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 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.

See Also

• Collection
• Set
• List
• Map

Field Summary

public static final List	EMPTY_LIST	The empty list (immutable).
public static final Map	EMPTY_MAP	The empty map (immutable).
public static final Set	EMPTY_SET	The empty set (immutable).

Public Method Summary

static <T> boolean	addAll(Collection<? super T> c, T... elements) Adds all of the specified elements to the specified collection.
static <T> Queue<T>	asLifoQueue(Deque<T> deque) Returns a view of a Deque as a Last-in-first-out (Lifo) Queue.
static <T> int	binarySearch(List<? extends T> list, T key, Comparator<? super T> c) Searches the specified list for the specified object using the binary search algorithm.
static <T> int	binarySearch(List<? extends Comparable<? super T>> list, T key) Searches the specified list for the specified object using the binary search algorithm.
static <E> Collection<E>	checkedCollection(Collection<E> c, Class<E> type) Returns a dynamically typesafe view of the specified collection.
static <E> List<E>	checkedList(List<E> list, Class<E> type) Returns a dynamically typesafe view of the specified list.
static <K, V> Map<K, V>	checkedMap(Map<K, V> m, Class<K> keyType, Class<V> valueType) Returns a dynamically typesafe view of the specified map.
static <K, V> NavigableMap<K, V>	checkedNavigableMap(NavigableMap<K, V> m, Class<K> keyType, Class<V> valueType) Returns a dynamically typesafe view of the specified navigable map.
static <E> NavigableSet<E>	checkedNavigableSet(NavigableSet<E> s, Class<E> type) Returns a dynamically typesafe view of the specified navigable set.
static <E> Queue<E>	checkedQueue(Queue<E> queue, Class<E> type) Returns a dynamically typesafe view of the specified queue.
static <E> Set<E>	checkedSet(Set<E> s, Class<E> type) Returns a dynamically typesafe view of the specified set.
static <K, V> SortedMap<K, V>	checkedSortedMap(SortedMap<K, V> m, Class<K> keyType, Class<V> valueType) Returns a dynamically typesafe view of the specified sorted map.
static <E> SortedSet<E>	checkedSortedSet(SortedSet<E> s, Class<E> type) Returns a dynamically typesafe view of the specified sorted set.
static <T> void	copy(List<? super T> dest, List<? extends T> src) Copies all of the elements from one list into another.
static boolean	disjoint(Collection<?> c1, Collection<?> c2) Returns true if the two specified collections have no elements in common.
static <T> Enumeration<T>	emptyEnumeration() Returns an enumeration that has no elements.
static <T> Iterator<T>	emptyIterator() Returns an iterator that has no elements.
final static <T> List<T>	emptyList() Returns an empty list (immutable).
static <T> ListIterator<T>	emptyListIterator() Returns a list iterator that has no elements.
final static <K, V> Map<K, V>	emptyMap() Returns an empty map (immutable).
final static <K, V> NavigableMap<K, V>	emptyNavigableMap() Returns an empty navigable map (immutable).
static <E> NavigableSet<E>	emptyNavigableSet() Returns an empty navigable set (immutable).
final static <T> Set<T>	emptySet() Returns an empty set (immutable).
final static <K, V> SortedMap<K, V>	emptySortedMap() Returns an empty sorted map (immutable).
static <E> SortedSet<E>	emptySortedSet() Returns an empty sorted set (immutable).
static <T> Enumeration<T>	enumeration(Collection<T> c) Returns an enumeration over the specified collection.
static <T> void	fill(List<? super T> list, T obj) Replaces all of the elements of the specified list with the specified element.
static int	frequency(Collection<?> c, Object o) Returns the number of elements in the specified collection equal to the specified object.
static int	indexOfSubList(List<?> source, List<?> target) Returns the starting position of the first occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence.
static int	lastIndexOfSubList(List<?> source, List<?> target) Returns the starting position of the last occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence.
static <T> ArrayList<T>	list(Enumeration<T> e) Returns an array list containing the elements returned by the specified enumeration in the order they are returned by the enumeration.
static <T> T	max(Collection<? extends T> coll, Comparator<? super T> comp) Returns the maximum element of the given collection, according to the order induced by the specified comparator.
static <T extends Object & Comparable<? super T>> T	max(Collection<? extends T> coll) Returns the maximum element of the given collection, according to the natural ordering of its elements.
