Doubt over reference when instantiating an object

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Doubt over reference when instantiating an object

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I have a question regarding the creation of objects, I will give 3 examples:

List<> nuevaLista = new ArrayList<>();

ArrayList<> nuevaLista= new ArrayList<>();

List <> nuevaLista = new LinkedList<>();

I know that List is an interface , and that any class that implements List should implement its abstract methods , but in this case, nuevaLista , How can it be a variable of type List if this is not a class?

Out of that doubt, I read that in 1 and 3, nuevaLista can only implement methods of the interface List , then what is the difference between instantiating it as ArrayList or LinkedList or any class that implements List ?

Thankful for your answers.

java netbeans

asked by Christian 04.08.2018 в 00:06

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score 1 · Answer 1

When you define List < > new list; you are declaring a variable of type List < > that can refer to objects of all classes that inherit from the List < & gt ;. This is called polymorphism.

Therefore, in your first new example, List can refer to instances of objects of different classes (all those that implement List < >) and in the second example newList can only point to objects of class ArrayList < > (to objects of his daughter classes, if he had them, too).

What can not be done in Java is to instantiate interfaces or abstract classes, but you can have a reference / variable that has one of them as a type. As I said before, this allows to make use of the polymorphism.

score 0 · Answer 2

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It can be done because LinkedList implements List as well as ArrayList, that you can check in the Java API

answered by 04.08.2018 в 00:27

score 0 · Answer 3

LinkedList as they said List as well as ArrayList em> , but unlike one another LinkedList is implemented with a double-linked list, instead ArrayList with an array that dynamically doubles its size when more space is needed.

Therefore we can consider that LinkedList for example allows me to delete or add elements with a complexity of O (1) while an ArrayList in O (n).
But for the search ArrayList is more efficient since it has a complexity of O (1) and LinkedList of O (n).

Conclusion: They are different implementations for the same purpose, so depending on the use you want to give you will choose one or the other.

I hope it helps you!

Greetings! :)