Java 8 list processing - add elements conditionally

ionut :

I have the following piece of code:

List<Object> list = new ArrayList<>();
list.addAll(method1());
if(list.isEmpty()) { list.addAll(method2()); }
if(list.isEmpty()) { list.addAll(method3()); }
if(list.isEmpty()) { list.addAll(method4()); }
if(list.isEmpty()) { list.addAll(method5()); }
if(list.isEmpty()) { list.addAll(method6()); }
return list;

Is there a nice way to add elements conditionally, maybe using stream operations? I would like to add elements from method2 only if the list is empty otherwise return and so on.

Edit: It's worth to mention that the methods contain heavy logic so need to be prevented from execution.

ernest_k :

I would simply use a stream of suppliers and filter on List.isEmpty:

Stream.<Supplier<List<Object>>>of(() -> method1(), 
                                  () -> method2(), 
                                  () -> method3(), 
                                  () -> method4(), 
                                  () -> method5(), 
                                  () -> method6())
    .map(Supplier<List<Object>>::get)
    .filter(l -> !l.isEmpty())
    .findFirst()
    .ifPresent(list::addAll);

return list;

findFirst() will prevent unnecessary calls to methodN() when the first non-empty list is returned by one of the methods.

EDIT:
As remarked in comments below, if your list object is not initialized with anything else, then it makes sense to just return the result of the stream directly:

return  Stream.<Supplier<List<Object>>>of(() -> method1(), 
                                          () -> method2(), 
                                          () -> method3(), 
                                          () -> method4(), 
                                          () -> method5(), 
                                          () -> method6())
    .map(Supplier<List<Object>>::get)
    .filter(l -> !l.isEmpty())
    .findFirst()
    .orElseGet(ArrayList::new);