File	Doc	Category	Size	Date	Package
Collections.java	API Doc	Java SE 6 API	139175	Tue Jun 10 00:25:52 BST 2008	java.util

Collections

• java.lang.Object

Fields Summary
private static final int	BINARYSEARCH_THRESHOLD
private static final int	REVERSE_THRESHOLD
private static final int	SHUFFLE_THRESHOLD
private static final int	FILL_THRESHOLD
private static final int	ROTATE_THRESHOLD
private static final int	COPY_THRESHOLD
private static final int	REPLACEALL_THRESHOLD
private static final int	INDEXOFSUBLIST_THRESHOLD
private static Random	r
public static final Set	EMPTY_SET The empty set (immutable). This set is serializable.
public static final List	EMPTY_LIST The empty list (immutable). This list is serializable.
public static final Map	EMPTY_MAP The empty map (immutable). This map is serializable.
private static final Comparator	REVERSE_ORDER

Constructors Summary
private Collections()

Methods Summary
public static boolean	addAll(java.util.Collection 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"); param c the collection into which elements are to be inserted param elements the elements to insert into c return true if the collection changed as a result of the call throws UnsupportedOperationException if c does not support the add operation throws NullPointerException if elements contains one or more null values and c does not permit null elements, or if c or elements are null throws IllegalArgumentException if some property of a value in elements prevents it from being added to c see Collection#addAll(Collection) since 1.5 boolean result = false; for (T element : elements) result |= c.add(element); return result;
public static java.util.Queue	asLifoQueue(java.util.Deque deque) Returns a view of a {@link Deque} as a Last-in-first-out (Lifo) {@link 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 {@link Queue#addAll addAll} method is implemented as a sequence of {@link Deque#addFirst addFirst} invocations on the backing deque. param deque the deque return the queue since 1.6 return new AsLIFOQueue<T>(deque);
public static int	binarySearch(java.util.List 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 {@link #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 {@link 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. param list the list to be searched. param key the key to be searched for. return 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. if (list instanceof RandomAccess || list.size()<BINARYSEARCH_THRESHOLD) return Collections.indexedBinarySearch(list, key); else return Collections.iteratorBinarySearch(list, key);
public static int	binarySearch(java.util.List list, T key, java.util.Comparator c) Searches the specified list for the specified object using the binary search algorithm. The list must be sorted into ascending order according to the specified comparator (as by the {@link #sort(List, Comparator) 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 {@link 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. param list the list to be searched. param key the key to be searched for. param c the comparator by which the list is ordered. A null value indicates that the elements' {@linkplain Comparable natural ordering} should be used. return 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. if (c==null) return binarySearch((List) list, key); if (list instanceof RandomAccess || list.size()<BINARYSEARCH_THRESHOLD) return Collections.indexedBinarySearch(list, key, c); else return Collections.iteratorBinarySearch(list, key, c);
public static java.util.Collection	checkedCollection(java.util.Collection c, java.lang.Class 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<String>(); may be replaced temporarily by this one: Collection<String> c = Collections.checkedCollection( new HashSet<String>(), 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. param c the collection for which a dynamically typesafe view is to be returned param type the type of element that c is permitted to hold return a dynamically typesafe view of the specified collection since 1.5 return new CheckedCollection<E>(c, type);
public static java.util.List	checkedList(java.util.List list, java.lang.Class 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 {@link #checkedCollection checkedCollection} method. The returned list will be serializable if the specified list is serializable. param list the list for which a dynamically typesafe view is to be returned param type the type of element that list is permitted to hold return a dynamically typesafe view of the specified list since 1.5 return (list instanceof RandomAccess ? new CheckedRandomAccessList<E>(list, type) : new CheckedList<E>(list, type));
public static java.util.Map	checkedMap(java.util.Map m, java.lang.Class keyType, java.lang.Class 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 {@link Map.Entry} instance obtained from the map's {@link Map#entrySet() 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 {@link #checkedCollection checkedCollection} method. The returned map will be serializable if the specified map is serializable. param m the map for which a dynamically typesafe view is to be returned param keyType the type of key that m is permitted to hold param valueType the type of value that m is permitted to hold return a dynamically typesafe view of the specified map since 1.5 return new CheckedMap<K,V>(m, keyType, valueType);
public static java.util.Set	checkedSet(java.util.Set s, java.lang.Class 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 {@link #checkedCollection checkedCollection} method. The returned set will be serializable if the specified set is serializable. param s the set for which a dynamically typesafe view is to be returned param type the type of element that s is permitted to hold return a dynamically typesafe view of the specified set since 1.5 return new CheckedSet<E>(s, type);