static <T extends Object & Comparable<? super T>> T	min(Collection<? extends T> coll) Returns the minimum element of the given collection, according to the natural ordering of its elements.
static <T> T	min(Collection<? extends T> coll, Comparator<? super T> comp) Returns the minimum element of the given collection, according to the order induced by the specified comparator.
static <T> List<T>	nCopies(int n, T o) Returns an immutable list consisting of n copies of the specified object.
static <E> Set<E>	newSetFromMap(Map<E, Boolean> map) Returns a set backed by the specified map.
static <T> boolean	replaceAll(List<T> list, T oldVal, T newVal) Replaces all occurrences of one specified value in a list with another.
static void	reverse(List<?> list) Reverses the order of the elements in the specified list.
static <T> Comparator<T>	reverseOrder() Returns a comparator that imposes the reverse of the natural ordering on a collection of objects that implement the Comparable interface.
static <T> Comparator<T>	reverseOrder(Comparator<T> cmp) Returns a comparator that imposes the reverse ordering of the specified comparator.
static void	rotate(List<?> list, int distance) Rotates the elements in the specified list by the specified distance.
static void	shuffle(List<?> list, Random rnd) Randomly permute the specified list using the specified source of randomness.
static void	shuffle(List<?> list) Randomly permutes the specified list using a default source of randomness.
static <T> Set<T>	singleton(T o) Returns an immutable set containing only the specified object.
static <T> List<T>	singletonList(T o) Returns an immutable list containing only the specified object.
static <K, V> Map<K, V>	singletonMap(K key, V value) Returns an immutable map, mapping only the specified key to the specified value.
static <T extends Comparable<? super T>> void	sort(List<T> list) Sorts the specified list into ascending order, according to the {@linkplain Comparable natural ordering} of its elements.
static <T> void	sort(List<T> list, Comparator<? super T> c) Sorts the specified list according to the order induced by the specified comparator.
static void	swap(List<?> list, int i, int j) Swaps the elements at the specified positions in the specified list.
static <T> Collection<T>	synchronizedCollection(Collection<T> c) Returns a synchronized (thread-safe) collection backed by the specified collection.
static <T> List<T>	synchronizedList(List<T> list) Returns a synchronized (thread-safe) list backed by the specified list.
static <K, V> Map<K, V>	synchronizedMap(Map<K, V> m) Returns a synchronized (thread-safe) map backed by the specified map.
static <K, V> NavigableMap<K, V>	synchronizedNavigableMap(NavigableMap<K, V> m) Returns a synchronized (thread-safe) navigable map backed by the specified navigable map.
static <T> NavigableSet<T>	synchronizedNavigableSet(NavigableSet<T> s) Returns a synchronized (thread-safe) navigable set backed by the specified navigable set.
static <T> Set<T>	synchronizedSet(Set<T> s) Returns a synchronized (thread-safe) set backed by the specified set.
static <K, V> SortedMap<K, V>	synchronizedSortedMap(SortedMap<K, V> m) Returns a synchronized (thread-safe) sorted map backed by the specified sorted map.
static <T> SortedSet<T>	synchronizedSortedSet(SortedSet<T> s) Returns a synchronized (thread-safe) sorted set backed by the specified sorted set.
static <T> Collection<T>	unmodifiableCollection(Collection<? extends T> c) Returns an unmodifiable view of the specified collection.
static <T> List<T>	unmodifiableList(List<? extends T> list) Returns an unmodifiable view of the specified list.
static <K, V> Map<K, V>	unmodifiableMap(Map<? extends K, ? extends V> m) Returns an unmodifiable view of the specified map.
static <K, V> NavigableMap<K, V>	unmodifiableNavigableMap(NavigableMap<K, ? extends V> m) Returns an unmodifiable view of the specified navigable map.
static <T> NavigableSet<T>	unmodifiableNavigableSet(NavigableSet<T> s) Returns an unmodifiable view of the specified navigable set.
static <T> Set<T>	unmodifiableSet(Set<? extends T> s) Returns an unmodifiable view of the specified set.
static <K, V> SortedMap<K, V>	unmodifiableSortedMap(SortedMap<K, ? extends V> m) Returns an unmodifiable view of the specified sorted map.
static <T> SortedSet<T>	unmodifiableSortedSet(SortedSet<T> s) Returns an unmodifiable view of the specified sorted set.

