList(List, int, int)
- Returns a List backed by the specified List that represents the portion
of the specified List whose index ranges from fromIndex (inclusive) to
toIndex (exclusive).
synchronizedCollection(Collection)
- Returns a synchronized (thread-safe) Collection backed by the specified
Collection.
synchronizedList(List)
- Returns a synchronized (thread-safe) List backed by the specified
List.
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.
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.
Collections
public Collections()
sort
public static void sort(List list)
- Sorts the specified List into ascending order, according
to the natural comparison method 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
typeMismatchException for any elements e1 and e2 in the List).
The sorting algorithm is a tuned quicksort, adapted from Jon
L. Bentley and M. Douglas McIlroy's "Engineering a Sort Function",
Software-Practice and Experience, Vol. 23(11) P. 1249-1265 (November
1993). This algorithm offers n*log(n) performance on many data sets
that cause other quicksorts to degrade to quadratic performance.
The specified List must be modifiable, but need not be resizable.
This implementation dumps the specified List into an List, sorts
the array, and iterates over the List resetting each element
from the corresponding position in the array. This avoids the
n^2*log(n) performance that would result from attempting
to sort a LinkedList in place.
- Parameters:
- list - the List to be sorted.
- Throws:
ClassCastException
- List contains elements that are not
mutually comparable (for example, Strings and
Integers).
- Throws:
UnsupportedOperationException
- The specified List's
ListIterator not support the set operation.
- 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 by the specified comparator (that is,
comparator.compare(e1, e2) must not throw a typeMismatchException for
any elements e1 and e2 in the List).
The sorting algorithm is a tuned quicksort, adapted from Jon
L. Bentley and M. Douglas McIlroy's "Engineering a Sort Function",
Software-Practice and Experience, Vol. 23(11) P. 1249-1265 (November
1993). This algorithm offers n*log(n) performance on many data sets
that cause other quicksorts to degrade to quadratic 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
n^2*log(n) performance that would result from attempting
to sort a LinkedList in place.
- Parameters:
- list - the List to be sorted.
- Throws:
ClassCastException
- List contains elements that are not
mutually comparable with the specified Comparator.
- Throws:
UnsupportedOperationException
- The specified List's did
ListIterator not support the set operation.
- 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 comparison method of its elements (as by
Sort(List), above) prior to making this call. If it is not sorted,
the results are undefined: in particular, the call may enter an infinite
loop. If the List contains multiple elements equal to the specified
Object, there is no guarantee which instance will be found.
This method will run in log(n) time for a "random access" List (which
provides near-constant-time positional access) like a Vector. 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
is an instanceof AbstracSequentialList, 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.
- 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 the point at which the value would
be inserted into the List: the index of the first element
greater than the value, or list.size(), if all elements in
the List are less than the specified value. Note that this
guarantees that the return value will be >= 0 if and only
if the Object is found.
- Throws:
ClassCastException
- 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.
- 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 Sort(List, Comparator),
above), prior to making this call.
This method will run in log(n) time for a "random access" List (which
provides near-constant-time positional access) like a Vector. 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
is an instanceof AbstracSequentialList, 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.
- 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 the point at which the value would
be inserted into the List: the index of the first element
greater than the value, or list.size(), if all elements in
the List are less than the specified value. Note that this
guarantees that the return value will be >= 0 if and only
if the Object is found.
- Throws:
ClassCastException
- List contains elements that are not
mutually comparable with the specified Comparator,
or the search key in not mutually comparable with the
elements of the List using this Comparator.
- See Also:
- Comparable, sort
min
public static Object min(Collection coll)
- Returns the minimum element of the given Collection, according
to the natural comparison method 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
typeMismatchException 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.
- Throws:
ClassCastException
- Collection contains elements that are not
mutually comparable (for example, Strings and
Integers).
- Throws:
NoSuchElementException
- Collection is empty.
- 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, comparator.compare(e1, e2) must not throw a
typeMismatchException 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.
- Throws:
ClassCastException
- Collection contains elements that are not
mutually comparable with the specified Comparator.
- Throws:
NoSuchElementException
- Collection is empty.
- See Also:
- Comparable
max
public static Object max(Collection coll)
- Returns the maximum element of the given Collection, according
to the natural comparison method 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
typeMismatchException 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.
- Throws:
ClassCastException
- Collection contains elements that are not
mutually comparable (for example, Strings and
Integers).
- Throws:
NoSuchElementException
- Collection is empty.
- 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, comparator.compare(e1, e2) must not throw a
typeMismatchException 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.
- Throws:
ClassCastException
- Collection contains elements that are not
mutually comparable with the specified Comparator.
- Throws:
NoSuchElementException
- Collection is empty.
- See Also:
- Comparable
subList
public static List subList(List list,
int fromIndex,
int toIndex)
- Returns a List backed by the specified List that represents the portion
of the specified List whose index ranges from fromIndex (inclusive) to
toIndex (exclusive). The returned List is not resizable. (Its size
is fixed at (toIndex - fromIndex).) The returned List is mutable iff
the specified List is mutable. Changes to the returned List "write
through" to the specified List, and vice-versa.
If the caller wants a List that is independent of the input List,
and free of the restrictions noted above, he should immediately
copy the returned List into a new List, for example:
Vector v = new Vector(Collections.subList(myList, 17, 42));
- Parameters:
- list - the List whose subList is to be returned.
- fromIndex - the index (in this List) of the first element to
appear in the subList.
- toIndex - the index (in this List) following the last element to
appear in the subList.
- Throws:
ArrayIndexOutOfBoundsException
- fromIndex or toIndex is out
of range (fromIndex < 0 || fromIndex > size ||
toIndex < 0 || toIndex > size).
- Throws:
IllegalArgumentException
- fromIndex > toIndex.
unmodifiableCollection
public static Collection unmodifiableCollection(Collection c)
- Returns an unmodifiable view of the specified Collection. This method
allow 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 case that the backing
Collection is a Set or a List.
- Parameters:
- c - the Collection for which an unmodifiable view is to be
returned.
unmodifiableSet
public static Set unmodifiableSet(Set s)
- Returns an unmodifiable view of the specified Set. This method allow
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.
- Parameters:
- s - the Set for which an unmodifiable view is to be returned.
unmodifiableList
public static List unmodifiableList(List list)
- Returns an unmodifiable view of the specified List. This method allow
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.
- Parameters:
- list - the List for which an unmodifiable view is to be returned.
unmodifiableMap
public static Map unmodifiableMap(Map m)
- Returns an unmodifiable view of the specified Map. This method
allow 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.
- Parameters:
- m - the Map for which an unmodifiable view is to be returned.
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 user manually synchronize on the returned
Collection when iterating over it:
Collection c = 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 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.
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 user manually synchronize on the returned
Set when iterating over it:
Set s = 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.
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 user manually synchronize on the returned
List when iterating over it:
List list = 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.
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 user manually synchronize on the returned
Map when iterating over any of its Collection views:
Map m = 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.
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 List.addAll to grow Lists.
- Parameters:
- n - the number of elements in the returned List.
- o - the element to appear repeatedly in the returned List.
- Throws:
IllegalArgumentException
- n < 0.
- See Also:
- addAll, addAll
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.
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