Java Doc for Collections.java in  » 6.0-JDK-Core » Collections-Jar-Zip-Logging-regex » java » util » Java Source Code / Java DocumentationJava Source Code and Java Documentation

The methods of this class all throw a NullPointerException if the collections or class objects 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.

This class is a member of the Java Collections Framework.
author:
   Josh Bloch
author:
   Neal Gafter
version:
   1.116, 07/23/07
See Also:   Collection
See Also:   Set
See Also:   List
See Also:   Map
since:
   1.2

Care must be exercised if this method is used on collections that do not comply with the general contract for Collection. Implementations may elect to iterate over either collection and test for containment in the other collection (or to perform any equivalent computation).

This method runs in linear time.

When elements are specified individually, this method provides a convenient way to add a few elements to an existing collection:

 Collections.addAll(flavors, "Peaches 'n Plutonium", "Rocky Racoon");
 

Each method invocation on the queue returned by this method results in exactly one method invocation on the backing deque, with one exception. The Queue.addAll addAll method is implemented as a sequence of Deque.addFirst addFirst invocations on the backing deque.
Parameters:
  deque - the deque the queue
since:
   1.6

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 interface and is large, this method will do an iterator-based binary search that performs O(n) link traversals and O(log n) element comparisons.
Parameters:
  list - the list to be searched.
Parameters:
  key - the key to be searched for. the index of the search key, if it is contained in the list;otherwise, (-(insertion point) - 1). Theinsertion point is defined as the point at which thekey would be inserted into the list: the index of the firstelement greater than the key, or list.size() if allelements in the list are less than the specified key. Notethat this guarantees that the return value will be >= 0 ifand only if the key is found.
throws:
  ClassCastException - if the list contains elements that are notmutually comparable (for example, strings andintegers), or the search key is not mutually comparablewith the elements of the list.

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 interface and is large, this method will do an iterator-based binary search that performs O(n) link traversals and O(log n) element comparisons.
Parameters:
  list - the list to be searched.
Parameters:
  key - the key to be searched for.
Parameters:
  c - the comparator by which the list is ordered.A null value indicates that the elements' should be used. the index of the search key, if it is contained in the list;otherwise, (-(insertion point) - 1). Theinsertion point is defined as the point at which thekey would be inserted into the list: the index of the firstelement greater than the key, or list.size() if allelements in the list are less than the specified key. Notethat this guarantees that the return value will be >= 0 ifand only if the key is found.
throws:
  ClassCastException - if the list contains elements that are notmutually comparable using the specified comparator,or the search key is not mutually comparable with theelements of the list using this comparator.

The generics mechanism in the language provides compile-time (static) type checking, but it is possible to defeat this mechanism with unchecked casts. Usually this is not a problem, as the compiler issues warnings on all such unchecked operations. There are, however, times when static type checking alone is not sufficient. For example, suppose a collection is passed to a third-party library and it is imperative that the library code not corrupt the collection by inserting an element of the wrong type.

Another use of dynamically typesafe views is debugging. Suppose a program fails with a ClassCastException, indicating that an incorrectly typed element was put into a parameterized collection. Unfortunately, the exception can occur at any time after the erroneous element is inserted, so it typically provides little or no information as to the real source of the problem. If the problem is reproducible, one can quickly determine its source by temporarily modifying the program to wrap the collection with a dynamically typesafe view. For example, this declaration:

 Collection<String> c = new HashSet<String>();
 

 Collection<String> c = Collections.checkedCollection(
 new HashSet<String>(), String.class);
 

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 a dynamically typesafe view is to bereturned
Parameters:
  type - the type of element that c is permitted to hold a dynamically typesafe view of the specified collection
since:
   1.5

A discussion of the use of dynamically typesafe views may be found in the documentation for the Collections.checkedCollection checkedCollection method.

The returned list will be serializable if the specified list is serializable.
Parameters:
  list - the list for which a dynamically typesafe view is to bereturned
Parameters:
  type - the type of element that list is permitted to hold a dynamically typesafe view of the specified list
since:
   1.5

Assuming a map contains no incorrectly typed keys or values prior to the time a dynamically typesafe view is generated, and that all subsequent access to the map takes place through the view (or one of its collection views), it is guaranteed that the map cannot contain an incorrectly typed key or value.

A discussion of the use of dynamically typesafe views may be found in the documentation for the Collections.checkedCollection checkedCollection method.

The returned map will be serializable if the specified map is serializable.
Parameters:
  m - the map for which a dynamically typesafe view is to bereturned
Parameters:
  keyType - the type of key that m is permitted to hold
Parameters:
  valueType - the type of value that m is permitted to hold a dynamically typesafe view of the specified map
since:
   1.5

A discussion of the use of dynamically typesafe views may be found in the documentation for the Collections.checkedCollection checkedCollection method.

