泛型类型java.util.List继承

Question

Lets say I've got two classes Person and Child . Child inherits from Person . I have also 2 other classes Class1 and Class2 .

Class1 has constructor with only one parameter - its a java.util. List of Person . Class2 has List of Child . I want to pass these childs to Class1 constructor. But I can't do it - Eclipse says that

The constructor Class1(List<Child>) is undefined.

I thought it would be possible because Child inherits from Person . What is the problem?

SSCCE (not compilable) may looks like this:

Somewhere in Class2

List<Child> childs = new ArrayList<Child>();
new Class1(childs);

Class1 constructor

public Class1(List<Person> persons)
{
//do nothing();
}

Answer 1

If a List<Apple> was a List<Fruit> , you could do the following:

List<Apple> apples = new ArrayList<>();
List<Fruit> fruits = apples; // this doesn't actually compile
fruits.add(new Banana()); 

And it would thus completely break the type-safety of generic types. If you want Class1 to accept a list of any type of Person, it should take a List<? extends Person> List<? extends Person> as argument.

Answer 2

If you define a constructor this way

public Class1(List<? extends Person> persons) {
}

you will be able to pass List<Child> to it