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;
}