链接列表存储对象

Question

这是我关于Stack Overflow的第一个问题。我必须实现一个存储对象的链表Node类（这是在线课的作业）。我已经实例化了对象并将它们作为参数传递给我的Node类，但是当我运行我的节点时，它返回为空（null）。对我做错的任何帮助？我是新来的！

public class Node {
Donor newDonor;
Node nextNode;
Node prevNode;
private Node head;


public Node() {

}

public Node(Donor newDonor) {

    nextNode = null;
    prevNode = null;
    this.head = null;

}


public Donor getDonor(){
    return newDonor;
}
public void setNewDonor(Donor newDonor) {
    this.newDonor = newDonor;
}
public void setDonor(Donor newDonor){
    this.newDonor = newDonor;
}
public Node getNextNode(){
    return nextNode;
}
public void setNextNode(Node nextNode){
    this.nextNode = nextNode;
}
public void setPrevNode(Node prevNode) {
    this.prevNode = prevNode;
}
public Node getPrevNode() {
    return prevNode;
}
public Node getHead() {
    return head;
}
public void setHead(Node head) {
    this.head = head;
}


@Override
public String toString() {
    return "Node{" + "newDonor=" + newDonor + ", nextNode=" + nextNode     + ",     prevNode=" + prevNode + '}';
}



public void addDonor(Donor newDonor) {

    Node newNode = new Node();
    if(head == null){
        newNode.setDonor(newDonor);
        newNode.setNextNode(head);
        head = newNode;
        System.out.println(toString());
    }
    else if(newDonor.getDonorID() < head.getDonor().getDonorID()){
        newNode.setNextNode(head);
        head = newNode;
    }
    Node current = head.getNextNode();
    Node previous = head;
    while (current != null && newNode.getDonor().getDonorID() >=current.getDonor().getDonorID()) {
        previous = current;
        current = current.getNextNode();
    }
    newNode.setNextNode(current);
    previous.setNextNode(newNode);
    //System.out.println(toString());