List Polymorphism

Question

I have these classes:

class Parent {
    public Parent() {
    }
}

class ChildA extends Parent {
    public ChildA() {
        super();
    }
}

class ChildB extends Parent {
    public ChildB() {
        super();
    }
}

public ListClas(List(Parent) list)   {
    this.list=list;
    }
}

And I want to run ListClas constructor as below.

List<ChildA> list_childA = new ArrayList<ChildA>();

List<ChildB> list_childB = new ArrayList<ChildB>();

ListClas listClasA = new ListClas(list_childA);

ListClas listClasB = new ListClas(list_childB);

But the compiler throws an error. How do I do this correctly using polymorphism?

Where is it you think you are using polymorphism? Generics and polymorphism aren't the same thing. Read [this](http://stackoverflow.com/questions/2745265/is-listdog-a-subclass-of-listanimal-why-arent-javas-generics-implicitly-p?rq=1). — Sotirios Delimanolis, Jan 07 '14 at 20:20
http://stackoverflow.com/questions/20940652/why-does-a-type-conversion-not-work-in-java/20940807#20940807 — MariuszS, Jan 07 '14 at 20:21
`public ListClas(List(Parent) list)` is not valid Java. Did you mean `public ListClas(List list)`? — Jim Garrison, Jan 07 '14 at 20:46

oceansize · Answer 1 · 2014-01-07T21:17:43.303

1

If you change to List<? extends Parent> list (also change ListClas.list field definition) in your ListClas then it will compile and work.

edited Jan 07 '14 at 21:17

answered Jan 07 '14 at 20:41

oceansize

623
3
16

That depends on what type `list` is. If it's just a `List` then it will work, but if it's a `List` then it won't work because you'd be assigning an instance of type `List` to a variable of type `List` – Mike B Jan 07 '14 at 20:57
`List` **can be assigned** to `List extends Parent>` as well as `List` and `List`. You can verify that or probably I misunderstood you. – oceansize Jan 07 '14 at 21:06
`List extends Parent> list = new ArrayList();` works, but `List list = new ArrayList();` doesn't. The OP doesn't show the entire `ListClas` class so it wasn't clear what type `this.list` is. Judging from the type of the `ListClas` constructor parameter `list` in the OP I assumed that the type of `this.list` was also `List` – Mike B Jan 07 '14 at 21:12
I assumed that `ListClas.list` field definition must be changed also. – oceansize Jan 07 '14 at 21:19

score 1 · Answer 2 · answered Jan 07 '14 at 20:48

1

If you want a function that accepts List containing subclasses of a superclass you should use syntax.

public ListClas(List<? extends Parent> list){
    this.list=list;
}

It will accept both of them.

ListClas listClasA = new ListClas(list_childA);

ListClas listClasB = new ListClas(list_childB);

answered Jan 07 '14 at 20:48

user2693979

2,482
4
24
23

1

That depends on what type `this.list` is. If it's just a `List` then it will work, but if it's a `List` then it won't work because you'd be assigning an instance of type `List` to a variable of type `List` – Mike B Jan 07 '14 at 21:01

score 0 · Answer 3 · edited May 23 '17 at 10:25

0

All of them: List<Parent>, List<ChildA> and List<ChildB> are different concrete parameterized type.

You can read more about this here: https://stackoverflow.com/a/20940807/516167

edited May 23 '17 at 10:25

Community

1
1

answered Jan 07 '14 at 20:22

MariuszS

30,646
12
114
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1

I think the issue here is more `List` being assigned to `List`. – Sotirios Delimanolis Jan 07 '14 at 20:25

Mike B · Answer 4 · 2014-01-07T21:27:15.227

I think you want your ListClas to look something like this:

class ListClas<T>
{
    private List<T> list;

    public ListClas(List<T> list)
    {
        this.list = list;
    }
}

Then to create them:

ListClas<ChildA> listClasA = new ListClas<ChildA>(list_childA);
ListClas<ChildB> listClasB = new ListClas<ChildB>(list_childB);

The T is a generic type designation. It allows you to use the same class for multiple runtime types. So you can do the above instead of having to write this:

class AListClas
{
    private List<ChildA> list;

    public AListClas(List<ChildA> list)
    {
        this.list = list;
    }
}

class BListClas
{
    private List<ChildB> list;

    public BListClas(List<ChildB> list)
    {
        this.list = list;
    }
}

If you don't need quite as much flexibility you could write your ListClas like this:

class ListClas
{
    private List<? extends Parent> list;

    public ListClas(List<? extends Parent> list)
    {
        this.list = list;
    }
}

and use it like this:

ListClas listClasA = new ListClas(list_childA);
ListClas listClasB = new ListClas(list_childB);

But don't understand T.What is it in this example? – user3170606 Jan 07 '14 at 20:38 — user3170606, Jan 07 '14 at 20:38
What JDK version are you using? – Mike B Jan 07 '14 at 20:51 — Mike B, Jan 07 '14 at 20:51
I added a 2nd implementation that might work for you. – Mike B Jan 07 '14 at 20:54 — Mike B, Jan 07 '14 at 20:54

List Polymorphism

4 Answers4