LinkedList实现

import java.util.Iterator;

public class MyLinkedList<T> implements Iterable<T>{

    private int theSize;
    private int modCount;
    private Node<T> beginMarker;
    private Node<T> endMarker;

    private static class Node<T>{
        public T data;
        public Node<T> prev;
        public Node<T> next;
        public Node(T d,Node<T> p,Node<T> n){
            this.data = d;
            this.prev = p;
            this.next = n;
        }
    }

    public MyLinkedList(){
        doClear();
    }

    public void clear(){
        doClear();
    }

    private void doClear() {
        beginMarker = new Node<T>(null,null,null);
        endMarker = new Node<T>(null,beginMarker,null);
        beginMarker.next = endMarker;
        theSize = 0;
        modCount++;
    }


    public int size(){
        return this.theSize;
    }

    public boolean isEmpty(){
        return this.size()==0;
    }

    public boolean add(T x){
        add(size(),x);
        return true;
    }

    public void add(int idx,T x){
        addBefore(getNode((idx),0,size()),x);
    }

    public T get(int idx){
        return getNode(idx).data;
    }

    public T set(int idx,T newVal){
        Node<T> p = getNode(idx);
        T oldVal = p.data;
        p.data = newVal;
        return oldVal;
    }

    public T remove(int idx){
        return remove(getNode(idx));
    }

    private void addBefore(Node<T> p,T x){
        Node<T> newNode = new Node<T>(x,p.prev,p);
        newNode.prev.next = newNode;
        p.prev = newNode;
        theSize++;
        modCount++;
    }

    private Node<T> getNode(int idx){
        return getNode(idx,0,size()-1);
    }

    private Node<T> getNode(int idx,int lower,int upper){
        Node<T> p;
        if(idx < lower || idx > upper)
            throw new IndexOutOfBoundsException();
        if(idx<size()/2){
            p = beginMarker.next;
            for(int i = 0;i < idx;i ++){
                p = p.next;
            }
        }else{
            p = endMarker;
            for(int i = size();i > idx;i --){
                p = p.prev;
            }
        }
        return p;
    }

    private T remove(Node<T> node){
        Node p = node.prev;
        p.next = node.next;
        node.next.prev = p;
        theSize--;
        modCount++;
        return node.data;
    }

    @Override
    public Iterator<T> iterator() {
        return new LinkedListIterator();
    }

    private class LinkedListIterator implements Iterator<T>{

        private Node<T> current = beginMarker;
        private int expectedModCount = modCount;
        private boolean okToRemove = false;

        @Override
        public boolean hasNext() {
            return current!=endMarker;
        }

        @Override
        public T next() {
            if(modCount!=expectedModCount)
                throw new java.util.ConcurrentModificationException();
            if(!hasNext())
                throw new java.util.NoSuchElementException();

            T nextItem = current.data;
            current = current.next;
            okToRemove = true;
            return nextItem;
        }

        @Override
        public void remove() {
            if(modCount!=expectedModCount)
                throw new java.util.ConcurrentModificationException();
            if(!okToRemove)
                throw new IllegalStateException();

            MyLinkedList.this.remove(current.prev);
            expectedModCount++;
            okToRemove = false;
        }
    }
}