Unmodifiable List in java

Although the accepted answer addresses the request of omitting / swallowing the UnsupportedOperationException, there should be an explanation of why it's an undesirable practice.

1) The overall intent to swallow an exception is a contradiction of the "fail-fast" principle. Instead of finding and catching defects early they are just allowed to go "under the carpet" turning into time bombs.

2) Not throwing UnsupportedOperationException is a direct violation of List's contract which says:

Throws:
    UnsupportedOperationException - if the add method is not supported by this list.

So the proper answer to what is written in the question's title: "Unmodifiable List in java" should still be:

return Collections.unmodifiableList(oListeRet);

Collections.unmodifiableList

Returns an unmodifiable view of the specified list. This method allows modules to provide users with "read-only" access to internal lists. Query operations on the returned list "read through" to the specified list, and attempts to modify the returned list, whether direct or via its iterator, result in an UnsupportedOperationException. The returned list will be serializable if the specified list is serializable. Similarly, the returned list will implement RandomAccess if the specified list does.

Java-9 provides a new methods to create unmodifiable/immutable List:

jshell> List<Integer> list = List.of(1,2,3);
list ==> [1, 2, 3]

jshell> list.add(10);
|  java.lang.UnsupportedOperationException thrown: 
|        at ImmutableCollections.uoe (ImmutableCollections.java:70)
|        at ImmutableCollections$AbstractImmutableList.add (ImmutableCollections.java:76)
|        at (#6:1)

List.of creates an immutable list containing an arbitrary number of elements.

You need java.util.Collections:

return Collections.unmodifiableList(oListeRet);

If you have to write your own, have that class implement the List interface and throw exceptions for the methods that modify contents.