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How to initialize List object in Java?


Question

I can not initialize a List as in the following code:

List<String> supplierNames = new List<String>();
supplierNames.add("sup1");
supplierNames.add("sup2");
supplierNames.add("sup3");
System.out.println(supplierNames.get(1));

I face the following error:

Cannot instantiate the type List<String>

How can I instantiate List<String>?

2014/10/17
1
416
10/17/2014 4:33:50 PM

Accepted Answer

If you check the API for List you'll notice it says:

Interface List<E>

Being an interface means it cannot be instantiated (no new List() is possible).

If you check that link, you'll find some classes that implement List:

All Known Implementing Classes:

AbstractList, AbstractSequentialList, ArrayList, AttributeList, CopyOnWriteArrayList, LinkedList, RoleList, RoleUnresolvedList, Stack, Vector

Those can be instantiated. Use their links to know more about them, I.E: to know which fits better your needs.

The 3 most commonly used ones probably are:

 List<String> supplierNames1 = new ArrayList<String>();
 List<String> supplierNames2 = new LinkedList<String>();
 List<String> supplierNames3 = new Vector<String>();

Bonus:
You can also instantiate it with values, in an easier way, using the Arrays class, as follows:

List<String> supplierNames = Arrays.asList("sup1", "sup2", "sup3");
System.out.println(supplierNames.get(1));

But note you are not allowed to add more elements to that list, as it's fixed-size.

2017/07/24
753
7/24/2017 8:33:01 AM

Can't instantiate an interface but there are few implementations:

JDK2

List<String> list = Arrays.asList("one", "two", "three");

JDK7

//diamond operator
List<String> list = new ArrayList<>();
list.add("one");
list.add("two");
list.add("three");

JDK8

List<String> list = Stream.of("one", "two", "three").collect(Collectors.toList());

JDK9

// creates immutable lists, so you can't modify such list 
List<String> immutableList = List.of("one", "two", "three");

// if we want mutable list we can copy content of immutable list 
// to mutable one for instance via copy-constructor (which creates shallow copy)
List<String> mutableList = new ArrayList<>(List.of("one", "two", "three"));

Plus there are lots of other ways supplied by other libraries like Guava.

List<String> list = Lists.newArrayList("one", "two", "three");
2019/06/10

List is an Interface, you cannot instantiate an Interface, because interface is a convention, what methods should have your classes. In order to instantiate, you need some realizations(implementations) of that interface. Try the below code with very popular implementations of List interface:

List<String> supplierNames = new ArrayList<String>(); 

or

List<String> supplierNames = new LinkedList<String>();
2017/01/13

You will need to use ArrayList<String> or such.

List<String> is an interface.

Use this:

import java.util.ArrayList;

...

List<String> supplierNames = new ArrayList<String>();
2012/11/15

List is an interface, and you can not initialize an interface. Instantiate an implementing class instead.

Like:

List<String> abc = new ArrayList<String>();
List<String> xyz = new LinkedList<String>();
2012/11/15

List is just an interface, a definition of some generic list. You need to provide an implementation of this list interface. Two most common are:

ArrayList - a list implemented over an array

List<String> supplierNames = new ArrayList<String>();

LinkedList - a list implemented like an interconnected chain of elements

List<String> supplierNames = new LinkedList<String>();
2012/11/15