Which is good practice - Modifying a List in the method, or returning a new List in the method?

Question

Example code:

modifyMyList(myList);  public void modifyMyList(List someList){     someList.add(someObject); } 

or:

List myList = modifyMyList(myList);  public List modifyMyList(List someList){     someList.add(someObject)     return someList; } 

There is also a 3rd option I believe: You can create a new List in modifyMyList method and return this new List...

( 3rd option is here, I was too lazy but someone already added it in the answers: )

List myList = modifyMyList(myList);  public List modifyMyList(List someList){     List returnList = new ArrayList();     returnList.addAll(someList);     returnList.add(someObject);     return Collections.unmodifiableList(returnList); } 

Is there any reason why I should choose one over another? What should be considered in such case?

Answer 1

I have a (self imposed) rule which is "Never mutate a method parameter in a public method". So, in a private method, it's ok to mutate a parameter (I even try to avoid this case too). But when calling a public method, the parameters should never be mutated and should be considered immutable.

I think that mutating method arguments is a bit hacky and can lead to bugs that are harder to see.

I have been known to make exceptions to this rule but I need a really good reason.