LinkedList的源码分析

1. LinkedList的定义

　　 1.1  继承于AbstractSequentialList的双向链表，可以被当作堆栈、队列或双端队列进行操作

1.2  有序，非线程安全的双向链表，默认使用尾部插入法

1.3 适用于频繁新增或删除场景，频繁访问场景请选用ArrayList

1.4 插入和删除时间复杂为O(1)，其余最差O(n)

1.5 由于实现Deque接口，双端队列相关方法众多

2. LinkedList的继承体系

      

• 继承 AbstractSequentialList，能被当作堆栈、队列或双端队列进行操作
• 实现 List 接口，能进行队列操作
• 实现 Deque 接口，能将LinkedList当作双端队列使用
• 实现 Cloneable 接口，重写 clone() ，能克隆(浅拷贝)
• 实现 java.io.Serializable 接口，支持序列化

3. LinkedList的源码实现

    

 public class LinkedList<E>
     extends AbstractSequentialList<E>
     implements List<E>, Deque<E>, Cloneable, java.io.Serializable {

     /**
      * 记录当前链表中元素的个数
      */
     transient int size = 0;

     /**
      * Pointer to first node.
      * Invariant: (first == null && last == null) ||
      *            (first.prev == null && first.item != null)
      * 链表的头部节点
      */
     transient Node<E> first;

     /**
      * Pointer to last node.
      * Invariant: (first == null && last == null) ||
      *            (last.next == null && last.item != null)
      * 链表的尾部节点
      */
     transient Node<E> last;

     /**
      * Constructs an empty list.
      * 默认空构造器 -- 注意LinkedList并不提供指定容量的构造器
      */
     public LinkedList() {
     }

     /**
      * 支持将一个Collection对象转换为linkedList对象
      */
     public LinkedList(Collection<? extends E> c) {
         this();
         addAll(c);
     }

     /**
      * 从头部开始添加一个元素
      */
     private void linkFirst(E e) {
         //获取头部节点
         final Node<E> f = first; 

        //组建新的节点，新的节点的prev为null,并赋值给头部节点
         final Node<E> newNode = new Node<>(null, e, f);
         first = newNode;

         //头节点==null.说明链表中没有元素
         if (f == null)
             last = newNode;
         else
             f.prev = newNode; //设置旧的头节点的前一个节点为新建的节点
         size++;
         modCount++;
     }

     /**
      * Links e as last element.
      * 从尾部开始添加一个元素
      */
     void linkLast(E e) {

         //获取尾部节点
         final Node<E> l = last;

         //组建新的节点，新节点的next节点为null,并将新的节点赋值给尾部节点
         final Node<E> newNode = new Node<>(l, e, null);
         last = newNode;

         //尾部节点为null,说明链表中没有元素
         if (l == null)
             first = newNode;
         else
             l.next = newNode; //设置旧的尾节点的next为新建的节点
         size++;
         modCount++;
     }

     /**
      * 在指定的节点之前添加元素
      */
     void linkBefore(E e, Node<E> succ) {

         //获取指定节点的prev
         final Node<E> pred = succ.prev;

         //构建新的节点
         final Node<E> newNode = new Node<>(pred, e, succ);
         //将新建的节点设置为指定节点的prev
         succ.prev = newNode;

         //判断prev是否为头结点
         if (pred == null)
             first = newNode;
         else
             pred.next = newNode; //设置新节点为prev的next节点
         size++;
         modCount++;
     }

     /**
      * 移除头部节点
      */
     private E unlinkFirst(Node<E> f) {
         // assert f == first && f != null;
         final E element = f.item;

         //获取头节点的next，并赋值给first
         final Node<E> next = f.next;
         f.item = null;
         f.next = null; // help GC
         first = next;

         //next == null,说明链表中已经没有元素
         if (next == null)
             last = null;
         else
             next.prev = null;  //设置头结点的prev为null
         size--;
         modCount++;
         return element;
     }

     /**
      * 移除尾部节点
      */
     private E unlinkLast(Node<E> l) {
         // assert l == last && l != null;
         final E element = l.item;

         //获取尾部节点的prev节点，并赋值为last节点
         final Node<E> prev = l.prev;
         l.item = null;
         l.prev = null; // help GC
         last = prev;

         //prev == null，说明链表中已经没有元素
         if (prev == null)
             first = null;
         else
             prev.next = null; //设置尾节点的next为空
         size--;
         modCount++;
         return element;
     }

     /**
      *  移除链表中的某个节点
      */
     E unlink(Node<E> x) {

         //获取要移除的元素
         final E element = x.item;
         final Node<E> next = x.next;
         final Node<E> prev = x.prev;

         //要移除的节点是否是头结点
         if (prev == null) {
             first = next;
         } else {
             //把当前节点的prev节点的next节点赋值为当前节点的next
             prev.next = next;
             //当前节点的prev置为null
             x.prev = null;
         }

         //要移除的节点是否是尾节点
         if (next == null) {
             last = prev;
         } else {
             //当前节点的next节点的prev节点赋值为当前节点的prev
             next.prev = prev;
             x.next = null;
         }

         //将当前节点置为null
         x.item = null;
         size--;
         modCount++;
         return element;
     }

     /**
      * 获取链表中的第一个元素
      */
     public E getFirst() {
         final Node<E> f = first;
         if (f == null)
             throw new NoSuchElementException();
         return f.item;
     }

     /**
      * 获取链表中的最后一个元素
      */
     public E getLast() {
         final Node<E> l = last;
         if (l == null)
             throw new NoSuchElementException();
         return l.item;
     }

