线性表和链表

单向链表利用了类的自引用，实现了类似指针的效果。
双向链表的实现，必须注意双向链表的head和back两个指针的正确指向，以及插入和删除过程中指向操作增减的有序性。

下面几图从java面向对象的角度说明了单向双向链表的逻辑结构，类的自引用使得逻辑指向成为可能。

以下两图说明了添加删除头尾节点时执行的顺序：

添加头结点时先加一个临时节点，建立此临时节点和原头节点后使此临时节点为新头结点即可。
删除尾节点时先使原次头结点为新头结点，然后删除原头节点和新头结点的连接后，再删除新头结点和原头结点的连接。

尾节点的处理方法类似。
这样的删除方法能够完全释放所有的占用空间。

下面是双向链表的实现过程，包括链表类NodeList的插入删除操作。

public class TestList 
{
    public static void main(String[] Args) 
    {
        NodeList OhMyGod = new NodeList();

        Boolean b = Boolean.TRUE;
        Character c = new Character('$');
        Integer i = new Integer(34567);
        String s = "hello";

        OhMyGod.insertFromBack(b);
        OhMyGod.print();
        OhMyGod.insertFromHead(c);
        OhMyGod.print();
        OhMyGod.insertFromBack(i);
        OhMyGod.print();
        OhMyGod.insertFromHead(s);
        OhMyGod.print();
        System.out.println(OhMyGod.deleteFromBack());
        OhMyGod.print();
        System.out.println(OhMyGod.deleteFromBack());
        OhMyGod.print();
        System.out.println(OhMyGod.deleteFromBack());
        OhMyGod.print();
        System.out.println(OhMyGod.deleteFromBack());
        OhMyGod.print();
    }
}

class Node ///定义节点，节点数据类型为Object，可以通过多态方式具象化为其他类型
{
    private Node headPointer;
    private Object data;
    private Node backPointer;


    public Node(Node hp,Object o,Node bp)
    {
        headPointer = hp;
        backPointer = bp;
        data = o;
    }
    public Node()
    {
        this(null,null,null);
    }
    public Node(Object o)
    {
        this(null,o,null);
    }
    public Node(Node hp,Object o)
    {
        this(hp,o,null);
    }
    public Node(Object o,Node bp)
    {
        this(null,o,bp);
    }

    public Node getHeadPointer() {
        return headPointer;
    }
    public Object getData() {
        return data;
    }
    public Node getBackPointer() {
        return backPointer;
    }
    public void setHeadPointer(Node headPointer) {
        this.headPointer = headPointer;
    }
    public void setBackPointer(Node backPointer) {
        this.backPointer = backPointer;
    }
}

class NodeListEmptyExcption extends RuntimeException
{

    private static final long serialVersionUID = 5130245130776457112L;
    public NodeListEmptyExcption(String name)
    {
        super("NodeList:"+name+" is Empty !");
    }
}

class NodeList
{
    private String listName;
    private Node headNode;
    private Node backNode;
    public NodeList()
    {
        this("default");
    }
    public NodeList(String listName) 
    {
        this.listName = listName;
        headNode = backNode = null;
    }
    public Boolean isEmpty()
    {
        return headNode == null;
    }
    ///////////////////////定义从头尾节点增加节点
    public void insertFromHead(Object o)
    {
        if(isEmpty())
            headNode = backNode = new Node(o);
        else 
        {
            //headNode = new Node(o,headNode);
            Node tempNode = new Node(o);
            tempNode.setBackPointer(headNode);
            headNode.setHeadPointer(tempNode);
            headNode = tempNode;


        }
    }
    public void insertFromBack(Object o)
    {
        if(isEmpty())
            headNode = backNode = new Node(o);
        else
        {
            Node tempNode = new Node(o);
            backNode.setBackPointer(tempNode);
            tempNode.setHeadPointer(backNode);
            backNode = tempNode;
        }
    }
    //////////////////////////定义从头尾节点删除节点
    public Object deleteFromHead() throws NodeListEmptyExcption
    {
        if(isEmpty())
            throw new NodeListEmptyExcption(listName);

        Object removedata = headNode.getData();

        if(headNode.equals(backNode))
        {
            headNode = backNode = null;
        }

        else 
        {
            headNode = headNode.getBackPointer();
            headNode.getHeadPointer().setBackPointer(null);
            headNode.setHeadPointer(null);  
        }
        return removedata;
    }
    public Object deleteFromBack() throws NodeListEmptyExcption
    {
        if(isEmpty())
            throw new NodeListEmptyExcption(listName);

        Object removedata = backNode.getData();
        if(headNode.equals(backNode))
        {
            headNode = backNode = null;
        }
        else 
        {
            backNode = backNode.getHeadPointer();
            backNode.getBackPointer().setHeadPointer(null);
            backNode.setBackPointer(null);
        }   
        return removedata;
    }
    public void print()
    {
        if(isEmpty())
        {
            System.out.println("Node List "+listName+" is empty !");
        return;
        }
        Node currentNode = headNode;
        while(currentNode!=null)
        {
            System.out.print(currentNode.getData().toString()+" ");
            currentNode  = currentNode.getBackPointer();
        }
        System.out.println("\n");
    }
}