Java Collections Interfaces - RichardDanielOliva/java-learning-wiki GitHub Wiki

Java collection interfaces are the foundation of the Java Collections Framework. Note that all the core collection interfaces are generic; for example public interface Collection. The syntax is for Generics and when we declare Collection, we should use it to specify the type of Object it can contain. It helps in reducing run-time errors by type-checking the Objects at compile-time.

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.

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.

The toArray methods are provided as a bridge between collections and older APIs that expect arrays on input.

Iterator interface provides methods to iterate over any Collection. We can get iterator instance from a Collection using iterator method. Iterator takes the place of Enumeration in the Java Collections Framework. Iterators allow the caller to remove elements from the underlying collection during the iteration. Iterators in collection classes implement Iterator Design Pattern.

The Set interface defines a collection that cannot contain duplicate elements. This interface contains only the methods inherited from Collection by adding the restriction that duplicate elements are prohibited. It is important to point out that, in order to check whether elements are duplicate elements or not, it is necessary that these elements have correctly implemented the equals and hashCode methods. To check if two Set are equal, it will be checked if all the elements that compose them are equal no matter in the order that these elements occupy.

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The Java platform contains three general-purpose Set implementations: HashSet, TreeSet, and LinkedHashSet. Set interface doesn’t allow random-access to an element in the Collection. You can use iterator or foreach loop to traverse the elements of a Set.

List is an ordered collection and can contain duplicate elements. You can access any element from its index. List is more like array with dynamic length. List is one of the most used Collection type. ArrayList and LinkedList are implementation classes of List interface.

List interface provides useful methods to add an element at a specific index, remove/replace element based on the index and to get a sub-list using the index.

Java Map is an object that maps keys to values. A map cannot contain duplicate keys: Each key can map to at most one value.

The Java platform contains three general-purpose Map implementations: HashMap, TreeMap, and LinkedHashMap.

The basic operations of Map are put, get, containsKey, containsValue, size, and isEmpty.

Queue is a collection used to hold multiple elements prior to processing. Besides basic Collection operations, a Queue provides additional insertion, extraction, and inspection operations.

Queues typically, but do not necessarily, order elements in a FIFO (first-in-first-out) manner. Among the exceptions are priority queues, which order elements according to a supplied comparator or the elements’ natural ordering. Whatever the ordering used, the head of the queue is the element that would be removed by a call to remove or poll. In a FIFO queue, all new elements are inserted at the tail of the queue.

A linear collection that supports element insertion and removal at both ends. The name deque is short for “double-ended queue” and is usually pronounced “deck”. Most Deque implementations place no fixed limits on the number of elements they may contain, but this interface supports capacity-restricted deques as well as those with no fixed size limit.

This interface defines methods to access the elements at both ends of the deque. Methods are provided to insert, remove, and examine the element.

An iterator for lists that allows the programmer to traverse the list in either direction, modify the list during iteration, and obtain the iterator’s current position in the list.

Java ListIterator has no current element; its cursor position always lies between the element that would be returned by a call to previous() and the element that would be returned by a call to next().

SortedSet is a Set that maintains its elements in ascending order. Several additional operations are provided to take advantage of the ordering. Sorted sets are used for naturally ordered sets, such as word lists and membership rolls.

Map that maintains its mappings in ascending key order. This is the Map analog of SortedSet. Sorted maps are used for naturally ordered collections of key/value pairs, such as dictionaries and telephone directories.

https://www.journaldev.com/1260/collections-in-java-tutorial