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Java collections class


The java.util.Collections class consists exclusively of static methods that operate on or return collections. It contains polymorphic algorithms that operate on collections, “wrappers”, which return a new collection backed by a specified collection, and a few other odds and ends.

Java Collections class fields

  • static List EMPTY_LIST: This is the empty list (immutable).
  • static Map EMPTY_MAP: This is the empty map (immutable).
  • static Set EMPTY_SET: This is the empty set (immutable).

Java Collections class methods

Method Description
static <T> boolean addAll(Collection<? super T> c, T… elements) This method adds all of the specified elements to the specified collection.
static <T> Queue<T> asLifoQueue(Deque<T> deque) This method returns a view of a Deque as a Last-in-first-out (Lifo) Queue.
static <T> int binarySearch(List<? extends Comparable<? super T>> list, T key) This method searches the specified list for the specified object using the binary search algorithm.
static <T> int binarySearch(List<? extends T> list, T key, Comparator<? super T< c) This method searches the specified list for the specified object using the binary search algorithm.
static <E> Collection<E> checkedCollection(Collection<E> c, Class<E> type) This method returns a dynamically typesafe view of the specified collection.
static <E> List<E> checkedList(List<E> list, Class<E> type) This method returns a dynamically typesafe view of the specified list.
static <K,V> Map<K,V>

checkedMap(Map<K,V> m,

Class<K> keyType, Class<V> valueType)

This method returns a dynamically typesafe view of the specified map.
static <E> Set<E> checkedSet(Set<E> s, Class<E> type) This method returns a dynamically typesafe view of the specified set.
static <K,V> SortedMap<K,V> checkedSortedMap(SortedMap<K,V> m, Class<K> keyType, Class<V> valueType) This method returns a dynamically typesafe view of the specified sorted map.
static <E> SortedSet<E>checkedSortedSet(SortedSet<E> s, Class<E> type) This method returns a dynamically typesafe view of the specified sorted set.
static <T> void copy(List<? super T> dest, List<? extends T> src) This method copies all of the elements from one list into another.
static boolean disjoint(Collection<?> c1, Collection<?> c2) This method returns true if the two specified collections have no elements in common.
static <T> List<T> emptyList() This method returns the empty list (immutable).
static <K,V> Map<K,V> emptyMap() This method returns the empty map (immutable).
static <T> Set<T> emptySet() This method returns the empty set (immutable).
static <T> Enumeration<T> enumeration(Collection<T> c) This method returns an enumeration over the specified collection.
static <T> void fill(List<? super T> list, T obj) This method replaces all of the elements of the specified list with the specified element.
static int frequency(Collection<?> c, Object o) This method returns the number of elements in the specified collection equal to the specified object.
static int indexOfSubList(List<?> source, List<?> target) This method returns the starting position of the first occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence.
static int lastIndexOfSubList(List<?> source, List<?> target) This method returns the starting position of the last occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence.
static <T> ArrayList<T> list(Enumeration<T> e) This method returns an array list containing the elements returned by the specified enumeration in the order they are returned by the enumeration.
static <T extends Object & Comparable<? super T> >T max(Collection<? extends T> coll) This method returns the maximum element of the given collection, according to the natural ordering of its elements.
static <T> T max(Collection<? extends T> coll, Comparator<? super T> comp) This method returns the maximum element of the given collection, according to the order induced by the specified comparator.
static <T extends Object & Comparable<? super T>>T min(Collection<? extends T> coll) This method Returns the minimum element of the given collection, according to the natural ordering of its elements.
static <T> T min(Collection<? extends T> coll, Comparator<? super T> comp) This method returns the minimum element of the given collection, according to the order induced by the specified comparator.
static <T> List<T> nCopies(int n, T o) This method returns an immutable list consisting of n copies of the specified object.
static <E> Set<E> newSetFromMap(Map<E,Boolean> map) This method returns a set backed by the specified map.
static <T> boolean replaceAll(List<T> list, T oldVal, T newVal) This method replaces all occurrences of one specified value in a list with another.
static void reverse(List<?> list) This method reverses the order of the elements in the specified list.
static <T> Comparator<T> reverseOrder() This method returns a comparator that imposes the reverse of the natural ordering on a collection of objects that implement the Comparable interface.
static <T> Comparator<T> reverseOrder(Comparator<T> cmp) This method returns a comparator that imposes the reverse ordering of the specified comparator.
static void rotate(List<?> list, int distance) This method rotates the elements in the specified list by the specified distance.
static void shuffle(List<?> list) This method randomly permutes the specified list using a default source of randomness.
static void shuffle(List<?> list, Random rnd) This method randomly permute the specified list using the specified source of randomness.
static <T> Set<T> singleton(T o) This method returns an immutable set containing only the specified object.
static <T> List<T> singletonList(T o) This method returns an immutable list containing only the specified object.
static <K,V> Map<K,V> singletonMap(K key, V value) This method returns an immutable map, mapping only the specified key to the specified value.
static <T extends Comparable<? super T>> void sort(List<T> list) This method sorts the specified list into ascending order, according to the natural ordering of its elements.
static <T> void sort(List<T> list, Comparator<? super T> c) This method sorts the specified list according to the order induced by the specified comparator.
static void swap(List<?> list, int i, int j) This method swaps the elements at the specified positions in the specified list.
static <T> Collection<T> synchronizedCollection(Collection<T> c) This method returns a synchronized (thread-safe) collection backed by the specified collection.
static <T> List<T>

synchronizedList(List<T> list)

This method returns a synchronized (thread-safe) list backed by the specified list.
static <K,V> Map<K,V> synchronizedMap(Map<K,V> m) This method returns a synchronized (thread-safe) map backed by the specified map.
static <T> Set<T> synchronizedSet(Set<T> s) This method returns a synchronized (thread-safe) set backed by the specified set.
static <K,V> SortedMap<K,V> synchronizedSortedMap(SortedMap<K,V> m) This method returns a synchronized (thread-safe) sorted map backed by the specified sorted map.
static <T> SortedSet<T> synchronizedSortedSet(SortedSet<T> s) This method returns a synchronized (thread-safe) sorted set backed by the specified sorted set.
static <T> Collection<T> unmodifiableCollection

(Collection<? extends T> c)

This method returns an unmodifiable view of the specified collection.
static <T> List<T>

unmodifiableList(List<? extends T> list)

This method returns an unmodifiable view of the specified list.
static <K,V> Map<K,V> unmodifiableMap(Map<? extends K,? extends V> m) This method returns an unmodifiable view of the specified map.
static <T> Set<T>

unmodifiableSet(Set<? extends T> s)

This method returns an unmodifiable view of the specified set.
static <K,V> SortedMap<K,V> unmodifiableSortedMap

(SortedMap<K,? extends V> m)

This method returns an unmodifiable view of the specified sorted map.
static <T> SortedSet<T> unmodifiableSortedSet(SortedSet<T> s) This method returns an unmodifiable view of the specified sorted set.

Java Collections class examples

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