LinkedList的实现

package com.nasuf.arraylist;

import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.NoSuchElementException;

public class MyLinkedList<AnyType> implements Iterable<AnyType> {
    
    private int theSize;
    private int modCount = 0;
    private Node<AnyType> beginMarker;
    private Node<AnyType> endMarker;
    
    private static class Node<AnyType> {
        
        public AnyType data;
        public Node<AnyType> prev;
        public Node<AnyType> next;
        
        public Node(AnyType d, Node<AnyType> p, Node<AnyType> n) {
            data = d;
            prev = p;
            next = n;
        }
    }
    
    public MyLinkedList() {
        clear();
    }
    
    public void clear() {
        beginMarker = new Node<AnyType> (null, null, null);
        endMarker = new Node<AnyType> (null, beginMarker, null);
        beginMarker.next = endMarker;
        
        theSize = 0;
        modCount++;
    }
    
    public int size() {
        return theSize;
    }
    
    public boolean isEmpty() {
        return size() == 0;
    }
    
    public boolean add(AnyType x) {
        add(size(), x);
        return true;
    }
    
    public void add(int idx, AnyType x) {
        addBefore(getNode(idx), x);
    }
    
    public AnyType get(int idx) {
        return getNode(idx).data;
    }
    
    public AnyType set(int idx, AnyType newVal) {
        Node<AnyType> p = getNode(idx);
        AnyType oldVal = p.data;
        p.data = newVal;
        return oldVal;
    }
    
    public AnyType remove(int idx) {
        return remove(getNode(idx));
    }
    
    private AnyType remove(Node<AnyType> p) {
        p.next.prev = p.prev;
        p.prev.next = p.next;
        theSize --;
        modCount ++;
        return p.data;
    }
    
    private Node<AnyType> getNode(int idx) {
        Node<AnyType> p;
        
        if(idx < 0 || idx > size()) {
            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 void addBefore(Node<AnyType> p, AnyType x) {
        Node<AnyType> newNode = new Node<AnyType>(x, p.prev, p);
        newNode.prev.next = newNode;
        p.prev = newNode;
        theSize++;
        modCount++;
    }

    @Override
    public Iterator<AnyType> iterator() {
        return new LinkedListIterator();
    }
    
    private class LinkedListIterator implements java.util.Iterator<AnyType> {
        
        private Node<AnyType> current = beginMarker.next;
        private int expectedModCount = modCount;
        private boolean okToRemove = false;
        
        public boolean hasNext() {
            return current != endMarker;
        }

        public AnyType next() {
            if (modCount != expectedModCount) 
                throw new ConcurrentModificationException();
            if (!hasNext()) 
                throw new NoSuchElementException();
            
            AnyType nextItem = current.data;
            current = current.next;
            okToRemove = true;
            return nextItem;
        }
        
        public void remove() {
            if (modCount != expectedModCount) 
                throw new ConcurrentModificationException();
            if (!okToRemove) 
                throw new IllegalStateException();
            
            MyLinkedList.this.remove(current.prev);
            okToRemove = false;
            expectedModCount ++;
        }
    }

}