public static java.util.SortedMap	checkedSortedMap(java.util.SortedMap m, java.lang.Class keyType, java.lang.Class 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 {@link Map.Entry} instance obtained from the map's {@link Map#entrySet() 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 {@link #checkedCollection checkedCollection} method. The returned map will be serializable if the specified map is serializable. param m the map for which a dynamically typesafe view is to be returned param keyType the type of key that m is permitted to hold param valueType the type of value that m is permitted to hold return a dynamically typesafe view of the specified map since 1.5 return new CheckedSortedMap<K,V>(m, keyType, valueType);
public static java.util.SortedSet	checkedSortedSet(java.util.SortedSet s, java.lang.Class 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 {@link #checkedCollection checkedCollection} method. The returned sorted set will be serializable if the specified sorted set is serializable. param s the sorted set for which a dynamically typesafe view is to be returned param type the type of element that s is permitted to hold return a dynamically typesafe view of the specified sorted set since 1.5 return new CheckedSortedSet<E>(s, type);
public static void	copy(java.util.List dest, java.util.List src) Copies all of the elements from one list into another. After the operation, the index of each copied element in the destination list will be identical to its index in the source list. The destination list must be at least as long as the source list. If it is longer, the remaining elements in the destination list are unaffected. This method runs in linear time. param dest The destination list. param src The source list. throws IndexOutOfBoundsException if the destination list is too small to contain the entire source List. throws UnsupportedOperationException if the destination list's list-iterator does not support the set operation. int srcSize = src.size(); if (srcSize > dest.size()) throw new IndexOutOfBoundsException("Source does not fit in dest"); if (srcSize < COPY_THRESHOLD || (src instanceof RandomAccess && dest instanceof RandomAccess)) { for (int i=0; i<srcSize; i++) dest.set(i, src.get(i)); } else { ListIterator<? super T> di=dest.listIterator(); ListIterator<? extends T> si=src.listIterator(); for (int i=0; i<srcSize; i++) { di.next(); di.set(si.next()); } }
public static boolean	disjoint(java.util.Collection c1, java.util.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 {@link SortedSet} whose ordering is not compatible with equals, or the key set of an {@link 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. param c1 a collection param c2 a collection throws NullPointerException if either collection is null since 1.5 /* * We're going to iterate through c1 and test for inclusion in c2. * If c1 is a Set and c2 isn't, swap the collections. Otherwise, * place the shorter collection in c1. Hopefully this heuristic * will minimize the cost of the operation. */ if ((c1 instanceof Set) && !(c2 instanceof Set) || (c1.size() > c2.size())) { Collection<?> tmp = c1; c1 = c2; c2 = tmp; } for (Object e : c1) if (c2.contains(e)) return false; return true;
public static final java.util.List	emptyList() Returns the empty list (immutable). This list is serializable. This example illustrates the type-safe way to obtain an empty list: List<String> s = Collections.emptyList(); Implementation note: Implementations of this method need not create a separate List object for each call. Using this method is likely to have comparable cost to using the like-named field. (Unlike this method, the field does not provide type safety.) see #EMPTY_LIST since 1.5 return (List<T>) EMPTY_LIST;
public static final java.util.Map	emptyMap() Returns the empty map (immutable). This map is serializable. This example illustrates the type-safe way to obtain an empty set: Map<String, Date> s = Collections.emptyMap(); Implementation note: Implementations of this method need not create a separate Map object for each call. Using this method is likely to have comparable cost to using the like-named field. (Unlike this method, the field does not provide type safety.) see #EMPTY_MAP since 1.5 return (Map<K,V>) EMPTY_MAP;
public static final java.util.Set	emptySet() Returns the 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(); Implementation note: Implementations of this method need not create a separate Set object for each call. Using this method is likely to have comparable cost to using the like-named field. (Unlike this method, the field does not provide type safety.) see #EMPTY_SET since 1.5 return (Set<T>) EMPTY_SET;
public static java.util.Enumeration	enumeration(java.util.Collection c) Returns an enumeration over the specified collection. This provides interoperability with legacy APIs that require an enumeration as input. param c the collection for which an enumeration is to be returned. return an enumeration over the specified collection. see Enumeration return new Enumeration<T>() { Iterator<T> i = c.iterator(); public boolean hasMoreElements() { return i.hasNext(); } public T nextElement() { return i.next(); } };
private static boolean	eq(java.lang.Object o1, java.lang.Object o2) Returns true if the specified arguments are equal, or both null. return (o1==null ? o2==null : o1.equals(o2));
