通用抽象类构造函数

Question

I'm trying to call a constructor for a generic abstract class within a method of said class. The code below shows this:

public abstract class Node<T> {
    public Collection <Node<T>> pars;   
    public Collection<Node<T>> interactors;
    private boolean target = false;
    private boolean multiple = false;
    private T value;

    //constructor for a simple node
    public Node(T val){
        this.value = val;
    }

    //constructor for a multiple interaction node
    public Node(Collection<Node<T>> inter, T val){
        this.interactors = inter;
        this.value = val;
        if (inter.size()>0){
            this.multiple = true;
        }
    }

    public void find_inters(){
        ArrayList<Collection<T>> multi_interactions = search();
        for (int i = 0; i < multi_interactions.size(); i++){
            Node<T> a = new Node<T>(multi_interactions.get(i), this.value);    <----i get compile error here
        }
    }
}

but I keep getting an error that I can't instantiate type Node. I want to create a new Node object within the function find_inters() but I can't. Anyone know why/possible solutions?

Answer 1

You've said that Node is abstract because you can't yet implement some of the methods, but the problem is that you want Node<T> a to be a Node object, but you have to decide what a will actually do when you call those methods.

You can have classes that don't have all their methods implemented -- that's how abstract classes work -- but you can't have objects that don't have all their methods implemented, because that object can have that method called, and your program needs to know what to do.

What you can do is write Node<T> a = new MySubNode<T>(...) , where MySubNode is a subclass of Node with all the methods filled in. Or you can write Node<T> a = new Node<T>(...) { implementations of methods go here } , which is essentially the same thing, except the implementation is in an anonymous class. But you can't instantiate an abstract class.

Answer 2

Just to add to Louis's answer. If search() is abstract, you can change it to return ArrayList<Node<T>> , so the implementing classes can do something like Node<T> a = new MySubNode<T>(...) (unless search() is used somewhere else that requires ArrayList<Collection<T>> ).