如何在Java中的双向链接列表中附加节点？

Question

I am beginning to study Java. As an assigment I have to implement a doubly linked list with given interfaces which i included in the code. My method insertAtTheEnd() seems not to work properly, since I have the value null on elements after inserting several ones. I checked similar questions on that topic and tried to apply the answers to my problem, but could not get any further. So any help is appreciated how i can setup this method to make it work. Thanks!

public interface IValueElement
{
    public String getName();
    public void setName(String paramName);
    public int getValue();
    public void setValue(int paramValue);
}

public interface IListElement
{
    public IValueElement getValueElement();
    public void setValueElement(IValueElement value);
    public IListElement getPredecessor();
    public void setPredecessor(IListElement predecessor);
    public IListElement getSuccessor();
    public void setSuccessor(IListElement successor);
}

public interface IList
{
    public IListElement getHead();
    public void insertAtTheEnd(IValueElement value);
//...
}

public class List implements IList
{
    public List()
    {
        if (head == null)
        {
            head = new ListElement(null);
        }
        else
        {
            return;
        }   
    }

    private IListElement head;

    public IListElement getHead()
    {
        return head;
    }

    public void insertAtTheEnd(IValueElement value)
    {
        if (head.getSuccessor() != null)
        {
            IListElement l = head;
            while (l.getSuccessor() != null)
                l = l.getSuccessor();
            IListElement q = new ListElement(value);
            l.setPredecessor(q);
        }
        else
        {
            IListElement q = new ListElement(value);
            q.setPredecessor(head);
            q.setSuccessor(null);
            head.setSuccessor(q);
            head.setPredecessor(q);
        }
    }
}

Additionally here are my implementations of ValueElement and ListElement:

//ListElement.java

public class ListElement implements IListElement
{
    public ListElement(IValueElement value)
    {
        this.valueElement = checkValueElementAttribute(value);
    }

    private IValueElement checkValueElementAttribute(IValueElement value)
    {
        return (value == null) ? new ValueElement(null, 0) : value;
    }

    private IValueElement valueElement;

    public IValueElement getValueElement()
    {
        return this.valueElement;
    }

    public void setValueElement(IValueElement value)
    {
        if (value != null)
        {
            this.valueElement = value;
        }
    }

    private IListElement predecessor;

    public IListElement getPredecessor()
    {
        return this.predecessor;
    }

    public void setPredecessor(IListElement predecessor)
    {
        this.predecessor = predecessor;
    }

    private IListElement successor;

    public IListElement getSuccessor()
    {
        return this.successor;
    }

    public void setSuccessor(IListElement successor)
    {
        this.successor = successor;
    }
}


// ValueElement.java
public class ValueElement implements IValueElement
{
    private String name;

    public String getName()
    {
        return this.name;
    }

    public void setName(String paramName)
    {
        if (paramName != null)
        {
            this.name = paramName;
        }
    }

    public ValueElement(String name, int value)
    {
        if (name == null || name.equals(""))
        {
            name = "default";
        }
        else
        {
            this.name = name;
        }
        this.value = value;
    }

    private int value;

    public int getValue()
    {
        return this.value;
    }

    public void setValue(int paramValue)
    {
        if (paramValue != 0)
        {
            this.value = paramValue;
        }
    }

    public String toString()
    {
        return "Name: " + this.name + " - Value: " + this.value;
    }

}

Answer 1

Have implementations of IValueElement and IListElement been provided for you, or are they something that you must also implement? From the code you have provided I cannot see implementations for them.

Regarding your method insertAtTheEnd, it looks like you have mostly the right idea, but maybe you could improve on it.

At the end of the if block, you set l's predecessor to q. I think that might be the wrong way round. l at that point is the last element in the list, having walked along the chain of successors until there is no more, it seems to me that the new element (q) should now be added as a successor to l NOT a predecessor. In addition, I think q would need to have it's predecessor set to l.

In your else block you correctly set q's predecessor and successor, and correctly set head's successor, however, why do you set head's predecessor to q? That would make q both a successor and predecessor to head. I think you should just remove that last setPredecessor call.

Answer 2

please provide the complete code so that we can run that and help you as of now we can only think of these problems:

1) you might not have called setValueElement on those nodes 2)your else block contains setting of both predecessor and successor to the same element so its a definite wrong line of code

shouldn't this be your code

public void insertAtTheEnd(IValueElement value)
    {
        if (head.getSuccessor() != null)
        {
            IListElement l = head;
            while (l.getSuccessor() != null)
                l = l.getSuccessor();
            IListElement q = new ListElement(value);
            l.setSuccessor(q);
        }       

    else
        {
            IListElement q = new ListElement(value);
            q.setPredecessor(head);
            q.setSuccessor(null);
            head.setSuccessor(q);
        }
    }