Declaring an ArrayList object as final for use in a constants file

Question

I am generating an ArrayList of objects. Following is the code

ArrayList someArrayList = new ArrayList();  Public ArrayList getLotOfData() { ArrayList someData = new ArrayList(); return someData;  }   someArrayList = eDAO.getLotOfData(); 

Once I have this ArrayList object "someArrayList", I would like to declare it public and final and store it in a Constants file so that it can be accessed globally. Is there a way I can do that? If I declare an Arraylist object public and final, then I would not be able to reassign any values to it. I tried the following

public final ArrayList anotherArrayList = new ArrayList();  anotherArrayList.addAll(someArrayList); 

I had hoped to store this "anotherArrayList" as a global ArrayList object and use it, but this returns a nullpointer exception. I want to use it just like a String constant "ConstantsFile.anotherArrayList". Any ideas???

Answer 1

You can easily make it public static final, but that won't stop people from changing the contents.

The best approach is to safely publish the "constant" by:

• wrapping it in an unmodifiable list
• using an instance block to populate it

Resulting in one neat final declaration with initialization:

public static final List<String> list = Collections.unmodifiableList(     new ArrayList<String>() {{         add("foo");         add("bar");         // etc     }}); 

or, similar but different style for simple elements (that don't need code)

public static final List<String> list =      Collections.unmodifiableList(Arrays.asList("foo", "bar"));