How to convert following method to java 8?

There is a critical flaw in your current code, i.e.

if(container!=null || !container.isEmpty())

this can still throw a NullPointerException (when container == null), unless the conditional operator is changed to &&. Post which the implementation below would be what I would suggest following.

---

It's almost correct, in the sense that you need to handle some default value if the conditions are not met :

DD detail = container.stream().findFirst().orElse(null); // or some default value instead of 'null'

If the container itself could be null, use

DD detail = container != null ? 
                container.stream().findFirst().orElse(null) : null;

---

In the case when you need the prescription from this object, use map as :

container.stream().findFirst().map(DD::getPrescription).orElse(null)
//                               ^^
//                               return type of prescription then

---

With Java-9, this could have been much simpler as :

A<DD, DI> basePrescription = Stream.ofNullable(container) // Java-9 API
                                   .flatMap(List::stream)
                                   .findFirst()
                                   .map(DD::getPrescription)
                                   .orElse(null);

This is way easier:

A<DD,DI> a = container.get(0).getPrescription();

While this is a direct translation of your original code, you probably intended something like that:

A<DD,DI> a = container != null && !container.isEmpty()
    ? container.get(0).getPrescription()
    : null;

As of JDK9, there is a new method T requireNonNullElse(T obj, T defaultObj) which essentially returns the first argument if it is non-null and otherwise returns the non-null second argument.

We can, therefore, simplify your code to:

Objects.requireNonNullElse(container, Collections.emptyList())
       .stream()
       .findFirst()
       .map(DD::getPrescription);

This returns an Optional<T> where T is whatever type getPrescription is. depending on the context and whether it's appropriate you might want to use .orElse(null); to get the value the optional contains or else a null value but there are also several other methods in the Optional<T> API which you might find more useful when extracting the value from the optional.