Question

public class Node {
    private Node nextNode;

    @Override
    public int hashCode() {
        //How to implement this?
        //Because you just have a attribute which is a reference.
        //I think the attribute is almost useless, because if you use the HashCode of the attribute, you will finally fall into a useless loop.
        //Thus, I think you should find a way to represent the HashCode of reference (instance) itself.
    }
}

From the comment in the code, my question is actually how to uniquely identify reference itself, like address in C.

Answer 1

If you want the Node‘s hash-code to represent its own reference — that is, if you’re not overriding equals(Object) — then you don’t need to override hashCode() at all.

If you want the Node‘s hash-code to represent the reference of its nextNode — that is, if your equals(Object) looks something like this:

@Override
public boolean equals(Object that)
{
    return ((that instanceof Node) && (nextNode == ((Node) that).nextNode));
}

— then you can use the JDK’s System.identityHashCode(Object) utility method:

@Override
public int hashCode()
{
    return System.identityHashCode(nextNode);
}