public static void main(String[] args) {
        List<Foo> foos = null;
        Optional.ofNullable(foos)
            .map(list -> list.stream().collect(Collectors.toMap(foo -> foo.getFoo(), foo -> foo)))
            .orElse(Collections.emptyMap());
    }
    private static class Foo {
        private String foo;
        public String getFoo() {
            return foo;
        }
    }

.map(list -> list.stream().collect(Collectors.toMap((Foo foo) -> foo.getFoo(), foo -> foo)))