Inherited Method Summary

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

Fields

public static final List EMPTY_LIST

The empty list (immutable). This list is serializable.

See Also

• emptyList()

public static final Map EMPTY_MAP

The empty map (immutable). This map is serializable.

See Also

• emptyMap()

public static final Set EMPTY_SET

The empty set (immutable). This set is serializable.

See Also

• emptySet()

Public Methods

public static boolean addAll (Collection<? super T> c, T... elements)

Adds all of the specified elements to the specified collection. Elements to be added may be specified individually or as an array. The behavior of this convenience method is identical to that of c.addAll(Arrays.asList(elements)), but this method is likely to run significantly faster under most implementations.

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");
 

Parameters

c	the collection into which elements are to be inserted
elements	the elements to insert into c

Returns

• true if the collection changed as a result of the call

Throws

UnsupportedOperationException	if c does not support the add operation
NullPointerException	if elements contains one or more null values and c does not permit null elements, or if c or elements are null
IllegalArgumentException	if some property of a value in elements prevents it from being added to c

See Also

• Collection.addAll(Collection)

public static Queue<T> asLifoQueue (Deque<T> deque)

Returns a view of a Deque as a Last-in-first-out (Lifo) Queue. Method add is mapped to push, remove is mapped to pop and so on. This view can be useful when you would like to use a method requiring a Queue but you need Lifo ordering.

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

Parameters

deque	the deque

Returns

• the queue

public static int binarySearch (List<? extends T> list, T key, Comparator<? super T> 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), 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). 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.
key	the key to be searched for.
c	the comparator by which the list is ordered. A null value indicates that the elements' {@linkplain Comparable natural ordering} should be used.

Returns

• the 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.

Throws

ClassCastException	if the list contains elements that are not mutually comparable using the specified comparator, or the search key is not mutually comparable with the elements of the list using this comparator.

public static int binarySearch (List<? extends Comparable<? super T>> list, T 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 {@linkplain Comparable natural ordering} of its elements (as by the sort(List) method) 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). 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.
key	the key to be searched for.

Returns

• the 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.

Throws

ClassCastException	if the list contains elements that are not mutually comparable (for example, strings and integers), or the search key is not mutually comparable with the elements of the list.

public static Collection<E> checkedCollection (Collection<E> c, Class<E> type)

Returns a dynamically typesafe view of the specified collection. Any attempt to insert an element of the wrong type will result in an immediate ClassCastException. Assuming a collection contains no incorrectly typed elements prior to the time a dynamically typesafe view is generated, and that all subsequent access to the collection takes place through the view, it is guaranteed that the collection cannot contain an incorrectly typed element.

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<>();
 

may be replaced temporarily by this one:

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

Running the program again will cause it to fail at the point where an incorrectly typed element is inserted into the collection, clearly identifying the source of the problem. Once the problem is fixed, the modified declaration may be reverted back to the original.

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.

Since null is considered to be a value of any reference type, the returned collection permits insertion of null elements whenever the backing collection does.

