Question

why is it
a method with the signature:

public void foo(List<String> string)

cant be called by guavas: foo(Lists.newArrayList())

or java7s: foo(new ArrayList<>())

Answer 1

Compiler is not capable of inferring correctly what type Lists.newArrayList() should return. You can tell compiler what type you want by

foo(Lists.<String>newArrayList());

Why can’t compiler infer the type? Consider these methods

<T> T newObject();
void bar(Integer i);
void bar(String s);

If you have bar(newObject()), then compiler can’t determine which method it should call. Compiler needs to know the type of the parameter before finding the method to call, even if there is only one possible method currently. Otherwise a new overloading method will break your code.