The returned set will be serializable if the specified set is serializable.
Parameters:
  s - the set for which a dynamically typesafe view is to bereturned
Parameters:
  type - the type of element that s is permitted to hold a dynamically typesafe view of the specified set
since:
   1.5

Assuming a map contains no incorrectly typed keys or values prior to the time a dynamically typesafe view is generated, and that all subsequent access to the map takes place through the view (or one of its collection views), it is guaranteed that the map cannot contain an incorrectly typed key or value.

A discussion of the use of dynamically typesafe views may be found in the documentation for the Collections.checkedCollection checkedCollection method.

The returned map will be serializable if the specified map is serializable.
Parameters:
  m - the map for which a dynamically typesafe view is to bereturned
Parameters:
  keyType - the type of key that m is permitted to hold
Parameters:
  valueType - the type of value that m is permitted to hold a dynamically typesafe view of the specified map
since:
   1.5

A discussion of the use of dynamically typesafe views may be found in the documentation for the Collections.checkedCollection checkedCollection method.

The returned sorted set will be serializable if the specified sorted set is serializable.
Parameters:
  s - the sorted set for which a dynamically typesafe view is to bereturned
Parameters:
  type - the type of element that s is permitted to hold a dynamically typesafe view of the specified sorted set
since:
   1.5

This method runs in linear time.
Parameters:
  dest - The destination list.
Parameters:
  src - The source list.
throws:
  IndexOutOfBoundsException - if the destination list is too smallto contain the entire source List.
throws:
  UnsupportedOperationException - if the destination list'slist-iterator does not support the set operation.

Care must be exercised if this method is used on collections that do not comply with the general contract for Collection. Implementations may elect to iterate over either collection and test for containment in the other collection (or to perform any equivalent computation). If either collection uses a nonstandard equality test (as does a SortedSet whose ordering is not compatible with equals, or the key set of an IdentityHashMap ), both collections must use the same nonstandard equality test, or the result of this method is undefined.

Note that it is permissible to pass the same collection in both parameters, in which case the method will return true if and only if the collection is empty.
Parameters:
  c1 - a collection
Parameters:
  c2 - a collection
throws:
  NullPointerException - if either collection is null
since:
   1.5

• Enumeration.hasMoreElements hasMoreElements always returns false .
• Enumeration.nextElement nextElement always throws NoSuchElementException .

Implementations of this method are permitted, but not required, to return the same object from multiple invocations. an empty enumeration
since:
   1.7

• Iterator.hasNext hasNext always returns false .
• Iterator.next next always throws NoSuchElementException .
• Iterator.remove remove always throws IllegalStateException .

Implementations of this method are permitted, but not required, to return the same object from multiple invocations. an empty iterator
since:
   1.7

This example illustrates the type-safe way to obtain an empty list:

 List<String> s = Collections.emptyList();
 

• Iterator.hasNext hasNext and ListIterator.hasPrevious hasPrevious always return false .
• Iterator.next next and ListIterator.previousprevious always throw NoSuchElementException .
• Iterator.remove remove and ListIterator.setset always throw IllegalStateException .
• ListIterator.add add always throws UnsupportedOperationException .
• ListIterator.nextIndex nextIndex always returns 0 .
• ListIterator.previousIndex previousIndex always returns -1 .

Implementations of this method are permitted, but not required, to return the same object from multiple invocations. an empty list iterator
since:
   1.7

This example illustrates the type-safe way to obtain an empty set:

 Map<String, Date> s = Collections.emptyMap();
 

This example illustrates the type-safe way to obtain an empty set:

 Set<String> s = Collections.emptySet();
 

This method runs in linear time.
Parameters:
  list - the list to be filled with the specified element.
Parameters:
  obj - The element with which to fill the specified list.
throws:
  UnsupportedOperationException - if the specified list or itslist-iterator does not support the set operation.

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.
Parameters:
  source - the list in which to search for the first occurrenceof target.
Parameters:
  target - the list to search for as a subList of source. the starting position of the first occurrence of the specifiedtarget list within the specified source list, or -1 if thereis no such occurrence.
since:
   1.4

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.
Parameters:
  source - the list in which to search for the last occurrenceof target.
Parameters:
  target - the list to search for as a subList of source. the starting position of the last occurrence of the specifiedtarget list within the specified source list, or -1 if thereis no such occurrence.
since:
   1.4

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. the maximum element of the given collection, accordingto the natural ordering of its elements.
throws:
  ClassCastException - if the collection contains elements that arenot mutually comparable (for example, strings andintegers).
throws:
  NoSuchElementException - if the collection is empty.
See Also:   Comparable

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.
Parameters:
  comp - the comparator with which to determine the maximum element.A null value indicates that the elements' naturalordering should be used. the maximum element of the given collection, accordingto the specified comparator.
throws:
  ClassCastException - if the collection contains elements that arenot mutually comparable using the specified comparator.
throws:
  NoSuchElementException - if the collection is empty.
See Also:   Comparable

