Also see:

emptySet()

Also see:

emptyList()

Also see:

emptyMap()
since: 1.3 -

 public static boolean addAll(Collection<? super T> c,
    T elements) 
{
        boolean result = false;
        for (T element : elements)
            result |= c.add(element);
        return result;
}

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");

 public static Queue<T> asLifoQueue(Deque<T> deque) 
{
        return new AsLIFOQueue<  >(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.

 public static int binarySearch(List<Comparable> list,
    T key) 
{
        if (list instanceof RandomAccess || list.size()< BINARYSEARCH_THRESHOLD)
            return Collections.indexedBinarySearch(list, key);
        else
            return Collections.iteratorBinarySearch(list, 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.

 public static int binarySearch(List<? extends T> list,
    T key,
    Comparator<? super T> c) 
{
        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);
}

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.

 public static Collection<E> checkedCollection(Collection<E> c,
    Class<E> type) 
{
        return new CheckedCollection<  >(c, 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 {@code 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:

 {@code
    Collection c = new HashSet();
}

may be replaced temporarily by this one:

 {@code
    Collection 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 {@code Object}'s {@code equals} and {@code 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 {@code null} is considered to be a value of any reference type, the returned collection permits insertion of null elements whenever the backing collection does.

 public static List<E> checkedList(List<E> list,
    Class<E> type) 
{
        return (list instanceof RandomAccess ?
                new CheckedRandomAccessList<  >(list, type) :
                new CheckedList<  >(list, 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 {@code null} is considered to be a value of any reference type, the returned list permits insertion of null elements whenever the backing list does.

 public static Map<K, V> checkedMap(Map<K, V> m,
    Class<K> keyType,
    Class<V> valueType) 
{
        return new CheckedMap<  >(m, keyType, 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 {@code 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.

 public static Set<E> checkedSet(Set<E> s,
    Class<E> type) 
{
        return new CheckedSet<  >(s, 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 {@code null} is considered to be a value of any reference type, the returned set permits insertion of null elements whenever the backing set does.

 public static SortedMap<K, V> checkedSortedMap(SortedMap<K, V> m,
    Class<K> keyType,
    Class<V> valueType) 
{
        return new CheckedSortedMap<  >(m, keyType, 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 {@code 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.

 public static SortedSet<E> checkedSortedSet(SortedSet<E> s,
    Class<E> type) 
{
        return new CheckedSortedSet<  >(s, 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 {@code 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.

 public static  void copy(List<? super T> dest,
    List<? extends T> src) 
{
        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());
            }
        }
}

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.

 public static boolean disjoint(Collection<?> c1,
    Collection<?> c2) 
{
        // The collection to be used for contains(). Preference is given to
        // the collection who's contains() has lower O() complexity.
        Collection< ? > contains = c2;
        // The collection to be iterated. If the collections' contains() impl
        // are of different O() complexity, the collection with slower
        // contains() will be used for iteration. For collections who's
        // contains() are of the same complexity then best performance is
        // achieved by iterating the smaller collection.
        Collection< ? > iterate = c1;
        // Performance optimization cases. The heuristics:
        //   1. Generally iterate over c1.
        //   2. If c1 is a Set then iterate over c2.
        //   3. If either collection is empty then result is always true.
        //   4. Iterate over the smaller Collection.
        if (c1 instanceof Set) {
            // Use c1 for contains as a Set's contains() is expected to perform
            // better than O(N/2)
            iterate = c2;
            contains = c1;
        } else if (!(c2 instanceof Set)) {
            // Both are mere Collections. Iterate over smaller collection.
            // Example: If c1 contains 3 elements and c2 contains 50 elements and
            // assuming contains() requires ceiling(N/2) comparisons then
            // checking for all c1 elements in c2 would require 75 comparisons
            // (3 * ceiling(50/2)) vs. checking all c2 elements in c1 requiring
            // 100 comparisons (50 * ceiling(3/2)).
            int c1size = c1.size();
            int c2size = c2.size();
            if (c1size == 0 || c2size == 0) {
                // At least one collection is empty. Nothing will match.
                return true;
            }
            if (c1size  > c2size) {
                iterate = c2;
                contains = c1;
            }
        }
        for (Object e : iterate) {
            if (contains.contains(e)) {
               // Found a common element. Collections are not disjoint.
                return false;
            }
        }
        // No common elements were found.
        return true;
}

Returns {@code 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 {@code 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 {@code true} if and only if the collection is empty.

 public static Enumeration<T> emptyEnumeration() 
{
        return (Enumeration< T >) EmptyEnumeration.EMPTY_ENUMERATION;
}

Returns an enumeration that has no elements. More precisely,
• hasMoreElements always returns {@code false}.
• nextElement always throws NoSuchElementException .

