generic class with an arraylist containing reference of generic class itself

Question

class Node<T>{

private T myValue;
private ArrayList<Object> next;



public <U> void addLink(U n){
        this.next.add(n);
}


}

in main i have:

    Node<String> myNode1 = new Node<String>("Ciao");
    Node<Integer> myNode2 = new Node<Integer>(12);

    myNode1.addLink(myNode2, true);

I need next in class Node to cointain pointers to the adiacent node

but java throws NullPointerException

Please help me.

Is myNode1.next actually set to a reference, or is it still null? — cdhowie
– cdhowie, Commented Nov 17, 2010 at 23:07
Also you can look at the stacktrace that java outputs with this error. It'll tell you exactly where the null pointer occured. — shoebox639
– shoebox639, Commented Nov 17, 2010 at 23:10

StriplingWarrior · Accepted Answer · 2010-11-17 23:07:00Z

4

this.next never got initialized. Try declaring it:

private ArrayList<Object> next = new ArrayList<Object>();

answered Nov 17, 2010 at 23:07

StriplingWarrior

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3 Comments

CurtainDog Over a year ago

Yes, although I think ArrayList<Object> is bad form in most circumstances. I suspect user496223 would be better off with an ArrayList<Node<?>>

Stephen C Over a year ago

Or maybe even Node<T> given the name of the next field.

StriplingWarrior Over a year ago

Yeah, I'm not exactly clear on what he had in mind with this. It almost looks like each node could potentially have a different type of object in it, which seems odd to me.

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