public static void	fill(java.util.List list, T obj) Replaces all of the elements of the specified list with the specified element. This method runs in linear time. param list the list to be filled with the specified element. param 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. int size = list.size(); if (size < FILL_THRESHOLD || list instanceof RandomAccess) { for (int i=0; i<size; i++) list.set(i, obj); } else { ListIterator<? super T> itr = list.listIterator(); for (int i=0; i<size; i++) { itr.next(); itr.set(obj); } }
public static int	frequency(java.util.Collection c, java.lang.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)). param c the collection in which to determine the frequency of o param o the object whose frequency is to be determined throws NullPointerException if c is null since 1.5 int result = 0; if (o == null) { for (Object e : c) if (e == null) result++; } else { for (Object e : c) if (o.equals(e)) result++; } return result;
private static T	get(java.util.ListIterator i, int index) Gets the ith element from the given list by repositioning the specified list listIterator. T obj = null; int pos = i.nextIndex(); if (pos <= index) { do { obj = i.next(); } while (pos++ < index); } else { do { obj = i.previous(); } while (--pos > index); } return obj;
public static int	indexOfSubList(java.util.List source, java.util.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. param source the list in which to search for the first occurrence of target. param target the list to search for as a subList of source. return 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. since 1.4 int sourceSize = source.size(); int targetSize = target.size(); int maxCandidate = sourceSize - targetSize; if (sourceSize < INDEXOFSUBLIST_THRESHOLD || (source instanceof RandomAccess&&target instanceof RandomAccess)) { nextCand: for (int candidate = 0; candidate <= maxCandidate; candidate++) { for (int i=0, j=candidate; i<targetSize; i++, j++) if (!eq(target.get(i), source.get(j))) continue nextCand; // Element mismatch, try next cand return candidate; // All elements of candidate matched target } } else { // Iterator version of above algorithm ListIterator<?> si = source.listIterator(); nextCand: for (int candidate = 0; candidate <= maxCandidate; candidate++) { ListIterator<?> ti = target.listIterator(); for (int i=0; i<targetSize; i++) { if (!eq(ti.next(), si.next())) { // Back up source iterator to next candidate for (int j=0; j<i; j++) si.previous(); continue nextCand; } } return candidate; } } return -1; // No candidate matched the target
private static int	indexedBinarySearch(java.util.List list, T key) int low = 0; int high = list.size()-1; while (low <= high) { int mid = (low + high) >>> 1; Comparable<? super T> midVal = list.get(mid); int cmp = midVal.compareTo(key); if (cmp < 0) low = mid + 1; else if (cmp > 0) high = mid - 1; else return mid; // key found } return -(low + 1); // key not found
private static int	indexedBinarySearch(java.util.List l, T key, java.util.Comparator c) int low = 0; int high = l.size()-1; while (low <= high) { int mid = (low + high) >>> 1; T midVal = l.get(mid); int cmp = c.compare(midVal, key); if (cmp < 0) low = mid + 1; else if (cmp > 0) high = mid - 1; else return mid; // key found } return -(low + 1); // key not found
private static int	iteratorBinarySearch(java.util.List l, T key, java.util.Comparator c) int low = 0; int high = l.size()-1; ListIterator<? extends T> i = l.listIterator(); while (low <= high) { int mid = (low + high) >>> 1; T midVal = get(i, mid); int cmp = c.compare(midVal, key); if (cmp < 0) low = mid + 1; else if (cmp > 0) high = mid - 1; else return mid; // key found } return -(low + 1); // key not found
private static int	iteratorBinarySearch(java.util.List list, T key) int low = 0; int high = list.size()-1; ListIterator<? extends Comparable<? super T>> i = list.listIterator(); while (low <= high) { int mid = (low + high) >>> 1; Comparable<? super T> midVal = get(i, mid); int cmp = midVal.compareTo(key); if (cmp < 0) low = mid + 1; else if (cmp > 0) high = mid - 1; else return mid; // key found } return -(low + 1); // key not found
public static int	lastIndexOfSubList(java.util.List source, java.util.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. param source the list in which to search for the last occurrence of target. param target the list to search for as a subList of source. return 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. since 1.4 int sourceSize = source.size(); int targetSize = target.size(); int maxCandidate = sourceSize - targetSize; if (sourceSize < INDEXOFSUBLIST_THRESHOLD || source instanceof RandomAccess) { // Index access version nextCand: for (int candidate = maxCandidate; candidate >= 0; candidate--) { for (int i=0, j=candidate; i<targetSize; i++, j++) if (!eq(target.get(i), source.get(j))) continue nextCand; // Element mismatch, try next cand return candidate; // All elements of candidate matched target } } else { // Iterator version of above algorithm if (maxCandidate < 0) return -1; ListIterator<?> si = source.listIterator(maxCandidate); nextCand: for (int candidate = maxCandidate; candidate >= 0; candidate--) { ListIterator<?> ti = target.listIterator(); for (int i=0; i<targetSize; i++) { if (!eq(ti.next(), si.next())) { if (candidate != 0) { // Back up source iterator to next candidate for (int j=0; j<=i+1; j++) si.previous(); } continue nextCand; } } return candidate; } } return -1; // No candidate matched the target
