Difference Betwixt A Listing Together With Arraylist Reference Variable Inwards Java?

Someone who is but starting amongst Java programming linguistic communication ofttimes has dubiety almost how nosotros are storing an ArrayList object inwards List variable, what is the deviation betwixt List in addition to ArrayList? Or why non but salvage the ArrayList object inwards ArrayList variable but similar nosotros produce for String, int, in addition to other information types. Well, the nous deviation betwixt List in addition to ArrayList is that List is an interface piece ArrayList is a class. Most importantly, it implements the List interface, which likewise agency that ArrayList is a subtype of List interface. In Java or whatever object-oriented language, supertype of a variable tin flame shop an object of subtype.

This is known every bit Polymorphism because whatever virtual method volition live on executed from subclass only, fifty-fifty though they were called from the supertype. This is the beginning, similar a shot let's meet those 2 questions every bit well.

Anyway, If you lot are non familiar amongst Inheritance in addition to other object-oriented programming concepts inwards Java, I propose you lot to initiative off acquire through a comprehensive course of education like The Complete Java Masterclass, which is the most up-to-date online course of education to acquire Java. Always updated for latest Java version similar Java 12. 

Why shop ArrayList object on List variable?

You powerfulness receive got seen something similar this:

List<Movie> listOfMovies = new ArrayList<Movie>()

Here nosotros are using a List every bit a type of variable to shop an object of ArrayList class, created using new() operator. This is known every bit programming for the interfaces. In fact, everywhere you lot involve to declare a reference variable, you lot should ever role the supertype, similar Map for passing HashMap, Set for giving HashSet in addition to List for passing ArrayList, LinkedList or Vector.

You should role interface every bit type on the supply type of method, type of arguments, etc. every bit shown below? Now the big query comes, Why should you lot produce that?

The reply is to accept payoff of Polymorphism. If you lot role interface than inwards futurity if the novel implementation is shipped, thence you lot are non required to alter your program.

For example, an application written using List volition run every bit expected whether you lot transcend a LinkedList, Vector, or ArrayList because they all implement List interface, they obey the contract exposed yesteryear List interface.

The solely deviation comes inwards performance, which is genuinely ane of the drivers for change. In short, if you lot programme using interface, tomorrow if a amend implementation of your interface is available thence you lot tin flame switch to that without making whatever farther alter on the client-side (part of the programme which uses that interface).


Similarly, you lot should role interface type on method arguments:

public void sort(List<? extends Comparable> input){        // logic to form the listing   }

in addition to role interface type on the supply type of methods:

public List<Employee> getEmployees(int departmentId){        // logic to supply employees for a given department }

If you lot shop ArrayList's object into a reference type of ArrayList, every bit shown below, thence your code volition non run if you lot transcend a LinkedList, because ArrayList IS NOT a LinkedList.

public List<Movies> getMovies(LinkedList<String> actors){        // this code volition non run if you lot transcend ArrayList }
 

but next code volition work, fifty-fifty if you lot transcend ArrayList, Vector, CopyOnWriteArrayList or LinkedList, because it expects a List, which is an interface:

public List<Movies> getMovies(List<String> actors){        // this code volition run if you lot transcend ArrayList, Vector etc }

Your programme is pretty much hardcoded, it lacks the flexibility offered yesteryear Polymorphism in addition to Inheritance. This reply is likewise applicable to questions similar the deviation betwixt Map in addition to HashMap or Set in addition to HashSet because ultimately they are the same question, the deviation betwixt an interface in addition to their implementation.

You must likewise follow SOLID Design Principles to write a amend programme inwards object-oriented programming languages similar Java or C++.

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