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

 public static Iterator<T> emptyIterator() 
{
        return (Iterator< T >) EmptyIterator.EMPTY_ITERATOR;
}

Returns an iterator that has no elements. More precisely,
• hasNext always returns {@code 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.

 public static final List<T> emptyList() 
{
        return (List< T >) EMPTY_LIST;
}

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.)

 public static ListIterator<T> emptyListIterator() 
{
        return (ListIterator< T >) EmptyListIterator.EMPTY_ITERATOR;
}

Returns a list iterator that has no elements. More precisely,
• hasNext and hasPrevious always return {@code false}.
• next and previous always throw NoSuchElementException .
• remove and set always throw IllegalStateException .
• add always throws UnsupportedOperationException .
• nextIndex always returns {@code 0} .
• previousIndex always returns {@code -1}.

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

 public static final Map<K, V> emptyMap() 
{
        return (Map< K,V >) EMPTY_MAP;
}

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.)

 public static final Set<T> emptySet() 
{
        return (Set< T >) EMPTY_SET;
}

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.)

 public static Enumeration<T> enumeration(Collection<T> c) 
{
        return new Enumeration< T >() {
            private final Iterator< T > i = c.iterator();
            public boolean hasMoreElements() {
                return i.hasNext();
            }
            public T nextElement() {
                return i.next();
            }
        };
}

Returns an enumeration over the specified collection. This provides interoperability with legacy APIs that require an enumeration as input.

 static boolean eq(Object o1,
    Object o2) 
{
        return o1==null ? o2==null : o1.equals(o2);
}

Returns true if the specified arguments are equal, or both null.

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

Replaces all of the elements of the specified list with the specified element.

This method runs in linear time.

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

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)).

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

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.

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

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.

 public static ArrayList<T> list(Enumeration<T> e) 
{
        ArrayList< T > l = new ArrayList<  >();
        while (e.hasMoreElements())
            l.add(e.nextElement());
        return l;
}

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.

 public static T max(Collection<? extends T> coll) 
{
        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;
}

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.

 public static T max(Collection<? extends T> coll,
    Comparator<? super T> comp) 
{
        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;
}

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.

 public static T min(Collection<? extends T> coll) 
{
        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;
}

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.

 public static T min(Collection<? extends T> coll,
    Comparator<? super T> comp) 
{
        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;
}

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.

 public static List<T> nCopies(int n,
    T o) 
{
        if (n <  0)
            throw new IllegalArgumentException("List length = " + n);
        return new CopiesList<  >(n, 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.

 public static Set<E> newSetFromMap(Map<E, Boolean> map) 
{
        return new SetFromMap<  >(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 Set implementation corresponding to any Map implementation. There is no need to use this method on a Map implementation that already has a corresponding Set implementation (such as HashMap or 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>());

 public static boolean replaceAll(List<T> list,
    T oldVal,
    T newVal) 
{
        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;
}

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.)

 public static  void reverse(List<?> list) 
{
        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);
            }
        }
}

Reverses the order of the elements in the specified list.

This method runs in linear time.

 public static Comparator<T> reverseOrder() 
{
        return (Comparator< T >) ReverseComparator.REVERSE_ORDER;
}

Returns a comparator that imposes the reverse of the natural ordering on a collection of objects that implement the {@code Comparable} interface. (The natural ordering is the ordering imposed by the objects' own {@code compareTo} method.) This enables a simple idiom for sorting (or maintaining) collections (or arrays) of objects that implement the {@code Comparable} interface in reverse-natural-order. For example, suppose {@code 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.

 public static Comparator<T> reverseOrder(Comparator<T> cmp) 
{
        if (cmp == null)
            return reverseOrder();
        if (cmp instanceof ReverseComparator2)
            return ((ReverseComparator2< T >)cmp).cmp;
        return new ReverseComparator2<  >(cmp);
}

Returns a comparator that imposes the reverse ordering of the specified comparator. If the specified comparator is {@code null}, this method is equivalent to #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 {@code null}).

 public static  void rotate(List<?> list,
    int distance) 
{
        if (list instanceof RandomAccess || list.size() <  ROTATE_THRESHOLD)
            rotate1(list, distance);
        else
            rotate2(list, 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 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 #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).

 public static  void shuffle(List<?> list) 
{
        Random rnd = r;
        if (rnd == null)
            r = rnd = new Random();
        shuffle(list, rnd);
}

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 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.

 public static  void shuffle(List<?> list,
    Random rnd) 
{
        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]);
            }
        }
}

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 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.

 public static Set<T> singleton(T o) 
{
        return new SingletonSet<  >(o);
}

Returns an immutable set containing only the specified object. The returned set is serializable.

 static Iterator<E> singletonIterator(E e) 
{
        return new Iterator< E >() {
            private boolean hasNext = true;
            public boolean hasNext() {
                return hasNext;
            }
            public E next() {
                if (hasNext) {
                    hasNext = false;
                    return e;
                }
                throw new NoSuchElementException();
            }
            public void remove() {
                throw new UnsupportedOperationException();
            }
        };
}

 public static List<T> singletonList(T o) 
{
        return new SingletonList<  >(o);
}

Returns an immutable list containing only the specified object. The returned list is serializable.

 public static Map<K, V> singletonMap(K key,
    V value) 
{
        return new SingletonMap<  >(key, value);
}

Returns an immutable map, mapping only the specified key to the specified value. The returned map is serializable.

 public static  void sort(List<T> list) 
{
        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]);
        }
}

Sorts the specified list into ascending order, according to the {@linkplain Comparable 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, {@code e1.compareTo(e2)} must not throw a {@code ClassCastException} for any elements {@code e1} and {@code 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.

