Java generics not allowing me to add another item to the list

Question

I have this generic class

public class BinTree<T> {

    T value;

    List<BinTree<? extends T>> branch = new ArrayList<BinTree<? extends T>>();

    public BinTree(T v){ value  = v;}

    public void addBranch(BinTree<? extends T> tree){
        if(branch.size() == 2){
            System.out.println("You can only have two childs");
        }else{
            branch.add(tree);
        }
    }

    public BinTree<? extends T> getBranch(int n){ return branch.get(n);}


}

And its implementation here

public static void main(String[] args){
        BinTree<Number> firstBinTree = new BinTree<Number>(0);
        firstBinTree.addBranch(new BinTree<Integer>(5));
        firstBinTree.addBranch(new BinTree<Double>(6.5));

        Number o = firstBinTree.getBranch(0).value;
        firstBinTree.getBranch(0).addBranch(new BinTree<Integer>(6));

    }

But this line

firstBinTree.getBranch(0).addBranch(new BinTree<Integer>(6));

Is not allowing me to add another BinTree of Type integer. why is that? I declared in my addBranch method that it can add any type as long that is ia subclass of Type(in this case number) it would be added in the list but how come I can't ? isn't Integer a sub class of Number?

score 2 · Accepted Answer · answered Jul 31 '12 at 01:54

2

It will not know if the

<? extends T>

in getBranch and addBranch are the same i think. So to it will be

<? extends ? extends T>

answered Jul 31 '12 at 01:54

Markus Mikkolainen

3,397
18
21

But how come on this line it worked adding branches? firstBinTree.addBranch(new BinTree(5)); firstBinTree.addBranch(new BinTree(6.5)); – user962206 Jul 31 '12 at 01:57
when you are adding branches you want a extends T) which is extends Number> in that case and that case is easy to see for the compiler. But when you call getBranch , it is <_something_ extends T> – Markus Mikkolainen Jul 31 '12 at 01:58
so the addbranch will be extends (_something_ extends T)> – Markus Mikkolainen Jul 31 '12 at 01:59
so if I call getBranch it doesn't specifically know what type of branch or BinTree will it turn? – user962206 Jul 31 '12 at 02:03
yes. it will know it extends Number. But it will not know what it is exactly. – Markus Mikkolainen Jul 31 '12 at 02:04
is there anyway I can add a branch to its sub branches? – user962206 Jul 31 '12 at 02:06
lie a bit and return from getBranch – Markus Mikkolainen Jul 31 '12 at 02:07
since you have limited amount of branches how about using the type parameters to define their types directly. – Markus Mikkolainen Jul 31 '12 at 02:09
class,>, then have method setleftBranch, setrightbranch etc. – Markus Mikkolainen Jul 31 '12 at 02:27

score 1 · Answer 2 · edited May 23 '17 at 12:23

It is because generics are not covariant.

In your example getBranch(0) can return BinTree<? extends Number> (for example BinTree<Integer> or BinTree<Double>) but compiler can't be sure about precise generic type that will be used. So from safety reasons it won't allow to use BinTree<Integer> in addBranch in case getBranch(0) will return something different then BinTree<Integer> like BinTree<Double>.

Java generics not allowing me to add another item to the list

2 Answers2