How to append a node in a doubly linked list in Java?

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;
    }

}