public class Node {
     Node prev;
     Object obj;
     Node next;

    public Node() {
    }

    public Node(Node prev, Object obj, Node next) {
        this.prev = prev;
        this.obj = obj;
        this.next = next;
    }

    public Node getPrev() {
        return prev;
    }

    public Object getObj() {
        return obj;
    }

    public Node getNext() {
        return next;
    }

    public void setPrev(Node prev) {
        this.prev = prev;
    }

    public void setObj(Object obj) {
        this.obj = obj;
    }

    public void setNext(Node next) {
        this.next = next;
    }

public class MyLinkedList {
    //第一個節點
    private Node first;
    //最后一個節點
    private Node last;

    private int size;

    public int size(){
        return  size;
    }

    public void add(Object obj){
        Node n=new Node();
        if(first==null){
            n.setPrev(null);
            n.setObj(obj);
            n.setNext(null);

            first=n;
            last=n;
        }else{
            n.setPrev(last);
            n.setObj(obj);
            n.setNext(null);

            last.setNext(n);

            last=n;
        }
        size++;
    }

    public void add(int index, Object obj){
        Node temp=node(index);

        Node newNode=new Node();
        newNode.obj=obj;
        if(temp!=null){
            Node preNode=temp.prev;
            preNode.next=newNode;
            newNode.prev=preNode;

            temp.prev=newNode;
            newNode.next=temp;

            size++;
        }
    }

    //在使用get方法的時候把Node的私有屬性去掉，方便理解
    public Object get(int index){
        rangeCheck(index);
        Node temp=node(index);
        if(temp!=null){
            return temp.obj;
        }else {
            return null;
        }
    }

    public void set(int index,Object obj){
        rangeCheck(index);
        Node temp=node(index);
        temp.obj=obj;
    }

    public void remove(int index){
        rangeCheck(index);
        Node temp=node(index);

        if(temp!=null){
            Node preNode=temp.prev;
            Node nexNode=temp.next;
            preNode.next=nexNode;
            nexNode.prev=preNode;
            size--;
        }

    }

    //獲得index處的節點
    private Node node(int index){
        Node temp=null;
        if(first!=null){
            temp=first;
            for(int i=0;i<index;i++){
                temp=temp.next;
            }
        }
        return temp;
    }

    //檢查索引
    private void rangeCheck(int index){
        if(index<0||index>=size){
            try {
                throw new Exception();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        MyLinkedList list=new MyLinkedList();
        list.add("aaa");
        list.add("bbb");
        list.add("ccc");
        list.add(1,"BBB");
        list.set(3,"CCC");
        System.out.println(list.get(1));

    }
}