Implementation note: This implementation is a stable, adaptive, iterative mergesort that requires far fewer than n lg(n) comparisons when the input array is partially sorted, while offering the performance of a traditional mergesort when the input array is randomly ordered. If the input array is nearly sorted, the implementation requires approximately n comparisons. Temporary storage requirements vary from a small constant for nearly sorted input arrays to n/2 object references for randomly ordered input arrays.

The implementation takes equal advantage of ascending and descending order in its input array, and can take advantage of ascending and descending order in different parts of the same input array. It is well-suited to merging two or more sorted arrays: simply concatenate the arrays and sort the resulting array.

The implementation was adapted from Tim Peters's list sort for Python ( TimSort). It uses techiques from Peter McIlroy's "Optimistic Sorting and Information Theoretic Complexity", in Proceedings of the Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, pp 467-474, January 1993.

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.

 public static  void sort(List<T> list,
    Comparator<? super T> c) 
{
        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]);
        }
}

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, {@code c.compare(e1, e2)} must not throw a {@code ClassCastException} for any elements {@code e1} and {@code 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.

Implementation note: This implementation is a stable, adaptive, iterative mergesort that requires far fewer than n lg(n) comparisons when the input array is partially sorted, while offering the performance of a traditional mergesort when the input array is randomly ordered. If the input array is nearly sorted, the implementation requires approximately n comparisons. Temporary storage requirements vary from a small constant for nearly sorted input arrays to n/2 object references for randomly ordered input arrays.

The implementation takes equal advantage of ascending and descending order in its input array, and can take advantage of ascending and descending order in different parts of the same input array. It is well-suited to merging two or more sorted arrays: simply concatenate the arrays and sort the resulting array.

The implementation was adapted from Tim Peters's list sort for Python ( TimSort). It uses techiques from Peter McIlroy's "Optimistic Sorting and Information Theoretic Complexity", in Proceedings of the Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, pp 467-474, January 1993.

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.

 public static  void swap(List<?> list,
    int i,
    int j) 
{
        final List l = list;
        l.set(i, l.set(j, l.get(i)));
}

Swaps the elements at the specified positions in the specified list. (If the specified positions are equal, invoking this method leaves the list unchanged.)

 public static Collection<T> synchronizedCollection(Collection<T> c) 
{
        return new SynchronizedCollection<  >(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.

 static Collection<T> synchronizedCollection(Collection<T> c,
    Object mutex) 
{
        return new SynchronizedCollection<  >(c, mutex);
}

 public static List<T> synchronizedList(List<T> list) 
{
        return (list instanceof RandomAccess ?
                new SynchronizedRandomAccessList<  >(list) :
                new SynchronizedList<  >(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.

 static List<T> synchronizedList(List<T> list,
    Object mutex) 
{
        return (list instanceof RandomAccess ?
                new SynchronizedRandomAccessList<  >(list, mutex) :
                new SynchronizedList<  >(list, mutex));
}

 public static Map<K, V> synchronizedMap(Map<K, V> m) 
{
        return new SynchronizedMap<  >(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.

 public static Set<T> synchronizedSet(Set<T> s) 
{
        return new SynchronizedSet<  >(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.

 static Set<T> synchronizedSet(Set<T> s,
    Object mutex) 
{
        return new SynchronizedSet<  >(s, mutex);
}

 public static SortedMap<K, V> synchronizedSortedMap(SortedMap<K, V> m) 
{
        return new SynchronizedSortedMap<  >(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.

 public static SortedSet<T> synchronizedSortedSet(SortedSet<T> s) 
{
        return new SynchronizedSortedSet<  >(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.

 public static Collection<T> unmodifiableCollection(Collection<? extends T> c) 
{
        return new UnmodifiableCollection<  >(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.

 public static List<T> unmodifiableList(List<? extends T> list) 
{
        return (list instanceof RandomAccess ?
                new UnmodifiableRandomAccessList<  >(list) :
                new UnmodifiableList<  >(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 RandomAccess if the specified list does.

 public static Map<K, V> unmodifiableMap(Map<? extends K, ? extends V> m) 
{
        return new UnmodifiableMap<  >(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.

 public static Set<T> unmodifiableSet(Set<? extends T> s) 
{
        return new UnmodifiableSet<  >(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.

 public static SortedMap<K, V> unmodifiableSortedMap(SortedMap<K, ? extends V> m) 
{
        return new UnmodifiableSortedMap<  >(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.

 public static SortedSet<T> unmodifiableSortedSet(SortedSet<T> s) 
{
        return new UnmodifiableSortedSet<  >(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.

 static T[] zeroLengthArray(Class<T> type) 
{
        return (T[]) Array.newInstance(type, 0);
}