If you go to the List or Set or Collection interfaces, you find some methods labelled "optional." Begging the question of whether you should have optional methods in interfaces in the first place, if you invoke those methods they don't do anything; obviously they have the short and sweet implementation below.You will have to find implementations which implement every method. Read the Collections#nCopies documentation and see what it returns. Then work out how you are supposed to use that List's add method. And reading the Collection#add(E) method's details will help too.