Can Java generic merges 2 lists of different sub-class into one list of super class?

Question

Say I have Animal class and Cat and Dog as its 2 subclass.

I want to return a list of both pets in one java method. So I tried the following:

public List<? extends Animal> getAllPets(...) {
    List<? extends Animal> allDogs = findAnimals(...someOtherArgs, Dog.class);
    List<? extends Animal> allCats = findAnimals(...someOtherArgs, Cat.class);
    List<? extends Animal> retObjs = allDogs;
    retObjs.addAll(allCats); // won't compile
    return retObjs;
}

Is it possible to do that in Java?

Answer 1

Simply leave out the wildcard, like this:

List<Integer> a = Arrays.asList(1);
List<Double>  b = Arrays.asList(1.0);
List<Number> c = new ArrayList<>();
c.addAll(a);
c.addAll(b);
System.out.println(c); 

The above compiles and runs fine, and prints

[1, 1.0]

Answer 2

Try such way:

public List<? extends Animal> getAllPets(...) {
    List<Dog> allDogs = findDogs();
    List<Cat> allCats = findCats();
    List<Animal> retObjs = new ArrayList<>(allDogs.size() + allCats.size());
    retObjs.addAll(allDogs);
    retObjs.addAll(allCats);
    return retObjs;
}

Answer 3

Polymorphism allows you to store all instances of any child classes in a List which was declared as list to store object of BaseClass . So, in your case, you need to declare List<Animal> , now you can store here any Cat or Dog .

Answer 4

It's no need to use ? in your code. Delete them as the code below:

public List<? extends Animal> getAllPets(...) {
List<Animal> allDogs = findDogs();
List<Animal> allCats = findCats();
List<Animal> retObjs = allDogs;
retObjs.addAll(allCats); // won't compile
return retObjs;

}