FancyList.java

NotEmptyList<T> getItems(); // always has at least one element.
List<T> maybeGetItems(); // might have zero elements.

T average(NotEmptyList<T> items) {
   T value = 0;
   for (T item: items.get()) {
       value += item;
   }
   return value / items.get().size(); 
}

println(average(getItems()));

List<T> myItems = maybeGetItems();
if (myItems.isEmpty()) {
   println("No items");
} else{
   println(average(NonEmptyList.of(myItems)));
}

Nulls.java

FoobarSource getFoobarSource(); // never returns NULL
FoobarSource maybeGetFoobarSource(); // might return NULL

int foobar(FoobarSource source) {
  return source.foo() + source.bar();
}

println(foobar(getFoobarSource()));

FoobarSource foobarSource = maybeGetFoobarSource();
if (foobarSource != null) {
  println(foobar(foobarSource));
}

Optional.java

FoobarSource getFoobarSource(); // never returns NULL
Optional<FoobarSource> maybeGetFoobarSource();

int foobar(FoobarSource source) {
  return source.foo() + source.bar();
}

println(foobar(getFoobarSource()));

FoobarSource foobarSource = maybeGetFoobarSource();
if (foobarSource.isPresent()) {
  println(foobar(foobarSource.get()));
}

PlainList.java

List<T> getItems(); // always has at least one element.
List<T> maybeGetItems(); // might have zero elements.

T average(List<T> items) {
   T value = 0;
   for (T item: items) {
       value += item;
   }
   return value / items.size(); 
}

println(average(getItems()));

List<T> myItems = maybeGetItems();
if (myItems.isEmpty()) {
   println("No items");
} else{
   println(average(myItems));
}