Introduction

This article will be looking into one of the most popular questions in Java Language – What is Collection in Java? Also, what do you mean by Collections in Java? Are Collection and Collections the same or different in Java?

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What is Collection?

Collection can simply be described as an Interface. With the help of Collection, we can easily group various objects into a single unit. Collection forms the root or head of the hierarchy of interfaces. If you are well aware of the C++ language, consider the concept of Collection to be very similar to Containers in C++ language. The other sub interfaces of this hierarchy are Sets, Lists, Maps, Queue, Deque, etc.

Java JDK does not allow us to directly manipulate the Container root interface but does allow us to have direct access to its sub-interfaces mentioned above. There are also sub-interfaces to which we can have indirect access. Examples of such are – ArrayList, PriorityQueue, Vector, HashSet, etc.

It is widely used to signify a group of individual objects in the form of a single unit. It is identical to the container used in the C++ language. It is the root interface of the collection framework. You can access various interfaces and classes to signify a group of individual objects as a distinct unit. Understanding the fundamentals of collection and collections is important before going through their differences.

The key sub-interface of the collection interface are Set, List, and Queue. The map interface is too a subset of the Java collection framework. However, it doesn’t inherit the collection of the interface. The prominent methods of the Collection interface in Java are add(), remove(),size(),clear(), and contains().

Java collection framework helps manipulate the object’s collection. The collection framework includes multiple convenience classes, wrapper classes, collection interfaces, and classes for legacy implementation like Hashtable and vector, etc.  These frameworks are different in collection and collections.

The collection interface in the java collection framework is categorized into two parts:

Java util collection: It includes classes like List, Set, Queue, etc.

Java util map: It includes classes like sortedMap, Map, etc.

The collection interface methods mentioned below help you to understand the difference between collection and collections easily:

• add(): It adds a new element to the stated collection.
• remove(Object o): It removes the existing element from the stated collection.
• clear(): It removes all elements from the stated collection.
• size(): It gets the number of elements in the stated collection.
• isEmpty(): It checks whether the stated collection includes any element or not. It would return “True” if one or more elements exist in the collection, else it returns “False”.

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Need For Collection?

These Collection interfaces help us in dealing with various objects by grouping them efficiently and performing operations accordingly. These objects might be similar or different from each other. Based on the objects and their way of dealing with individual objects, we have the above-mentioned sub-interfaces.

Let us look into detail some of the most common Collection interfaces:

• Set – The most important thing about Sets is that Sets should never contain any duplicate element or object. This Set Interface can be obtained from java. util package. Sets do not return their elements in their collection in any predictable order. It is unordered, and many Java developers often forget this fact and thus face bugs in their code.

EnumSet, HashSet, LinkedHashSet, TreeSet, ConcurrentSkipList Set etc are some classes which can be implemented on the Set interface.

• List – List implementation can be thought of as something similar to Python Lists, but they do have some differences. List interfaces in Java are collections of ordered elements or objects. They are sorted. Moreover, unlike Sets, Lists can allow duplicate elements in their collection. Like an Array, a List’s elements can be accessed based on its elements’ index or position. Some basic operations using Lists are – Searching for an element, Accessing an element from the list, etc.

Vector, Stack, LinkedList, ArrayList, CopyOnWriteArrayList are some of the most commonly used classes which can be used with List interfaces.

• Map – For those well acquainted with Python, Map Interface is very similar to the Dictionary data structure in Python Language. Using the Map interface, we can group and store data elements in key-value pairs. As evident from the fact, each key is unique in a Map, and hence there are no duplicate keys. Using the key, its corresponding value is returned.

HashMap, HashTable, EnumMap, TreeMap, IdentityHashMap, WeakHashMap are some classes that can be implemented with the Map interface.

• Stack – Most of the programmers are already familiar with Stacks and Queues (Queues discussed shortly). Stack interface is implemented based on the usual LIFO data structure, i.e., Last In First Out. In a Stack, elements are pushed into one end of the stack, and elements pop out from the same end of that stack.

• Queue – Queue interface, based on the usual Queue Data Structure, implements a FIFO method. FIFO stands for First In First Out. In simple words, elements are pushed in one end of the Queue and popped out from the other end of that Queue.

Some classes of Collection are discussed below.

ArrayList:

It uses a dynamic array for elements’ storage. It allows duplicates. Moreover, it is index-based, so it is a perfect choice for those willing to insert data sequentially, retrieve it according to its index, and access data randomly.