Parameters

c	the collection for which a dynamically typesafe view is to be returned
type	the type of element that c is permitted to hold

Returns

• a dynamically typesafe view of the specified collection

public static List<E> checkedList (List<E> list, Class<E> type)

Returns a dynamically typesafe view of the specified list. Any attempt to insert an element of the wrong type will result in an immediate ClassCastException. Assuming a list contains no incorrectly typed elements prior to the time a dynamically typesafe view is generated, and that all subsequent access to the list takes place through the view, it is guaranteed that the list cannot contain an incorrectly typed element.

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

The returned list will be serializable if the specified list is serializable.

Since null is considered to be a value of any reference type, the returned list permits insertion of null elements whenever the backing list does.

Parameters

list	the list for which a dynamically typesafe view is to be returned
type	the type of element that list is permitted to hold

Returns

• a dynamically typesafe view of the specified list

public static Map<K, V> checkedMap (Map<K, V> m, Class<K> keyType, Class<V> valueType)

Returns a dynamically typesafe view of the specified map. Any attempt to insert a mapping whose key or value have the wrong type will result in an immediate ClassCastException. Similarly, any attempt to modify the value currently associated with a key will result in an immediate ClassCastException, whether the modification is attempted directly through the map itself, or through a Map.Entry instance obtained from the map's entry set view.

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 checkedCollection method.

The returned map will be serializable if the specified map is serializable.

Since null is considered to be a value of any reference type, the returned map permits insertion of null keys or values whenever the backing map does.

Parameters

m	the map for which a dynamically typesafe view is to be returned
keyType	the type of key that m is permitted to hold
valueType	the type of value that m is permitted to hold

Returns

• a dynamically typesafe view of the specified map

public static NavigableMap<K, V> checkedNavigableMap (NavigableMap<K, V> m, Class<K> keyType, Class<V> valueType)

Returns a dynamically typesafe view of the specified navigable map. Any attempt to insert a mapping whose key or value have the wrong type will result in an immediate ClassCastException. Similarly, any attempt to modify the value currently associated with a key will result in an immediate ClassCastException, whether the modification is attempted directly through the map itself, or through a Map.Entry instance obtained from the map's entry set view.

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 checkedCollection method.

The returned map will be serializable if the specified map is serializable.

Since null is considered to be a value of any reference type, the returned map permits insertion of null keys or values whenever the backing map does.

Parameters

m	the map for which a dynamically typesafe view is to be returned
keyType	the type of key that m is permitted to hold
valueType	the type of value that m is permitted to hold

Returns

• a dynamically typesafe view of the specified map

public static NavigableSet<E> checkedNavigableSet (NavigableSet<E> s, Class<E> type)

Returns a dynamically typesafe view of the specified navigable set. Any attempt to insert an element of the wrong type will result in an immediate ClassCastException. Assuming a navigable set contains no incorrectly typed elements prior to the time a dynamically typesafe view is generated, and that all subsequent access to the navigable set takes place through the view, it is guaranteed that the navigable set cannot contain an incorrectly typed element.

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

The returned navigable set will be serializable if the specified navigable set is serializable.

Since null is considered to be a value of any reference type, the returned navigable set permits insertion of null elements whenever the backing sorted set does.

Parameters

s	the navigable set for which a dynamically typesafe view is to be returned
type	the type of element that s is permitted to hold

Returns

• a dynamically typesafe view of the specified navigable set

public static Queue<E> checkedQueue (Queue<E> queue, Class<E> type)

Returns a dynamically typesafe view of the specified queue. Any attempt to insert an element of the wrong type will result in an immediate ClassCastException. Assuming a queue contains no incorrectly typed elements prior to the time a dynamically typesafe view is generated, and that all subsequent access to the queue takes place through the view, it is guaranteed that the queue cannot contain an incorrectly typed element.

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

The returned queue will be serializable if the specified queue is serializable.

Since null is considered to be a value of any reference type, the returned queue permits insertion of null elements whenever the backing queue does.

Parameters

queue	the queue for which a dynamically typesafe view is to be returned
type	the type of element that queue is permitted to hold

Returns

• a dynamically typesafe view of the specified queue

public static Set<E> checkedSet (Set<E> s, Class<E> type)

Returns a dynamically typesafe view of the specified set. Any attempt to insert an element of the wrong type will result in an immediate ClassCastException. Assuming a set contains no incorrectly typed elements prior to the time a dynamically typesafe view is generated, and that all subsequent access to the set takes place through the view, it is guaranteed that the set cannot contain an incorrectly typed element.

