Polymorphism and ArrayLists in Java

Question

I have a class Cell and a class Neighbour extending Cell. But I get an error when I try to pass an ArrayList<Neighbour> to a function expecting an ArrayList<Cell>. What have I missed?

class Cell {
    PVector pos;

    Cell(PVector pPos) {
        pos = pPos.get();
    }
}

class Neighbour extends Cell {
    int borders = 0;

    Neighbour(PVector pPos) {
        super(pPos);
    }
}

private int inSet(PVector pPos, ArrayList<Cell> set) {
    [...]

    return -1;
}

[...]

ArrayList<Neighbour> neighbours = new ArrayList<Neighbour>();
PVector pPos = new PVector(0, 0);

[...]

inSet(pPos, neighbours);

The last line throws the error `The method iniSet(PVector, ArrayList) is not applicable for the arguments (PVector, ArrayList);

Thanks for your help!

This may help to understand : http://stackoverflow.com/questions/17131664/inheritance-in-java-with-generics/17131699#17131699 — user2336315, Nov 08 '13 at 07:54

score 3 · Accepted Answer · answered Nov 08 '13 at 07:53

3

that is because

List<A> != List<B> ... even if B extends A.

What you need to do is modify the function to the following

private int inSet(PVector pPos, ArrayList<? extends Cell> set) {
    [...]
    return -1;
}

Hope that helps.

answered Nov 08 '13 at 07:53

Harsha R

707
6
12

score 2 · Answer 2 · answered Nov 08 '13 at 07:52

2

Try with:

private int inSet(PVector pPos, List<? extends Cell> set)

answered Nov 08 '13 at 07:52

Jean Logeart

52,687
11
83
118

Polymorphism and ArrayLists in Java

2 Answers2