public static java.util.ArrayList	list(java.util.Enumeration 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. param e enumeration providing elements for the returned array list return an array list containing the elements returned by the specified enumeration. since 1.4 see Enumeration see ArrayList ArrayList<T> l = new ArrayList<T>(); while (e.hasMoreElements()) l.add(e.nextElement()); return l;
public static T	max(java.util.Collection coll) Returns the maximum element of the given collection, according to the natural ordering of its elements. All elements in the collection must implement the Comparable interface. Furthermore, all elements in the collection must be mutually comparable (that is, e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 in the collection). This method iterates over the entire collection, hence it requires time proportional to the size of the collection. param coll the collection whose maximum element is to be determined. return 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). throws NoSuchElementException if the collection is empty. see Comparable Iterator<? extends T> i = coll.iterator(); T candidate = i.next(); while (i.hasNext()) { T next = i.next(); if (next.compareTo(candidate) > 0) candidate = next; } return candidate;
public static T	max(java.util.Collection coll, java.util.Comparator comp) Returns the maximum element of the given collection, according to the order induced by the specified comparator. All elements in the collection must be mutually comparable by the specified comparator (that is, comp.compare(e1, e2) must not throw a ClassCastException for any elements e1 and e2 in the collection). This method iterates over the entire collection, hence it requires time proportional to the size of the collection. param coll the collection whose maximum element is to be determined. param comp the comparator with which to determine the maximum element. A null value indicates that the elements' natural ordering should be used. return 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. throws NoSuchElementException if the collection is empty. see Comparable if (comp==null) return (T)max((Collection<SelfComparable>) (Collection) coll); Iterator<? extends T> i = coll.iterator(); T candidate = i.next(); while (i.hasNext()) { T next = i.next(); if (comp.compare(next, candidate) > 0) candidate = next; } return candidate;
public static T	min(java.util.Collection coll) Returns the minimum element of the given collection, according to the natural ordering of its elements. All elements in the collection must implement the Comparable interface. Furthermore, all elements in the collection must be mutually comparable (that is, e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 in the collection). This method iterates over the entire collection, hence it requires time proportional to the size of the collection. param coll the collection whose minimum element is to be determined. return the minimum 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). throws NoSuchElementException if the collection is empty. see Comparable Iterator<? extends T> i = coll.iterator(); T candidate = i.next(); while (i.hasNext()) { T next = i.next(); if (next.compareTo(candidate) < 0) candidate = next; } return candidate;
public static T	min(java.util.Collection coll, java.util.Comparator comp) Returns the minimum element of the given collection, according to the order induced by the specified comparator. All elements in the collection must be mutually comparable by the specified comparator (that is, comp.compare(e1, e2) must not throw a ClassCastException for any elements e1 and e2 in the collection). This method iterates over the entire collection, hence it requires time proportional to the size of the collection. param coll the collection whose minimum element is to be determined. param comp the comparator with which to determine the minimum element. A null value indicates that the elements' natural ordering should be used. return the minimum 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. throws NoSuchElementException if the collection is empty. see Comparable if (comp==null) return (T)min((Collection<SelfComparable>) (Collection) coll); Iterator<? extends T> i = coll.iterator(); T candidate = i.next(); while (i.hasNext()) { T next = i.next(); if (comp.compare(next, candidate) < 0) candidate = next; } return candidate;
public static java.util.List	nCopies(int n, T o) Returns an immutable list consisting of n copies of the specified object. The newly allocated data object is tiny (it contains a single reference to the data object). This method is useful in combination with the List.addAll method to grow lists. The returned list is serializable. param n the number of elements in the returned list. param o the element to appear repeatedly in the returned list. return an immutable list consisting of n copies of the specified object. throws IllegalArgumentException if n < 0. see List#addAll(Collection) see List#addAll(int, Collection) if (n < 0) throw new IllegalArgumentException("List length = " + n); return new CopiesList<T>(n, o);
public static java.util.Set	newSetFromMap(java.util.Map map) Returns a set backed by the specified map. The resulting set displays the same ordering, concurrency, and performance characteristics as the backing map. In essence, this factory method provides a {@link Set} implementation corresponding to any {@link Map} implementation. There is no need to use this method on a {@link Map} implementation that already has a corresponding {@link Set} implementation (such as {@link HashMap} or {@link TreeMap}). 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>()); param map the backing map return the set backed by the map throws IllegalArgumentException if map is not empty since 1.6 return new SetFromMap<E>(map);
