Almost all collections in Java are derived from the java.util.Collection interface. Collection defines the basic parts of all collections. The interface states the add() and remove() methods for adding to and removing from a collection respectively. Also required is the toArray() method, which converts the collection into a simple array of all the elements in the collection. Finally, the contains() method checks if a specified element is in the collection. The Collection interface is a subinterface of java.lang.Iterable, so any Collection may be the target of a for-each statement. (The Iterable interface provides the iterator() method used by for-each statements.) All collections have an iterator that goes through all of the elements in the collection. Additionally, Collection is a generic. Any collection can be written to store any class. For example, Collection can hold strings, and the elements from the collection can be used as strings without any casting required.

Java Collections Interfaces

To keep the number of core collection interfaces manageable, the Java platform doesn’t provide separate interfaces for each variant of each collection type. If an unsupported operation is invoked, a collection implementation throws an UnsupportedOperationException.

Collection Interface

This is the root of the collection hierarchy. A collection represents a group of objects known as its elements. The Java platform doesn’t provide any direct implementations of this interface.

The interface has methods to tell you how many elements are in the collection (size, isEmpty), to check whether a given object is in the collection (contains), to add and remove an element from the collection (add, remove), and to provide an iterator over the collection (iterator).

Collection interface also provides bulk operations methods that work on entire collection – containsAll, addAll, removeAll, retainAll, clear.

Synchronized Wrappers

The synchronization wrappers add automatic synchronization (thread-safety) to an arbitrary collection. Each of the six core collection interfaces — Collection, Set, List, Map, SortedSet, and SortedMap — has one static factory method.

public static Collection synchronizedCollection(Collection c);
public static Set synchronizedSet(Set s);
public static List synchronizedList(List list);
public static <K,V> Map<K,V> synchronizedMap(Map<K,V> m);
public static SortedSet synchronizedSortedSet(SortedSet s);
public static <K,V> SortedMap<K,V> synchronizedSortedMap(SortedMap<K,V> m);

Each of these methods returns a synchronized (thread-safe) Collection backed up by the specified collection.

Thread-Safe Collections

Java 1.5 Concurrent package (java.util.concurrent) contains thread-safe collection classes that allow collections to be modified while iterating. By design, the iterator is fail-fast and throws ConcurrentModificationException. Some of these classes are CopyOnWriteArrayList, ConcurrentHashMapCopyOnWriteArraySet.

Collections API Algorithms

Java Collections Framework provides algorithm implementations that are commonly used such as sorting and searching. Collections class contain these method implementations. Most of these algorithms work on List but some of them are applicable for all kinds of collections.


The sort algorithm reorders a List so that its elements are in ascending order according to an ordering relationship. Two forms of the operation are provided. The simple form takes a List and sorts it according to its elements’ natural ordering. The second form of sort takes a Comparator in addition to a List and sorts the elements with the Comparator.


The shuffle algorithm destroys any trace of order that may have been present in a List. That is, this algorithm reorders the List based on input from a source of randomness such that all possible permutations occur with equal likelihood, assuming a fair source of randomness. This algorithm is useful in implementing games of chance.


The binarySearch algorithm searches for a specified element in a sorted List. This algorithm has two forms. The first takes a List and an element to search for (the “search key”). This form assumes that the List is sorted in ascending order according to the natural ordering of its elements. The second form takes a Comparator in addition to the List and the search key, and assumes that the List is sorted into ascending order according to the specified Comparator. The sort algorithm can be used to sort the List prior to calling binarySearch.


The frequency and disjoint algorithms test some aspect of the composition of one or more Collections.

  • frequency: counts the number of times the specified element occurs in the specified collection
  • disjoint: determines whether two Collections are disjoint; that is, whether they contain no elements in common

Min and Max values

The min and the max algorithms return, respectively, the minimum and maximum element contained in a specified Collection. Both of these operations come in two forms. The simple form takes only a Collection and returns the minimum (or maximum) element according to the elements’ natural ordering.
The second form takes a Comparator in addition to the Collection and returns the minimum (or maximum) element according to the specified Comparator.

Collection classes in a Nutshell

Below table provides basic details of commonly used collection classes.

CollectionOrderingRandom AccessKey-ValueDuplicate ElementsNull ElementThread Safety


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