实现简单版的LinkedList

相比ArrayList，双链表的数据结构就复杂多了，想要弄清代码的意思还是要搞清数据结构层面的变化。

 package cn.sp.chapter03;

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

 /**
  * Created by 2YSP on 2017/10/9.
  * 实现自己的双链表
  */
 public class MyLinkedList<AnyType> implements Iterable<AnyType> {

     private static class Node<AnyType> {

         public AnyType data;
         public Node<AnyType> prev;//指向的前一个节点
         public Node<AnyType> next;//指向的后一个节点

         public Node(AnyType d, Node<AnyType> p, Node<AnyType> n) {
             this.data = d;
             this.prev = p;
             this.next = n;
         }
     }

     public MyLinkedList() {
         doClear();
     }

     public void clear() {
         doClear();
     }

     private void doClear() {
         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, 0, size()), 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));
     }

     /**
      * @param p 添加在该节点前
      * @param x 要添加的数据
      */
     private void addBefore(Node<AnyType> p, AnyType x) {
         Node<AnyType> newNode = new Node<>(x, p.prev, p);
         newNode.prev.next = newNode;
         p.prev = newNode;
         theSize++;
         modCount++;
     }

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

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

     private Node<AnyType> getNode(int idx, int lower, int upper) {
         Node<AnyType> 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;
     }

     @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;

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

         @Override
         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;
         }

         @Override
         public void remove() {
             if (modCount != expectedModCount) {
                 throw new ConcurrentModificationException();
             }

             if (!okToRemove) {
                 throw new IllegalStateException();
             }

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

     private int theSize;
     private int modCount = 0;
     private Node<AnyType> beginMarker;
     private Node<AnyType> endMarker;
 }