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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

Constructor Index

o Collections()

Method Index

o binarySearch(List, Object)
Searches the specified List for the specified Object using the binary search algorithm.
o binarySearch(List, Object, Comparator)
Searches the specified List for the specified Object using the binary search algorithm.
o enumeration(Collection)
Returns an Enumeration over the specified Collection.
o max(Collection)
Returns the maximum element of the given Collection, according to the natural comparison method of its elements.
o max(Collection, Comparator)
Returns the maximum element of the given Collection, according to the order induced by the specified Comparator.
o min(Collection)
Returns the minimum element of the given Collection, according to the natural comparison method of its elements.
o min(Collection, Comparator)
Returns the minimum element of the given Collection, according to the order induced by the specified Comparator.
o nCopies(int, Object)
Returns an immutable List consisting of n copies of the specified Object.
o sort(List)
Sorts the specified List into ascending order, according to the natural comparison method of its elements.
o sort(List, Comparator)
Sorts the specified List according to the order induced by the specified Comparator.
o 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).
o synchronizedCollection(Collection)
Returns a synchronized (thread-safe) Collection backed by the specified Collection.
o synchronizedList(List)
Returns a synchronized (thread-safe) List backed by the specified List.
o synchronizedMap(Map)
Returns a synchronized (thread-safe) Map backed by the specified Map.
o synchronizedSet(Set)
Returns a synchronized (thread-safe) Set backed by the specified Set.
o unmodifiableCollection(Collection)
Returns an unmodifiable view of the specified Collection.
o unmodifiableList(List)
Returns an unmodifiable view of the specified List.
o unmodifiableMap(Map)
Returns an unmodifiable view of the specified Map.
o unmodifiableSet(Set)
Returns an unmodifiable view of the specified Set.

Constructors

o Collections 
public Collections()

Methods

o 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
o 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
o 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
o 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
o 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
o 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
o 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
o 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
o 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.
o 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.
o 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.
o 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.
o 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.
o 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.
o 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.
o 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.
o 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.
o 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
o 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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