List immutablity best practice

Question

Hello,
I have a singleton bean in my app that basically maintains Lists of all my codes tables in the system. Other objects as well as JSPs can call getter methods on this object to get the list of codes. However, I obviously want to make this object immutable so that a calling object cannot modify a list it gets and thereby affect the entire application and any other object using said list.
What is the best way to implement immutablity? The way I have it now is to instantiate a new object in the getter like this:
public List<SelectItem> getJobStatusList() {
return new ArrayList<SelectItem>(this.jobStatusList);
}However, a friend of mine suggested this instead:
public List<SelectItem> getJobStatusList() {
return Collections.unmodifiableList(this.jobStatusList);
}Which is the better approach or are they interchangeable? In the 2nd approach, it leaves it up to the calling object to instantiate a new list for modification whereas the first one does this automatically. I'm not a fan of the UnsupportedOperationException being called on the getter, but if that is the preferred approach I will deal with it through javadoc.
Any suggestions or is this a matter of preference?

Answer 1

Both options could be used but have down sides.
In the first case you are taking a mutable copy. This is not the same as an immutable copy but it does means that any changes made to the original don't change your copy.
In the second case you are returning an immutable facade. This means the collection cannot be changed using this, however if the original is changed, your facade will see those changes.
The safest option (which may be overkill for your needs is to do both)
return Collections.unmodifiableList(new ArrayList<SelectItem>(this.jobStatusList));
{code]
This returns an independent copy which is immutable.