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It is a collection of collection classes in the java API. It can used to handle the data structure in java language or collection classes in java API used to manage the data very efficiently like inserting, deleting, updating, retrieving, sorting the data etc.

It is one of the standardized mechanisms which allow us to group multiple values either of same types or different type or both the types in a single variable with dynamic size in nature. This single variable is known as collection variable.
All the operations that we want to perform on a data such as searching, sorting, insertion, deletion etc. Before storing or moving the data to permanent memory location, can be performed by Java Collection Framework.

High-level diagram showing collection hierarchy

The Java collection framework is part of the java.util package and it has a collection interface on the top of the hierarchy. We can see the complete hierarchy below, List, Queue, and Set implements the Collection interface. The Map interface is not inherited from the Collection interface and unlike List, Queue, or Set, it operates on key/value pair.

 Java Collections Main Entities

There are four interfaces in Java Collection Framework, and they are List, Queue, Set, and Map.

List

The List stores a sequence of elements. We can access any element by its position in the List. The List interface has three implementing classes; ArrayList, Vector, and LinkedList.

Queue

The Queue maintains the order of the elements; it follows FIFO (first in, first out) concept. The Queue interface has two implementing classes; LinkedList and PriorityQueue  

Set

The set is a collection of unique elements, and it doesn’t allow duplicate elements. The classes HashSet and LinkedHashSet implements this interface, and extended by SortedSet class extends this interface. TreeSet implements SortedSet.                                                                                                                                                                                                                                                                                                                                                                       

Map

The Map interface is not inherited from the Collection interface. It is a collection of elements as key/value pair. The classes HashMap and LinkedHashMap implements this interface, and extended by SortedMap class extends this interface. TreeMap implements SortedMap.

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            

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