COLLECTIONS FRAMEWORK IN JAVA

Collections

➢ Collections are the containers that group multiple items into a single unit.

➢ Collections are used to store, retrieve and manipulate data and to transmit data from one method to another.

Java Collections Framework

➢ A collections framework is a unified architecture for representing and manipulating collections.

➢ Java Collections framework provides many interfaces and classes in order to store data.

Benefits of a Collections Framework

🔸 It reduces programming effort.

🔸 It increases program speed and quality.

🔸 It allows interoperability among unrelated APIs.

🔸 It fosters software reuse.

🔸 It reduces effort to design new APIs.

Java Collections Framework Hierarchy

➢ A set of APIs linked together as a parent-child relationship.

Java Interfaces

➢ In interfaces in Java, you write methods which must be implemented by Java objects as rules.

➢ Interfaces are the reference types which are similar to classes but contains only abstract methods as rules.

Iterable

➢ The Iterable interface is the root interface for all the Collection classes.

➢ The Collection interface along with all its subclasses also implement the Iterable interface.

Methods: Iterator <T> iterator()

➢ From the above method, we get reference of Iterator and we can iterate in the forward direction in any data structure.

Collection

➢ Collection interface is implemented by all the classes in the collection framework and declares the methods that every collection contain.

➢ This achieves Runtime Polymorphism.

Methods: Boolean add (Object obj)
         Boolean addAll (Object obj)
         void clear()                                   etc.

Lists

➢ Java list is an interface that extends the Collection.

➢ It contains an ordered collection of elements including duplicate values.

➢ Supports redundancy.

1. ArrayList

✽ ArrayList is the implementation of List Interface where the elements can be dynamically added or removed from the list.

✽ The size of the list is increased dynamically if the elements are added more than the initial size.

                 ArrayList object = new ArrayList();

2. LinkedList

✽ LinkedList is a sequence of links which contains items.

✽ Data Storage / Structure → Reference to other link

✽ Each link contains a connection to another link.

                Linkedlist object = new Linkedlist();

2 different types of LinkedLists.

1. Singly Linked List

● Each node in this list stores the data of the node and a pointer or reference to the next node in the list.

2. Doubly Linked List

● There are 2 references.

1. To next node
2. To previous node

Node ➜ Contains data

Next ➜ Points to next node

Previous ➜ Points to previous node

3. Vector

✽ This is an API that implements List Interface.

✽ Vectors are similar to arrays, where the elements of the vector object can be accessed via an index into the vector.

✽ A Vector is synchronized (thread safe) and implements a dynamic array that is not limited to specific size.

             Vector object = new Vector(size, increment);

Java Queue

➢ Queue in Java follows a FIFO (First In First Out) approach.

First In First Out — Orders the elements according to First in First out manner

➢ The first element is removed first and last element is removed in the end.

                Queue <Integer> q = new LinkedList<>();

Java Set

➢A set refers to a collection that cannot contain duplicate elements.

➢ Focuses on Uniqueness.

➢ It is mainly used to model the mathematical set abstraction.

➢ Uses hashing technique by which we will be having unique element coming in.

➢ Set has its implementation in various classes.

1. HashSet
2. LinkedHashSet
3. TreeSet

1. HashSet

✽ Java HashSet class creates a collection that use a hash table for storage.

✽ HashSet only contain unique elements and it inherits the AbstractSet Class and implements Set Interface.

✽ It uses a hashing mechanism to store the elements.

Hashing function → Generates a hashed code for the element

              HashSet <String> al = new HashSet();

2. LinkedHashSet

✽ Java LinkedHashSet class is a Hash table and Linkedlist implementation of the set interface.

✽ It contains only unique elements.

✽ It provides all optional set operations and maintains insertion order. (LinkedHashSet maintains how the data is getting inserted for us. HashSet is not able to do that, it gives us random/unordered output)

         LinkedHashSet <String> al = new LinkedHashSet();

✽ This class inherits methods from other classes.

3. TreeSet

✽ A sorted version of HashSet.

✽ TreeSet class implements the Set Interface that uses a tree for storage.

✽ The objects of this class are unique and are stored in the ascending order.

✽ It inherits AbstractSet class and implements NavigableSet interface.

           TreeSet <String> al = new TreeSet <String>();

Java Map

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

➢ Cannot traverse a Map, therefore it should be converted into Set using keySet() or entrySet() method.

           HashMap< , > object1 = new HashMap < , > ();

References

Collections in Java - javatpoint