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

The returned set will be serializable if the specified set is serializable.

Since null is considered to be a value of any reference type, the returned set permits insertion of null elements whenever the backing set does.

Parameters

s	the set for which a dynamically typesafe view is to be returned
type	the type of element that s is permitted to hold

Returns

• a dynamically typesafe view of the specified set

public static SortedMap<K, V> checkedSortedMap (SortedMap<K, V> m, Class<K> keyType, Class<V> valueType)

Returns a dynamically typesafe view of the specified sorted map. Any attempt to insert a mapping whose key or value have the wrong type will result in an immediate ClassCastException. Similarly, any attempt to modify the value currently associated with a key will result in an immediate ClassCastException, whether the modification is attempted directly through the map itself, or through a Map.Entry instance obtained from the map's entry set view.

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 checkedCollection method.

The returned map will be serializable if the specified map is serializable.

Since null is considered to be a value of any reference type, the returned map permits insertion of null keys or values whenever the backing map does.

Parameters

m	the map for which a dynamically typesafe view is to be returned
keyType	the type of key that m is permitted to hold
valueType	the type of value that m is permitted to hold

Returns

• a dynamically typesafe view of the specified map

public static SortedSet<E> checkedSortedSet (SortedSet<E> s, Class<E> type)

Returns a dynamically typesafe view of the specified sorted set. Any attempt to insert an element of the wrong type will result in an immediate ClassCastException. Assuming a sorted set contains no incorrectly typed elements prior to the time a dynamically typesafe view is generated, and that all subsequent access to the sorted set takes place through the view, it is guaranteed that the sorted set cannot contain an incorrectly typed element.

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

The returned sorted set will be serializable if the specified sorted set is serializable.

Since null is considered to be a value of any reference type, the returned sorted set permits insertion of null elements whenever the backing sorted set does.

Parameters

s	the sorted set for which a dynamically typesafe view is to be returned
type	the type of element that s is permitted to hold

Returns

• a dynamically typesafe view of the specified sorted set

public static void copy (List<? super T> dest, List<? extends T> 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.

Throws

IndexOutOfBoundsException	if the destination list is too small to contain the entire source List.
UnsupportedOperationException	if the destination list's list-iterator does not support the set operation.

public static boolean disjoint (Collection<?> c1, Collection<?> c2)

Returns true if the two specified collections have no elements in common.

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.

Care must also be exercised when using collections that have restrictions on the elements that they may contain. Collection implementations are allowed to throw exceptions for any operation involving elements they deem ineligible. For absolute safety the specified collections should contain only elements which are eligible elements for both collections.

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
c2	a collection

Returns

• true if the two specified collections have no elements in common.

Throws

NullPointerException	if either collection is null.
NullPointerException	if one collection contains a null element and null is not an eligible element for the other collection. (optional)
ClassCastException	if one collection contains an element that is of a type which is ineligible for the other collection. (optional)

public static Enumeration<T> emptyEnumeration ()

Returns an enumeration that has no elements. More precisely,

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

Implementations of this method are permitted, but not required, to return the same object from multiple invocations.

Returns

• an empty enumeration

public static Iterator<T> emptyIterator ()

Returns an iterator that has no elements. More precisely,

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

Implementations of this method are permitted, but not required, to return the same object from multiple invocations.

Returns

• an empty iterator

public static final List<T> emptyList ()

Returns an empty list (immutable). This list is serializable.

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

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

Returns

• an empty immutable list

See Also

• EMPTY_LIST

public static ListIterator<T> emptyListIterator ()

Returns a list iterator that has no elements. More precisely,

• hasNext and hasPrevious always return false.
• next and previous always throw NoSuchElementException.
• remove and set always throw IllegalStateException.
• add always throws UnsupportedOperationException.
• nextIndex always returns 0.
• previousIndex always returns -1.

