CODEWITHSUNDEEP
javagenerics
Adam
Adam

Reputation: 637

Java List vs ArrayList

As a C++ oldtimer I have managed to solve my problem but I can not wrap my head around the underlying Java mechanisms here: 

Vector<Object> x = new Vector<Object>();        // OK
ArrayList<Object> y = new ArrayList<Object>();  // OK
List<Object> zzz = new ArrayList<Object>();     // OK solves problem below but question remains
List<Object> z = new List<Object>();            // WHY? Compiler error: Cannot instantiate

Upvotes: 9

Views: 8036

Answers (7)

Benjamin
Benjamin

Reputation: 628

"Interface" is like a protocol that an object must comply with.

Upvotes: 1

Bryan Giovanny
Bryan Giovanny

Reputation: 424

Yes. Because List is an Interface and in Java you cannot instantiate an Interface. You can only instantiate a class.

ArrayList is a class that's implementing List<> that's why you can instantiate it. :)

Upvotes: 3

Azodious
Azodious

Reputation: 13872

List is an interface and an interface can't be instantiated.

It's used to implement polymorphism. i.e. a reference of interface type can hold object of any class that implements it.

List<Object> zzz = new ArrayList<Object>();

it works, cause ArrayList implements List.

Upvotes: 0

Santosh
Santosh

Reputation: 17903

The List is an interface. You cannot create in instance of an interface using new operator. That's why the line List<Object> z = new List<Object>(); gives error. Only classes can be instantiated.

Upvotes: 2

Pratik
Pratik

Reputation: 30855

List isn't class it's an Interface and you can't instantiate the interface object.

ArrayList is the class which was implement the List interface so can able to instantiate the ArrayList object and assign to the List object

Upvotes: 1

MByD
MByD

Reputation: 137322

List is an interface, you cannot initialize it. ArrayList implements List, so you can assign an ArrayList instance to a List variable.

Upvotes: 5

Daniel Earwicker
Daniel Earwicker

Reputation: 116674

List is an interface, somewhat like a class with some = 0 methods in C++. You can't instantiate it.

But ArrayList<T> "inherits" List<T> (or in Java terms, implements it), so those references are assignment-compatible.

Upvotes: 15

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