For example, you can create a program that employs a mobile phone to store, revise, remove, query contact names, and store the contacts in ArrayList.

LinkedList:

It utilizes a doubly linked list for elements’ storage. Suppose a group of friends want to create a chain in a LinkedList. So, you can quickly accept new friends and remove the existing friends. In this case, LinkedList works such that it holds the address reference of your right and/or left side friends based on your location.

Any newcomers are welcomed, and existing friends are allowed to leave when they want.  When somebody leaves the chain, right and left elements delete their address and links to the previous and/or next friends. Note that nobody in the chain has to proceed to fill the space abandoned by the previous member(s). it means that the operation will be faster than that in an ArrayList

HashSet:

It uses hashing for elements’ storage. Duplicates are not allowed, the insertion order is not sustained, and a null value is permitted. Immutability is extremely significant because if you choose non-primitive data types in the array, the alteration of an object/object class might provide unnecessary results.

You must override hashCode() and equals()  methods to store your objects in the HashSet. You can better understand such classes of collection and collections in Java with an example. Suppose, you employ a student class with name and school as class members. Firstly, you must know your equality criteria before inserting every student’s data into HashSet. You can define which students are identical and which are not. Consequently, HashSet will not add duplicates that are identical to the list. 

LinkedHashSet:

Apart from the features inherited from the HashSet class, you can maintain the insertion order.

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What is Collections?

Collections is simply a utility class that is found in java. util package. The methods coming under the Collections class are compulsorily static methods.

Collections are utility class in java.util.package. It defines various utility methods like searching and sorting to work on the collection. It owns all static methods that offer enough convenience to developers to work with Collection Framework successfully.

For example, Collections have a method sort() to sort the collection elements as per the default sorting order. It also contains methods like min() and max() to determine the minimum and maximum values correspondingly in the collection elements. This is one of the important differences between collections and collection in Java.

The key purpose of Collections is to offer a convenient method to the developers. It only supports static methods that execute on and return collection. Moreover, the class methods provide a NullPointerException if the class or collections objects assigned to them are null.

Before going into the difference between collection and collections in java, let’s understand the need for collections.

Need For Collections?

Collections provide an easier go-to method for developers to perform certain basic operations on elements without needing to get into the nitty-gritty details of that operation. The developers can focus their attention on much bigger tasks or operations. Hence, Collections classes are extremely convenient among experienced developers.

For example – Collections class has a method to search for a particular element in a collection. It also has a method to perform sorting operations on the elements of the Collection interface.

Some commonly used Collections class are:

• Collections.binarySearch() – Searches for the desired element in a collection using the popular Binary Search Algorithm.
• Collections.sort() – Performs sorting operation on the specified Collection.
• Collections.max() – As evident from the name, it returns the Maximum element from a specified Collection.
• Collections.min() – Returns the Minimum element from a specified Collection.
• Collections.reverse() – Reverses the order of the elements present in the specified Collection.
• Collections.copy() – As the name suggests, using this, elements from one collection are copied to another collection.
• Collections.shuffle(): It randomly shuffles the elements in the stated collection.
• Collections.synchronizedCollection(): It synchronizes the stated collection.
• Collections.disjoint(): It returns “True” if two given collections don’t share any elements in common, else it returns “False”.We will go through the difference between collection and collections after understanding an example of Collections in Java.It is better to understand collections and collection in java with an example. Suppose you have a Theatre class with Seat inner class.You can create an ArrayList with Seats with the following command.List theList = new ArrayList<>(theatre.getSeats());The following command rotates the elements in the list:

  Collections.rotate(myList, 4);

  The following command reverses the elements in the list:

  Collections.reverse(myList);

   Collection vs. Collections in Java:

  The following table highlights the difference between collection and collections in Java.

  Collection	Collections
  It is an interface.	It is a utility class.
  It is utilized to represent a group of individual objects in the form of a single unit.	It specifies several utility methods that are used to work on the collection.
  The Collection is an interface that includes a static method since the launch of Java8. The Interface can contain default and abstract methods.	It contains static methods only.
  It extends the iterable interface.	It extends the Object class.

  It is easy to choose the most suitable one based on the above difference between collection and collections in Java.        

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Conclusion

So in Java, Collection and Collections are not at all the same thing. Collection is the interface where you group objects into a single unit. Collections is a utility class that has some set of operations you perform on Collection. Collection does not have all static methods in it, but Collections consist of methods that are all static.

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