Adding elements to a list of strings in java

Question

I and more used to how things are done in C# than how things are done in java. The following lines are in java:

List <String> cIDs = null;
cIDs.add("something");

This shows a warning in eclipse. It says that the variable cIDs can only be null at this location. This tell me that the add will fail.

I tried writing some instantiate code:

cIDs = new List<String>()

but this does not work in java.

How do I add elements to a list of strings in java?

`List` is the interface. If you want to instanciate it, you need to use a class that implements the interface, such as `ArrayList`. — AntonH, Aug 06 '14 at 23:15

score 2 · Answer 1 · answered Aug 06 '14 at 23:15

2

Try this:

    import java.util.ArrayList;
    ...

    cIDs = new ArrayList<String>();

List is an interface and thus cannot be instantiated. ArrayList is a concrete class that implements that interface. Thus is can be instantiated.

answered Aug 06 '14 at 23:15

EJK

score 2 · Answer 2 · answered Aug 06 '14 at 23:26

List is an interface, it is abstract, not to be used to create/instantiate real object. So you have to use its "implementation" either your own:

public YourListimplements List<String>{
....
}
List<String> aList = new YourList();

( anonymously inline)

   List<String> aList= new List<String>() {
   ....
   }

or existing one :

public class java.util.ArrayList extends java.util.AbstractList implements java.util.List ....{
...
}

List<String> aList= new ArrayList();

score 1 · Answer 3 · answered Aug 06 '14 at 23:16

1

List is an interface, this means it cannot be instantiated. Instead, use the ArrayList implementation:

List<String> cIDs = new ArrayList<>();
cIDs.add("something");

If the diamond operator (<>) isn't supported, use new ArrayList<String>().

answered Aug 06 '14 at 23:16

August

3 Answers3