How to initialize List String object in Java

Question

I can not initialize a List as in the following code   List lt String gt  supplierNames   new List lt String gt     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 lt String gt    How can I instantiate List lt String gt

User · Answer

Depending on what kind of List you want to use, something like

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

should get you going.

List is the interface, ArrayList is one implementation of the List interface. More implementations that may better suit your needs can be found by reading the JavaDocs of the List interface.

User · Answer

Can t instantiate an interface but there are few implementations   JDK2  List lt String gt  list   Arrays asList  one    two    three      JDK7    diamond operator List lt String gt  list   new ArrayList lt  gt     list add  one    list add  two    list add  three      JDK8  List lt String gt  list   Stream of  one    two    three   collect Collectors toList       JDK9      creates immutable lists  so you can t modify such list  List lt String gt  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 lt String gt  mutableList   new ArrayList lt  gt  List of  one    two    three       Plus there are lots of other ways supplied by other libraries like Guava   List lt String gt  list   Lists newArrayList  one    two    three

User · Answer

If you just want to create an immutable List lt T gt  with only one object in it  you can use this API   List lt String gt  oneObjectList   Collections singletonList  theOnlyObject        More info  docs

User · Answer

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 lt String gt  supplierNames   new ArrayList lt String gt       LinkedList - a list implemented like an interconnected chain of elements  List lt String gt  supplierNames   new LinkedList lt String gt

User · Answer

If you check the API for List you ll notice it says   Interface List lt E gt    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 lt String gt  supplierNames1   new ArrayList lt String gt      List lt String gt  supplierNames2   new LinkedList lt String gt      List lt String gt  supplierNames3   new Vector lt String gt         Bonus  You can also instantiate it with values  in an easier way  using the Arrays class  as follows   List lt String gt  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

User · Answer

You will need to use ArrayList lt String gt  or such   List lt String gt  is an interface   Use this     import java util ArrayList        List lt String gt  supplierNames   new ArrayList lt String gt

User · Answer

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 lt String gt  supplierNames   new ArrayList lt String gt        or  List lt String gt  supplierNames   new LinkedList lt String gt

User · Answer

List is an interface  and you can not initialize an interface  Instantiate an implementing class instead   Like   List lt String gt  abc   new ArrayList lt String gt     List lt String gt  xyz   new LinkedList lt String gt

User · Answer

We created soyuz-to to simplify 1 problem  how to convert X to Y  e g  String to Integer   Constructing of an object is also kind of conversion so it has a simple function to construct Map  List  Set    import io thedocs soyuz to   List lt String gt  names   to list  John    Fedor      Please check it - it has a lot of other useful features

User · Answer

In most cases you want simple ArrayList - an implementation of List  Before JDK version 7  List lt String gt  list   new ArrayList lt String gt       JDK 7 and later you can use the diamond operator  List lt String gt  list   new ArrayList lt  gt       Further informations are written here Oracle documentation - Collections

User · Answer

List is an Interface   You cant use List to initialize it     List lt String gt  supplierNames   new ArrayList lt String gt       These are the some of List impelemented classes   ArrayList  LinkedList  Vector   You could use any of this as per your requirement  These each classes have its own features

User · Answer

Just in case  any one still lingering around this question  Because  i see one or two new users again asking the same question and everyone telling then   No you can t do that  Dear Prudence  Apart from all the answers given here  I would like to provide additional Information -  Yes you can actually do  List list   new List    But at the cost of writing implementations of all the methods of Interfaces   The notion is not simply List list   new List    but   List lt Integer gt  list   new List lt Integer gt               Override         public int size                    TODO Auto-generated method stub             return 0                      Override         public boolean isEmpty                    TODO Auto-generated method stub             return false                      Override         public boolean contains Object o                   TODO Auto-generated method stub             return false                    and So on  Cant write all methods    This is an example of Anonymous class  Its correct when someone states   No you cant instantiate an interface  and that s right  But you can never say   You CANT write List list   new List    but  evidently you can do that and that s a hard statement to make that You can t do

[java] How to initialize List<String> object in Java?

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