public static boolean	replaceAll(java.util.List list, T oldVal, T newVal) Replaces all occurrences of one specified value in a list with another. More formally, replaces with newVal each element e in list such that (oldVal==null ? e==null : oldVal.equals(e)). (This method has no effect on the size of the list.) param list the list in which replacement is to occur. param oldVal the old value to be replaced. param newVal the new value with which oldVal is to be replaced. return true if list contained one or more elements e such that (oldVal==null ? e==null : oldVal.equals(e)). throws UnsupportedOperationException if the specified list or its list-iterator does not support the set operation. since 1.4 boolean result = false; int size = list.size(); if (size < REPLACEALL_THRESHOLD || list instanceof RandomAccess) { if (oldVal==null) { for (int i=0; i<size; i++) { if (list.get(i)==null) { list.set(i, newVal); result = true; } } } else { for (int i=0; i<size; i++) { if (oldVal.equals(list.get(i))) { list.set(i, newVal); result = true; } } } } else { ListIterator<T> itr=list.listIterator(); if (oldVal==null) { for (int i=0; i<size; i++) { if (itr.next()==null) { itr.set(newVal); result = true; } } } else { for (int i=0; i<size; i++) { if (oldVal.equals(itr.next())) { itr.set(newVal); result = true; } } } } return result;
public static void	reverse(java.util.List list) Reverses the order of the elements in the specified list. This method runs in linear time. param list the list whose elements are to be reversed. throws UnsupportedOperationException if the specified list or its list-iterator does not support the set operation. int size = list.size(); if (size < REVERSE_THRESHOLD || list instanceof RandomAccess) { for (int i=0, mid=size>>1, j=size-1; i<mid; i++, j--) swap(list, i, j); } else { ListIterator fwd = list.listIterator(); ListIterator rev = list.listIterator(size); for (int i=0, mid=list.size()>>1; i<mid; i++) { Object tmp = fwd.next(); fwd.set(rev.previous()); rev.set(tmp); } }
public static java.util.Comparator	reverseOrder() Returns a comparator that imposes the reverse of the natural ordering on a collection of objects that implement the Comparable interface. (The natural ordering is the ordering imposed by the objects' own compareTo method.) This enables a simple idiom for sorting (or maintaining) collections (or arrays) of objects that implement the Comparable interface in reverse-natural-order. For example, suppose a is an array of strings. Then: Arrays.sort(a, Collections.reverseOrder()); sorts the array in reverse-lexicographic (alphabetical) order. The returned comparator is serializable. return a comparator that imposes the reverse of the natural ordering on a collection of objects that implement the Comparable interface. see Comparable return (Comparator<T>) REVERSE_ORDER;
public static java.util.Comparator	reverseOrder(java.util.Comparator cmp) Returns a comparator that imposes the reverse ordering of the specified comparator. If the specified comparator is null, this method is equivalent to {@link #reverseOrder()} (in other words, it returns a comparator that imposes the reverse of the natural ordering on a collection of objects that implement the Comparable interface). The returned comparator is serializable (assuming the specified comparator is also serializable or null). return a comparator that imposes the reverse ordering of the specified comparator. since 1.5 if (cmp == null) return reverseOrder(); return new ReverseComparator2<T>(cmp);
public static void	rotate(java.util.List list, int distance) Rotates the elements in the specified list by the specified distance. After calling this method, the element at index i will be the element previously at index (i - distance) mod list.size(), for all values of i between 0 and list.size()-1, inclusive. (This method has no effect on the size of the list.) 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); To make this concrete, suppose list comprises [a, b, c, d, e]. To move the element at index 1 (b) forward two positions, perform the following invocation: Collections.rotate(l.subList(1, 4), -1); The resulting list is [a, c, d, b, e]. 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 {@link 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 {@link #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). param list the list to be rotated. param distance the distance to rotate the list. There are no constraints on this value; it may be zero, negative, or greater than list.size(). throws UnsupportedOperationException if the specified list or its list-iterator does not support the set operation. since 1.4 if (list instanceof RandomAccess || list.size() < ROTATE_THRESHOLD) rotate1((List)list, distance); else rotate2((List)list, distance);
private static void	rotate1(java.util.List list, int distance) int size = list.size(); if (size == 0) return; distance = distance % size; if (distance < 0) distance += size; if (distance == 0) return; for (int cycleStart = 0, nMoved = 0; nMoved != size; cycleStart++) { T displaced = list.get(cycleStart); int i = cycleStart; do { i += distance; if (i >= size) i -= size; displaced = list.set(i, displaced); nMoved ++; } while(i != cycleStart); }
private static void	rotate2(java.util.List list, int distance) int size = list.size(); if (size == 0) return; int mid = -distance % size; if (mid < 0) mid += size; if (mid == 0) return; reverse(list.subList(0, mid)); reverse(list.subList(mid, size)); reverse(list);
