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Using of Collections.emptyList() the right way in Java 1.5+

Posted by JOKe on 7/27/2010 04:07:00 PM

It is little strange that no one really uses emptyList like he should in Java.
So this is small post showing how to use the emptyList method in the Collections class.

Question: How to create an empty list ?
lets assume that we have a class Book with multiple titles.
So the class in our examples will be:

import java.util.List;

public class Book {

      private List<String> titles;

      public void setTitles(List<String> titles) {

            this.titles = titles;

      }

      public List<String> getTitles() {

            return titles;

      }

}

Lets say that in our snippet we have something like :

  Book myCrazyBook=new Book();

and we want to assign an empty list to the titles in this book. This is a common scenario if you want to set something to be emptyList in specific case instead of null.

So of course your first try will be something like:
Answer 1:

  myCrazyBook.setTitles(new ArrayList()); 

-WARRNING - ArrayList is a raw type. References to generic type ArrayList<E> should be parameterized

Ok changing to:

  myCrazyBook.setTitles(new ArrayList<String>());   

WTF ? the problem in this answer is not small one :
- you are creating empty ArrayList which basiclly is not needed. ( what if you are in a loop 1000+ empty ArrayLists ? )

Answer 2.

   myCrazyBook.setTitles(Collections.EMPTY_LIST);

-WARNING : Type safety: The expression of type List needs unchecked conversion to conform to List<String>, because the EMPTY_LIST is raw type, which was heavily used before Java 1.5.
The definition of this list is :

    /**

     * The empty list (immutable).  This list is serializable.

     *

     * @see #emptyList()

     */

    public static final List EMPTY_LIST = new EmptyList();

So you see it is not ArrayList or LinkedList or MyCrazyCustomListWhichIUseEveryWhereList.

Answer 3:
Eclipse will tell you this answer : Replace with Collections.emptyList();

      myCrazyBook.setTitles(Collections.emptyList());

COMPILE TIME ERROR: why ? because emptyList invoked like that means List of Objects not List of String. The other thing is that you cannot cast to List<String> because you cannot cast List<Object>  to List<String>.

Answer 4:
You know that there is Collections.emptyList() AND YOU KNOW THAT IT RETURNS List<T>
so your answer is:

            Book myCrazyBook=new Book();

            List<String> titles = Collections.emptyList();

            myCrazyBook.setTitles(titles);

           

And this will work WITHOUT any WARNINGS or ERRORS. The type of the List is taken from the type of the variable that you are trying to assign to.
Of course why you need this local variable ? I want to remove it it is not needed and used, but if you remove it you are in Answer 3. So what can you do ?

Answer 5: The right but strange looking answer.

            Book myCrazyBook=new Book();

            myCrazyBook.setTitles(Collections.<String>emptyList());

Yes this is not a joke there is a dot (.) then the generic type before the method name. This compiles, runs, no warnings thats the real way.
It is a little strange that most people dont know this aspect of generics in Java even I didn't know it, I was thinking that you cannot pass a generic type like this but as it looks you can.

So if you have some static method like this one:

public class Main{

      public static <G> List<G> someGreatMethod(){

            List<G> gList= new ArrayList<G>();

            return gList;

      }

}

you can invoke it with Main.<String>someGreatMethod(); for example.

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