Difference betwixt List as well as ArrayList variable inwards Java?

Someone who is merely starting amongst Java programming linguistic communication oftentimes has incertitude close how nosotros are storing an ArrayList object inward List variable, what is the divergence betwixt List together with ArrayList? or why non merely shop the ArrayList object inward ArrayList variable merely similar nosotros create for String, int, together with other information types. Well, the top dog divergence betwixt List together with ArrayList is that List is an interface piece ArrayList is a class. Most importantly, it implements the List interface, which too way that ArrayList is a subtype of List interface. In Java or whatever object oriented language, super type of variable tin sack 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 super type. This is the beginning, forthwith let's run into those 2 questions every bit well.

Why shop ArrayList object on List variable?

You mightiness conduct maintain seen something similar this:

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

hither 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 take away to declare a reference variable, you lot should ever purpose the supertype e.g. Map for passing HashMap, Set for passing HashSet together with List for passing ArrayList, LinkedList or Vector. You should purpose interface every bit type on the furnish type of method, type of arguments etc every bit shown below? Now big query comes, Why should you lot create that?

The answer is to conduct maintain wages of Polymorphism. If you lot purpose interface than inward futurity if the novel implementation is shipped together with thence you lot are non required to alter your program. For example, whatever programme written using List volition travel every bit expected whether you lot travel past times a LinkedList, Vector or ArrayList, because they all implements List interface, they obey the contract exposed past times List interface.

The solely divergence comes inward performance, which is genuinely on of the driver for change. In short, if you lot programme using interface, tomorrow if a improve implementation of your interface is available together with thence you lot tin sack switch to that without making nay farther alter on the customer side (part of the programme which uses that interface).

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

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

together with purpose interface type on the furnish type of methods:

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

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

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

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

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

Your programme is pretty much difficult coded, it lack the flexibility offered past times Polymorphism together with Inheritance. This answer is too applicable to questions similar divergence betwixt Map together with HashMap or Set together with HashSet because ultimately they are the same question, the divergence betwixt interface together with their implementation. You must follow SOLID principles to write a improve programme inward object oriented programming languages similar Java or C++.

Here are 10 object oriented principles Every Java Developer should know