Cast List to List

Question

I have a simple POJO named "Father" and another one named "Son" which extends "Father", the simplest class inheritance example.

Now I have a List<Son> and I need to cast it to a List<Father>.

How can I do?

EDIT

Sorry for the bad naming, I didn't explain myself. Person and Employee would have been a better example. Or Product and Computer, too.

Why would you want to cast "down"? A `Son` is a `Father`, so it can do anything a `Father` can do... — Lukas Knuth, Feb 27 '13 at 12:12
[You can't](http://stackoverflow.com/questions/2745265/is-listdog-a-subclass-of-listanimal-why-arent-javas-generics-implicit) => you will need to copy the content. — assylias, Feb 27 '13 at 12:12
There is a logical flaw here. Not every `Son` is a `Father`. However it can be true in a parallel universe. — Adam Arold, Feb 27 '13 at 12:14
A `Son` is a `Father`? A son **has-a** father. The real world example cries for composition instead of inheritance! — Andreas Dolk, Feb 27 '13 at 12:15
Inheritance in object oriented programming has a completely different meaning than in biology. In OO, inheritance means **specialization**: a subclass *is a specialized version of* a superclass. Using names such as `Father` and `Son` for the classes confuses it with the biological meaning of inheritance. — Jesper, Feb 27 '13 at 12:26
@Andreas_D obviously... the right example would be "Person" and "SalesPerson" — Fabio B., Feb 27 '13 at 13:21

assylias · Answer 1 · 2013-02-27T12:35:21.440

3

You can't use a cast here* as commented above. You could write a small helper method to do the conversion (i.e.the copy):

private static List<Father> getListFather(List<? extends Father> list) {
    return new ArrayList<> (list);
}

_{* Actually you can - cf the other answer: List<Father> listFather = (List<Father>) (List<? extends Father>) listSons;}

edited Feb 27 '13 at 12:35

answered Feb 27 '13 at 12:19

assylias

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This code misses `` from the method signature, and still needs to be called with a generic hint, but it's the only typesafe way I know. – Petr Janeček Feb 27 '13 at 12:33
@Slanec What do you mean "This code misses from the method signature" ? (I've changed A into Father but the rest is the same) – assylias Feb 27 '13 at 12:34
With `Father` it works well. I thought your `A` was a generic type for the method to work with all the possible types. And in that case, it would need to be `private static List getList(List extends A> list)` and also would need a generic hint when called. – Petr Janeček Feb 27 '13 at 12:42

score 3 · Answer 2 · answered Feb 27 '13 at 12:19

Assume for a moment you could do that with a cast, it would lead to the following problem:

List<Son> ls = ...;
List<Father> lf = (List<Son>) ls;
lf.add(new Father());

Both ls and lf point to the same instance so you have just added a Father object into a list of Sons.

frIT · Accepted Answer · 2013-02-27T12:28:32.453

3

2 suggestions:

Have an Interface, say Person, that Father (and thus Son) implements. Use List<Person> for both.

Create a new List<Father> with the collection Constructor, e.g. List<Father> fathers = new ArrayList<Father>(sons);

edited Feb 27 '13 at 12:28

answered Feb 27 '13 at 12:21

frIT

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score 2 · Answer 4 · answered Feb 27 '13 at 12:20

2

This seems to work:

static class Father {};
static class Son extends Father{};

public void test() {
  List<Son> sons = new ArrayList<>();
  // Not allowed.
  //List<Father> sons2 = (List<Father>)sons;
  // Two step.
  List<? extends Father> sons3 = sons;
  List<Father> sons4 = (List<Father>)sons3;
  // Direct.
  List<Father> sons5 = (List<Father>)((List<? extends Father>)sons);
}

answered Feb 27 '13 at 12:20

OldCurmudgeon

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Also, `List fathers = (List)(List super Son>)sons;`. But beware, you're avoiding the Java type safety which could end up backfiring easily! – Petr Janeček Feb 27 '13 at 12:21
Not sure I would recommended it but nice ;-) – assylias Feb 27 '13 at 12:22

Cast List to List

4 Answers4