public static void	shuffle(java.util.List list) Randomly permutes the specified list using a default source of randomness. All permutations occur with approximately equal likelihood. The hedge "approximately" is used in the foregoing description because default source of 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 {@link 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. param list the list to be shuffled. throws UnsupportedOperationException if the specified list or its list-iterator does not support the set operation. if (r == null) { r = new Random(); } shuffle(list, r);
public static void	shuffle(java.util.List list, java.util.Random rnd) Randomly permute the specified list using the specified source of randomness. All permutations occur with equal likelihood assuming that the source of randomness is fair. This implementation traverses the list backwards, from the last element up to the second, repeatedly swapping a randomly selected element into the "current position". Elements are randomly selected from the portion of the list that runs from the first element to the current position, inclusive. This method runs in linear time. If the specified list does not implement the {@link 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. param list the list to be shuffled. param rnd the source of randomness to use to shuffle the list. throws UnsupportedOperationException if the specified list or its list-iterator does not support the set operation. int size = list.size(); if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) { for (int i=size; i>1; i--) swap(list, i-1, rnd.nextInt(i)); } else { Object arr[] = list.toArray(); // Shuffle array for (int i=size; i>1; i--) swap(arr, i-1, rnd.nextInt(i)); // Dump array back into list ListIterator it = list.listIterator(); for (int i=0; i<arr.length; i++) { it.next(); it.set(arr[i]); } }
public static java.util.Set	singleton(T o) Returns an immutable set containing only the specified object. The returned set is serializable. param o the sole object to be stored in the returned set. return an immutable set containing only the specified object. return new SingletonSet<T>(o);
public static java.util.List	singletonList(T o) Returns an immutable list containing only the specified object. The returned list is serializable. param o the sole object to be stored in the returned list. return an immutable list containing only the specified object. since 1.3 return new SingletonList<T>(o);
public static java.util.Map	singletonMap(K key, V value) Returns an immutable map, mapping only the specified key to the specified value. The returned map is serializable. param key the sole key to be stored in the returned map. param value the value to which the returned map maps key. return an immutable map containing only the specified key-value mapping. since 1.3 return new SingletonMap<K,V>(key, value);
public static void	sort(java.util.List list) Sorts the specified list into ascending order, according to the natural ordering of its elements. All elements in the list must implement the Comparable interface. Furthermore, all elements in the list must be mutually comparable (that is, e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 in the list). This sort is guaranteed to be stable: equal elements will not be reordered as a result of the sort. The specified list must be modifiable, but need not be resizable. The sorting algorithm is a modified mergesort (in which the merge is omitted if the highest element in the low sublist is less than the lowest element in the high sublist). This algorithm offers guaranteed n log(n) performance. 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. param list the list to be sorted. throws ClassCastException if the list contains elements that are not mutually comparable (for example, strings and integers). throws UnsupportedOperationException if the specified list's list-iterator does not support the set operation. see Comparable Object[] a = list.toArray(); Arrays.sort(a); ListIterator<T> i = list.listIterator(); for (int j=0; j<a.length; j++) { i.next(); i.set((T)a[j]); }
public static void	sort(java.util.List list, java.util.Comparator c) Sorts the specified list according to the order induced by the specified comparator. All elements in the list must be mutually comparable using the specified comparator (that is, c.compare(e1, e2) must not throw a ClassCastException for any elements e1 and e2 in the list). This sort is guaranteed to be stable: equal elements will not be reordered as a result of the sort. The sorting algorithm is a modified mergesort (in which the merge is omitted if the highest element in the low sublist is less than the lowest element in the high sublist). This algorithm offers guaranteed n log(n) performance. 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. param list the list to be sorted. param c the comparator to determine the order of the list. A null value indicates that the elements' natural ordering should be used. throws ClassCastException if the list contains elements that are not mutually comparable using the specified comparator. throws UnsupportedOperationException if the specified list's list-iterator does not support the set operation. see Comparator Object[] a = list.toArray(); Arrays.sort(a, (Comparator)c); ListIterator i = list.listIterator(); for (int j=0; j<a.length; j++) { i.next(); i.set(a[j]); }
public static void	swap(java.util.List list, int i, int j) Swaps the elements at the specified positions in the specified list. (If the specified positions are equal, invoking this method leaves the list unchanged.) param list The list in which to swap elements. param i the index of one element to be swapped. param j the index of the other element to be swapped. throws IndexOutOfBoundsException if either i or j is out of range (i < 0 || i >= list.size() || j < 0 || j >= list.size()). since 1.4 final List l = list; l.set(i, l.set(j, l.get(i)));
