Generic type java.util.List inheritance

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();
}

List<Child> is not a sub class of List<Person>

Eran
– Eran

2013-12-24 13:42:07 +00:00
Commented Dec 24, 2013 at 13:42 — Eran
– Eran, Commented Dec 24, 2013 at 13:42

JB Nizet · Accepted Answer · 2013-12-24 13:48:22Z

4

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> as argument.

answered Dec 24, 2013 at 13:48

JB Nizet

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1 Comment

Jerome Over a year ago

better answer because there is an explanation why this is impossible

Evgeniy Dorofeev · Accepted Answer · 2013-12-24 13:47:41Z

2

If you define a constructor this way

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

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

answered Dec 24, 2013 at 13:47

Evgeniy Dorofeev

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