Question

Why does this line cause a NullPointerException:

    List<Foo> foos = new ArrayList<Foo>(userDetailsService.getFoos(currentUser));

The getFoos method simply returns a Collection of Foos:

    public Collection<Foo> getFoos(final String username)

I can’t tell if the NullPointerException refers to getFoos returning null or some problem with the cast from a Collection to an ArrayList.

Answer 1

Depends on the stacktrace. Head to the first line. If it points to java.util.ArrayList.<init>, then getFoos() has returned null. Or if it points to the particular line in your question, then userDetailsService is null.