Implementations of this method are permitted, but not required, to return the same object from multiple invocations.

Returns

• an empty list iterator

public static final Map<K, V> emptyMap ()

Returns an empty map (immutable). This map is serializable.

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

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

Returns

• an empty map

See Also

• EMPTY_MAP

public static final NavigableMap<K, V> emptyNavigableMap ()

Returns an empty navigable map (immutable). This map is serializable.

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

 NavigableMap<String, Date> s = Collections.emptyNavigableMap();
 

Returns

• an empty navigable map

public static NavigableSet<E> emptyNavigableSet ()

Returns an empty navigable set (immutable). This set is serializable.

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

 NavigableSet<String> s = Collections.emptyNavigableSet();
 

Returns

• the empty navigable set

public static final Set<T> emptySet ()

Returns an empty set (immutable). This set is serializable. Unlike the like-named field, this method is parameterized.

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

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

Returns

• the empty set

See Also

• EMPTY_SET

public static final SortedMap<K, V> emptySortedMap ()

Returns an empty sorted map (immutable). This map is serializable.

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

 SortedMap<String, Date> s = Collections.emptySortedMap();
 

Returns

• an empty sorted map

public static SortedSet<E> emptySortedSet ()

Returns an empty sorted set (immutable). This set is serializable.

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

 SortedSet<String> s = Collections.emptySortedSet();
 

Returns

• the empty sorted set

public static Enumeration<T> enumeration (Collection<T> c)

Returns an enumeration over the specified collection. This provides interoperability 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.

See Also

• Enumeration

public static void fill (List<? super T> list, T obj)

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.
obj	The element with which to fill the specified list.

Throws

UnsupportedOperationException	if the specified list or its list-iterator does not support the set operation.

public static int frequency (Collection<?> c, Object o)

Returns the number of elements in the specified collection equal to the specified object. More formally, returns the number of elements e in the collection such that (o == null ? e == null : o.equals(e)).

Parameters

c	the collection in which to determine the frequency of o
o	the object whose frequency is to be determined

Returns

• the number of elements in c equal to o

Throws

NullPointerException	if c is null

public static int indexOfSubList (List<?> source, List<?> target)

Returns the starting position of the first occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence. More formally, returns the lowest index i such that source.subList(i, i+target.size()).equals(target), or -1 if there is no such index. (Returns -1 if target.size() > source.size())

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 occurrence of target.
target	the list to search for as a subList of source.

Returns

• the starting position of the first occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence.

public static int lastIndexOfSubList (List<?> source, List<?> target)

Returns the starting position of the last occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence. More formally, returns the highest index i such that source.subList(i, i+target.size()).equals(target), or -1 if there is no such index. (Returns -1 if target.size() > source.size())

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 occurrence of target.
target	the list to search for as a subList of source.

Returns

• the starting position of the last occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence.

public static ArrayList<T> list (Enumeration<T> e)

Returns an array list containing the elements returned by the specified enumeration in the order they are returned by the enumeration. This method provides interoperability between legacy APIs that return enumerations and new APIs that require collections.

Parameters

e	enumeration providing elements for the returned array list

Returns

• an array list containing the elements returned by the specified enumeration.

See Also

• Enumeration
• ArrayList

public static T max (Collection<? extends T> coll, Comparator<? super T> 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.
comp	the comparator with which to determine the maximum element. A null value indicates that the elements' natural ordering should be used.

Returns

• the maximum element of the given collection, according to the specified comparator.

Throws

ClassCastException	if the collection contains elements that are not mutually comparable using the specified comparator.
NoSuchElementException	if the collection is empty.

See Also

• Comparable

public static T max (Collection<? extends T> 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.

Throws

ClassCastException	if the collection contains elements that are not mutually comparable (for example, strings and integers).
NoSuchElementException	if the collection is empty.

See Also

• Comparable

public static T min (Collection<? extends T> 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