private static void	swap(java.lang.Object[] arr, int i, int j) Swaps the two specified elements in the specified array. Object tmp = arr[i]; arr[i] = arr[j]; arr[j] = tmp;
public static java.util.Collection	synchronizedCollection(java.util.Collection c) Returns a synchronized (thread-safe) collection backed by the specified collection. In order to guarantee serial access, it is critical that all access to the backing collection is accomplished through the returned collection. It is imperative that the user manually synchronize on the returned collection when iterating over it: Collection c = Collections.synchronizedCollection(myCollection); ... synchronized(c) { Iterator i = c.iterator(); // Must be in the synchronized block while (i.hasNext()) foo(i.next()); } Failure to follow this advice may result in non-deterministic behavior. The returned collection does not pass the hashCode and equals operations through to the backing collection, but relies on Object's equals and hashCode methods. This is necessary to preserve the contracts of these operations in the case that the backing collection is a set or a list. The returned collection will be serializable if the specified collection is serializable. param c the collection to be "wrapped" in a synchronized collection. return a synchronized view of the specified collection. return new SynchronizedCollection<T>(c);
static java.util.Collection	synchronizedCollection(java.util.Collection c, java.lang.Object mutex) return new SynchronizedCollection<T>(c, mutex);
public static java.util.List	synchronizedList(java.util.List list) Returns a synchronized (thread-safe) list backed by the specified list. In order to guarantee serial access, it is critical that all access to the backing list is accomplished through the returned list. It is imperative that the user manually synchronize on the returned list when iterating over it: List list = Collections.synchronizedList(new ArrayList()); ... synchronized(list) { Iterator i = list.iterator(); // Must be in synchronized block while (i.hasNext()) foo(i.next()); } Failure to follow this advice may result in non-deterministic behavior. The returned list will be serializable if the specified list is serializable. param list the list to be "wrapped" in a synchronized list. return a synchronized view of the specified list. return (list instanceof RandomAccess ? new SynchronizedRandomAccessList<T>(list) : new SynchronizedList<T>(list));
static java.util.List	synchronizedList(java.util.List list, java.lang.Object mutex) return (list instanceof RandomAccess ? new SynchronizedRandomAccessList<T>(list, mutex) : new SynchronizedList<T>(list, mutex));
public static java.util.Map	synchronizedMap(java.util.Map m) Returns a synchronized (thread-safe) map backed by the specified map. In order to guarantee serial access, it is critical that all access to the backing map is accomplished through the returned map. It is imperative that the user manually synchronize on the returned map when iterating over any of its collection views: Map m = Collections.synchronizedMap(new HashMap()); ... Set s = m.keySet(); // Needn't be in synchronized block ... synchronized(m) { // Synchronizing on m, not s! Iterator i = s.iterator(); // Must be in synchronized block while (i.hasNext()) foo(i.next()); } Failure to follow this advice may result in non-deterministic behavior. The returned map will be serializable if the specified map is serializable. param m the map to be "wrapped" in a synchronized map. return a synchronized view of the specified map. return new SynchronizedMap<K,V>(m);
public static java.util.Set	synchronizedSet(java.util.Set s) Returns a synchronized (thread-safe) set backed by the specified set. In order to guarantee serial access, it is critical that all access to the backing set is accomplished through the returned set. It is imperative that the user manually synchronize on the returned set when iterating over it: Set s = Collections.synchronizedSet(new HashSet()); ... synchronized(s) { Iterator i = s.iterator(); // Must be in the synchronized block while (i.hasNext()) foo(i.next()); } Failure to follow this advice may result in non-deterministic behavior. The returned set will be serializable if the specified set is serializable. param s the set to be "wrapped" in a synchronized set. return a synchronized view of the specified set. return new SynchronizedSet<T>(s);
static java.util.Set	synchronizedSet(java.util.Set s, java.lang.Object mutex) return new SynchronizedSet<T>(s, mutex);
public static java.util.SortedMap	synchronizedSortedMap(java.util.SortedMap m) Returns a synchronized (thread-safe) sorted map backed by the specified sorted map. In order to guarantee serial access, it is critical that all access to the backing sorted map is accomplished through the returned sorted map (or its views). It is imperative that the user manually synchronize on the returned sorted map when iterating over any of its collection views, or the collections views of any of its subMap, headMap or tailMap views. SortedMap m = Collections.synchronizedSortedMap(new 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()); } or: 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()); } Failure to follow this advice may result in non-deterministic behavior. The returned sorted map will be serializable if the specified sorted map is serializable. param m the sorted map to be "wrapped" in a synchronized sorted map. return a synchronized view of the specified sorted map. return new SynchronizedSortedMap<K,V>(m);
