How to initialize a static list <T>?

Question

I can not understand what is happening in this declaration.

public static List<Vertex> vertices; // where Vertex is a class with a default constructor public static void main ( String [] arg ) throws IOException { vertices = new List<Vertex>(); // eclipse complains } 

Where and how should I initialize this list ..... Because of this, when I keep adding to the list, it complains about null pointer exception ..... Can someone tell me what I'm doing wrong .. ..

Answer 1

A list is an abstract type that is expanded and implemented by various types of lists. Try the following:

  public static void main ( String [] arg ) throws IOException { vertices = new ArrayList<Vertex>(); } 

Answer 2

A list is an interface and cannot be created. Use an ArrayList or LinkedList instead.

 vertices = new ArrayList<Vertex>(); 

Answer 3

A list is an interface. You need to use a class that implements List, such as ArrayList.

Answer 4

Try:

 vertices = new ArrayList<Vertex>(); 

List is an interface in Java, so you need to use one of its implementations.

http://download.oracle.com/javase/6/docs/api/java/util/List.html

Answer 5

List not a class, but an interface. Since an interface is not a complete concrete implementation of something, it can be created. You can only make new, not abstract classes. So try creating an instance of ArrayList or another implementation.

Answer 6

You need to use the List implementation, for example:

 vertices = new ArrayList<Vertex>(); 

Answer 7

Eclipse complains because List cannot be created, because it is an interface, not a concrete class. You have 2 options here -

Option1:

 public static List<Vertex> vertices; // where Vertex is a class with a default constructor public static void main ( String [] arg ) throws IOException { vertices = new ArrayList<Vertex>(); // eclipse does not complain } 

Option2:

 public static List<Vertex> vertices=new ArrayList<Vertex>(); // where Vertex is a class with a default constructor public static void main ( String [] arg ) throws IOException { v }