Convert an ArrayList to an unmodifiable ArrayList

Question

I'm trying to get a unmodifiable ArrayList to a final variable EX_FIELDS. exList is an existing ArrayList object.

EX_FIELDS = (ArrayList<String>) Collections.unmodifiableList(exList);

This code is present in a static block.When the class loads, I get the following error.

java.lang.ClassCastException: java.util.Collections$UnmodifiableRandomAccessList cannot be cast to java.util.ArrayList

I need to use the EX_FIELDS to support random access.Is there any other way to accomplish it?Thanks for any help in advance

Answer 1

EX_FIELDS should have type List<String>, not ArrayList<String>, and you shouldn't need to do any cast: you should just write

EX_FIELDS = Collections.unmodifiableList(exList);

This is one instance of the much more general rule that you should program to the interface, not the implementation.

Answer 2

You get a ClassCastException because Collections.unmodifiableList() doesn't return an ArrayList, it returns a List<T> (interface) which can be any backing class that implements the List interface. As you can see from the exception, it is actually returning an UnmodifiableRandomAccessList.

When you create the variable EX_FIELDS you should declare it like

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

That is, EX_FIELDS is a List and you have chosen an ArrayList for the actual instance. Later on you'll do

EX_FIELDS = Collections.unmodifiableList(exList);

unmodifiableList() returns a List, but you don't care what kind it actually is, as long as it conforms to the List interface.