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. the minimum element of the given collection, accordingto the natural ordering of its elements.
throws:
  ClassCastException - if the collection contains elements that arenot mutually comparable (for example, strings andintegers).
throws:
  NoSuchElementException - if the collection is empty.
See Also:   Comparable

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.
Parameters:
  comp - the comparator with which to determine the minimum element.A null value indicates that the elements' naturalordering should be used. the minimum element of the given collection, accordingto the specified comparator.
throws:
  ClassCastException - if the collection contains elements that arenot mutually comparable using the specified comparator.
throws:
  NoSuchElementException - if the collection is empty.
See Also:   Comparable

Each method invocation on the set returned by this method results in exactly one method invocation on the backing map or its keySet view, with one exception. The addAll method is implemented as a sequence of put invocations on the backing map.

The specified map must be empty at the time this method is invoked, and should not be accessed directly after this method returns. These conditions are ensured if the map is created empty, passed directly to this method, and no reference to the map is retained, as illustrated in the following code fragment:

 Set<Object> weakHashSet = Collections.newSetFromMap(
 new WeakHashMap<Object, Boolean>());
 

This method runs in linear time.
Parameters:
  list - the list whose elements are to be reversed.
throws:
  UnsupportedOperationException - if the specified list orits list-iterator does not support the set operation.

 Arrays.sort(a, Collections.reverseOrder());
 

The returned comparator is serializable. a comparator that imposes the reverse of the naturalordering on a collection of objects that implementthe Comparable interface.
See Also:   Comparable

The returned comparator is serializable (assuming the specified comparator is also serializable or null). a comparator that imposes the reverse ordering of thespecified comparator
since:
   1.5

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);
 

 Collections.rotate(l.subList(1, 4), -1);
 

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 Collections.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).
Parameters:
  list - the list to be rotated.
Parameters:
  distance - the distance to rotate the list. There are noconstraints on this value; it may be zero, negative, orgreater than list.size().
throws:
  UnsupportedOperationException - if the specified list orits list-iterator does not support the set operation.
since:
   1.4

The hedge "approximately" is used in the foregoing description because default source of randomness 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.
Parameters:
  list - the list to be shuffled.
throws:
  UnsupportedOperationException - if the specified list orits list-iterator does not support the set operation.

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.
Parameters:
  list - the list to be shuffled.
Parameters:
  rnd - the source of randomness to use to shuffle the list.
throws:
  UnsupportedOperationException - if the specified list or itslist-iterator does not support the set operation.

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.
Parameters:
  list - the list to be sorted.
throws:
  ClassCastException - if the list contains elements that are notmutually comparable (for example, strings and integers).
throws:
  UnsupportedOperationException - if the specified list'slist-iterator does not support the set operation.
See Also:   Comparable

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.
Parameters:
  list - the list to be sorted.
Parameters:
  c - the comparator to determine the order of the list. Anull value indicates that the elements' naturalordering should be used.
throws:
  ClassCastException - if the list contains elements that are notmutually comparable using the specified comparator.
throws:
  UnsupportedOperationException - if the specified list'slist-iterator does not support the set operation.
See Also:   Comparator

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());
 }
 

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. a synchronized view of the specified collection.

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());
 }
 

The returned list will be serializable if the specified list is serializable.
Parameters:
  list - the list to be "wrapped" in a synchronized list. a synchronized view of the specified list.

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());
 }
 

The returned map will be serializable if the specified map is serializable.
Parameters:
  m - the map to be "wrapped" in a synchronized map. a synchronized view of the specified map.

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());
 }
 

The returned set will be serializable if the specified set is serializable.
Parameters:
  s - the set to be "wrapped" in a synchronized set. a synchronized view of the specified set.

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 TreeMap());
 ...
 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());
 }
 

 SortedMap m = Collections.synchronizedSortedMap(new TreeMap());
 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());
 }
 

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. a synchronized view of the specified sorted map.

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 TreeSet());
 ...
 synchronized(s) {
 Iterator i = s.iterator(); // Must be in the synchronized block
 while (i.hasNext())
 foo(i.next());
 }
 

 SortedSet s = Collections.synchronizedSortedSet(new TreeSet());
 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());
 }
 

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. a synchronized view of the specified sorted set.

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 bereturned. an unmodifiable view of the specified collection.

The returned list will be serializable if the specified list is serializable. Similarly, the returned list will implement RandomAccess if the specified list does.
Parameters:
  list - the list for which an unmodifiable view is to be returned. an unmodifiable view of the specified list.

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. an unmodifiable view of the specified map.

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. an unmodifiable view of the specified set.

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 bereturned. an unmodifiable view of the specified sorted map.

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 bereturned. an unmodifiable view of the specified sorted set.