Java Collections Class

Updated March 27, 2023

Introduction to Java Collections Class

In Java, an individual object’s group is represented as one unit known as Collection. This framework defines multiple classes and interfaces to denote a group of objects as a single unit. In addition to that, several other features are also present for the Java Collections Class. It includes:

• Two important root interfaces are the Collection interface and Map interface of the packages java.util.Collection and java.util.Map respectively.
• If the collections of objects of the class are null, then NullPointerException will be thrown.
• Polymorphic algorithms are supported.

Declaration and Methods of Java Collections Class

More details on the Java Collections Class will be discussed in the following sections.

Declaration

Java Collections Class can be declared using the below syntax:

Syntax:

public class Collections extends Object

Methods

Now, let us see some of the commonly used methods in the Java Collections Class.

• addAll(Collection<? super T> c, T… elements): Every element will be added to the mentioned collection on calling this method.
• binarySearch(List<? extends Comparable<? super T>> list, T key): Mentioned list will be searched for the specified object with the help of a binary search algorithm.
• binarySearch(List<? extends T> list, T key, Comparator<? super T> c): Mentioned list will be searched for the specified object with the help of a binary search algorithm.
• asLifoQueue(Deque<T> deque): Deque view will be returned similar to LIFO(Last-in-First-Out) queue.
• checkedCollection(Collection<E> c, Class<E> type): The typesafe view, which is dynamic, will be returned for the collection mentioned.
• checkedList(List<E> list, Class<E> type): The typesafe view, which is dynamic, will be returned for the list mentioned.
• checkedMap(Map<K,V> m, Class<K> keyType, Class<V> valueType): The typesafe view which is dynamic will be returned for the map mentioned.
• checkedSet(Set<E> s, Class<E> type): The typesafe view, which is dynamic, will be returned for the set mentioned.
• checkedSortedMap(SortedMap<K,V> m, Class<K> keyType, Class<V> valueType): The typesafe view which is dynamic will be returned for the sorted map mentioned.
• checkedSortedSet(SortedSet<E> s, Class<E> type): The typesafe view, which is dynamic, will be returned for the sorted set mentioned.
• emptyEnumeration(): An enumeration will be returned which has no elements.
• emptyIterator(): An iterator will be returned which has no elements.
• emptyList(): An empty list will be returned, which is immutable.
• emptyListIterator(): A list iterator will be returned which has no elements.
• emptyMap(): An empty map will be returned, which is immutable.
• emptySet(): An empty set will be returned, which is immutable.
• copy(List<? super T> dest, List<? extends T> src): Elements from one list will be copied into another.
• disjoint(Collection<?> c1, Collection<?> c2): If no elements are common for the mentioned two collections, true will be returned.
• enumeration(Collection<T> c): An enumeration will be returned over the collection mentioned.
• fill(List<? super T> list, T obj): Elements of the list mentioned will be replaced with the element mentioned.
• frequency(Collection<?> c, Object o): Count of the elements in the collection will be returned that are equal to the object mentioned.
• lastIndexOfSubList(List<?> source, List<?> target): The beginning position of the last occurrence of the target list mentioned will be returned. If no such occurrences are there, -1 will be returned.
• indexOfSubList(List<?> source, List<?> target): The beginning position of the first occurrence of the target list mentioned will be returned. If no such occurrences are there, -1 will be returned.
• list(Enumeration<T> e): An array list will be returned, which contains the elements returned by the enumeration mentioned in the order where the enumeration is returned.
• unmodifiableCollection (Collection<? extends T> c): An unmodifiable view will be returned for the collection mentioned.
• unmodifiableList (List<? extends T> list): An unmodifiable view will be returned for the list mentioned.
• unmodifiableMap (Map<? extends K,? extends V> m): An unmodifiable view will be returned for the map mentioned.
• unmodifiableSet (Set<? extends T> s): An unmodifiable view will be returned for the set mentioned.
• unmodifiableSortedMap (SortedMap<K,? extends V> m): An unmodifiable view will be returned for the sorted map mentioned.
• unmodifiableSortedSet (SortedSet<T> s): An unmodifiable view will be returned for the sorted set mentioned.
• max(Collection<? extends T> coll): The largest element of the collection will be returned based on the element’s natural ordering.
• max(Collection<? extends T> coll, Comparator<? super T> comp): The largest element of the collection will be returned based on the comparator provided.
• min(Collection<? extends T> coll): The minimum element of the collection will be returned based on the element’s natural ordering.
• min(Collection<? extends T> coll, Comparator<? super T> comp): The minimum element of the collection will be returned based on the comparator provided.
• replaceAll(List<T> list, T oldVal, T newVal): Every occurrence of the value mentioned in the list will be replaced with another.
• nCopies(int n, T o): An immutable list will be returned, containing n copies of the object mentioned.
• newSetFromMap(Map< E,Boolean> map): A set will be returned, which is backed by the map mentioned.
• reverse(List<?> list): Elements in the list mentioned will be reversed.
• reverseOrder(): A comparator will be returned that makes the objects ordering reverse that can implement a Comparable interface.
• shuffle(List<?> list): The list mentioned will be shuffled based on the randomness.
• reverseOrder (Comparator<T> cmp): A comparator will be returned that makes the specified comparator’s ordering reverse.
• rotate(List<?> list, int distance): Elements in the list mentioned will be rotated by the distance specified.
• shuffle(List<?> list, Random rnd): The list mentioned will be permuted randomly based on the randomness.
• singleton(T o): An immutable set will be returned that contains only the object mentioned.
• singletonList(T o): An immutable list will be returned that contains only the object mentioned.
• singletonMap(K key, V value): An immutable map will be returned that maps only the key mentioned to a particular value.
• sort(List<T> list): The list mentioned will be sorted based on the natural ordering.
• sort(List<T> list, Comparator<? super T> c): The list mentioned will be sorted based on the comparator mentioned.
• swap(List<?> list, int i, int j): Elements in the list mentioned will be swapped based on the positions mentioned.
• synchronizedCollection (Collection<T> c): A synchronized collection that is thread-safe will be returned, which is backed by the collection mentioned.
• synchronizedList (List<T> list): A synchronized list that is thread-safe will be returned, which is backed by the list mentioned.
• synchronizedMap (Map<K,V> m): A synchronized map that is thread-safe will be returned, which is backed by the map mentioned.
• synchronizedSet (Set<T> s): A synchronized set that is thread-safe will be returned, which is backed by the set mentioned.
• synchronizedSortedMap (SortedMap<K,V> m): A synchronized sorted map that is thread-safe will be returned, which is backed by the sorted map mentioned.
• synchronizedSortedSet (SortedSet<T> s): A synchronized sorted set that is thread-safe will be returned, which is backed by the sorted set mentioned.

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