public static java.util.SortedSet	synchronizedSortedSet(java.util.SortedSet s) Returns a synchronized (thread-safe) sorted set backed by the specified sorted set. In order to guarantee serial access, it is critical that all access to the backing sorted set is accomplished through the returned sorted set (or its views). It is imperative that the user manually synchronize on the returned sorted set when iterating over it or any of its subSet, headSet, or tailSet views. SortedSet s = Collections.synchronizedSortedSet(new TreeSet()); ... synchronized(s) { Iterator i = s.iterator(); // Must be in the synchronized block while (i.hasNext()) foo(i.next()); } or: 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()); } Failure to follow this advice may result in non-deterministic behavior. The returned sorted set will be serializable if the specified sorted set is serializable. param s the sorted set to be "wrapped" in a synchronized sorted set. return a synchronized view of the specified sorted set. return new SynchronizedSortedSet<T>(s);
public static java.util.Collection	unmodifiableCollection(java.util.Collection c) Returns an unmodifiable view of the specified collection. This method allows modules to provide users with "read-only" access to internal collections. Query operations on the returned collection "read through" to the specified collection, and attempts to modify the returned collection, whether direct or via its iterator, result in an UnsupportedOperationException. The returned collection does not pass the hashCode and equals operations through to the backing collection, but relies on Object's equals and hashCode methods. This is necessary to preserve the contracts of these operations in the case that the backing collection is a set or a list. The returned collection will be serializable if the specified collection is serializable. param c the collection for which an unmodifiable view is to be returned. return an unmodifiable view of the specified collection. return new UnmodifiableCollection<T>(c);
public static java.util.List	unmodifiableList(java.util.List list) Returns an unmodifiable view of the specified list. This method allows modules to provide users with "read-only" access to internal lists. Query operations on the returned list "read through" to the specified list, and attempts to modify the returned list, whether direct or via its iterator, result in an UnsupportedOperationException. The returned list will be serializable if the specified list is serializable. Similarly, the returned list will implement {@link RandomAccess} if the specified list does. param list the list for which an unmodifiable view is to be returned. return an unmodifiable view of the specified list. return (list instanceof RandomAccess ? new UnmodifiableRandomAccessList<T>(list) : new UnmodifiableList<T>(list));
public static java.util.Map	unmodifiableMap(java.util.Map m) Returns an unmodifiable view of the specified map. This method allows modules to provide users with "read-only" access to internal maps. Query operations on the returned map "read through" to the specified map, and attempts to modify the returned map, whether direct or via its collection views, result in an UnsupportedOperationException. The returned map will be serializable if the specified map is serializable. param m the map for which an unmodifiable view is to be returned. return an unmodifiable view of the specified map. return new UnmodifiableMap<K,V>(m);
public static java.util.Set	unmodifiableSet(java.util.Set s) Returns an unmodifiable view of the specified set. This method allows modules to provide users with "read-only" access to internal sets. Query operations on the returned set "read through" to the specified set, and attempts to modify the returned set, whether direct or via its iterator, result in an UnsupportedOperationException. The returned set will be serializable if the specified set is serializable. param s the set for which an unmodifiable view is to be returned. return an unmodifiable view of the specified set. return new UnmodifiableSet<T>(s);
public static java.util.SortedMap	unmodifiableSortedMap(java.util.SortedMap m) Returns an unmodifiable view of the specified sorted map. This method allows modules to provide users with "read-only" access to internal sorted maps. Query operations on the returned sorted map "read through" to the specified sorted map. Attempts to modify the returned sorted map, whether direct, via its collection views, or via its subMap, headMap, or tailMap views, result in an UnsupportedOperationException. The returned sorted map will be serializable if the specified sorted map is serializable. param m the sorted map for which an unmodifiable view is to be returned. return an unmodifiable view of the specified sorted map. return new UnmodifiableSortedMap<K,V>(m);
public static java.util.SortedSet	unmodifiableSortedSet(java.util.SortedSet s) Returns an unmodifiable view of the specified sorted set. This method allows modules to provide users with "read-only" access to internal sorted sets. Query operations on the returned sorted set "read through" to the specified sorted set. Attempts to modify the returned sorted set, whether direct, via its iterator, or via its subSet, headSet, or tailSet views, result in an UnsupportedOperationException. The returned sorted set will be serializable if the specified sorted set is serializable. param s the sorted set for which an unmodifiable view is to be returned. return an unmodifiable view of the specified sorted set. return new UnmodifiableSortedSet<T>(s);