     /**
      * 移除链表中的第一个元素并返回
      */
     public E removeFirst() {
         final Node<E> f = first;
         if (f == null)
             throw new NoSuchElementException();

         //执行移除头部节点的方法
         return unlinkFirst(f);
     }

     /**
      * 移除链表中的最后一个元素并返回
      */
     public E removeLast() {
         final Node<E> l = last;
         if (l == null)
             throw new NoSuchElementException();

         //执行移除尾部节点的方法
         return unlinkLast(l);
     }

     /**
      * 在链表的头部添加节点
      */
     public void addFirst(E e) {
         linkFirst(e);
     }

     /**
      * 在链表的尾部添加节点
      */
     public void addLast(E e) {
         linkLast(e);
     }

     /**
      * 链表中是否包含某个元素
      */
     public boolean contains(Object o) {
         return indexOf(o) != -1;
     }

     /**
      * 链表中元素的个数
      */
     public int size() {
         return size;
     }

     /**
      * 添加一个元素到链表中
      * 默认是尾部添加
      * 成功则返回true
      */
     public boolean add(E e) {
         linkLast(e);
         return true;
     }

     /**
      *  移除链表中的某个元素
      */
     public boolean remove(Object o) {
         if (o == null) {
             //遍历查找null元素,并进行移除
             for (Node<E> x = first; x != null; x = x.next) {
                 if (x.item == null) {
                     //执行移除的方法
                     unlink(x);
                     return true;
                 }
             }
         } else {
             //遍历找到要移除的元素
             for (Node<E> x = first; x != null; x = x.next) {
                 if (o.equals(x.item)) {
                     //执行移除的方法
                     unlink(x);
                     return true;
                 }
             }
         }
         return false;
     }

     /**
      *  将一个collection对象转换为链表中的节点
      */
     public boolean addAll(Collection<? extends E> c) {
         return addAll(size, c);
     }

     /**
      *
      */
     public boolean addAll(int index, Collection<? extends E> c) {

         //检查索引的合理性
         checkPositionIndex(index);

         //将collection对象转换为数组，数组为空则直接返回
         Object[] a = c.toArray();
         int numNew = a.length;
         if (numNew == 0)
             return false;

         //判断是在指定位置插入节点还是在尾部插入节点
         Node<E> pred, succ;
         if (index == size) {
             succ = null;
             pred = last;
         } else {
             succ = node(index);
             pred = succ.prev;
         }

         //遍历数组，进行插入
         for (Object o : a) {
             @SuppressWarnings("unchecked") E e = (E) o;
             Node<E> newNode = new Node<>(pred, e, null);
             //判断链表书否为空
             if (pred == null)
                 first = newNode;
             else
                 pred.next = newNode;
             pred = newNode;
         }

         //如果succ为null,则是在尾部进行节点的插入
         if (succ == null) {
             last = pred;
         } else {
             //将新插入的节点和后面的节点进行链接
             pred.next = succ;
             succ.prev = pred;
         }

         size += numNew;
         modCount++;
         return true;
     }

     /**
      * 清空链表中的元素
      */
     public void clear() {

         //循环设置链表的值为null,加快gc
         for (Node<E> x = first; x != null; ) {
             Node<E> next = x.next;
             x.item = null;
             x.next = null;
             x.prev = null;
             x = next;
         }
         first = last = null;
         size = 0;
         modCount++;
     }

     /**
      * 获取指定位置的元素
      */
     public E get(int index) {
         //检查索引的合理性
         checkElementIndex(index);
         return node(index).item;
     }

     /**
      * 替换指定位置上的元素并返回旧的元素
      */
     public E set(int index, E element) {
         checkElementIndex(index);
         //找到指定元素所在的节点
         Node<E> x = node(index);
         E oldVal = x.item;
         x.item = element;
         return oldVal;
     }

     /**
      * 在指定的位置添加元素
      */
     public void add(int index, E element) {
         checkPositionIndex(index);

         if (index == size)
             linkLast(element);
         else
             linkBefore(element, node(index));
     }

     /**
      *  移除指定位置的元素
      */
     public E remove(int index) {
         //检查索引的合理性
         checkElementIndex(index);
         //进行指定位置上的节点移除
         return unlink(node(index));
     }

     private boolean isElementIndex(int index) {
         return index >= 0 && index < size;
     }

     private boolean isPositionIndex(int index) {
         return index >= 0 && index <= size;
     }

     private String outOfBoundsMsg(int index) {
         return "Index: "+index+", Size: "+size;
     }

     /**
      * 检查索引是否合理
      * @param index
      */
     private void checkElementIndex(int index) {
         if (!isElementIndex(index))
             throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
     }

     private void checkPositionIndex(int index) {
         if (!isPositionIndex(index))
             throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
     }

     /**
      * 返回某个位置上的节点元素
      */
     Node<E> node(int index) {

         //二分查找
         if (index < (size >> 1)) {
             Node<E> x = first;
             for (int i = 0; i < index; i++)
                 x = x.next;
             return x;
         } else {
             Node<E> x = last;
             for (int i = size - 1; i > index; i--)
                 x = x.prev;
             return x;
         }
     }

     /**
      * 元素存在于链表中，则返回所在的节点的位置
      * 否则返回-1
      */
     public int indexOf(Object o) {
         //初始化为0
         int index = 0;
         //如果元素为null,返回第一个查找到的null的位置
         if (o == null) {
             //遍历链表，查找null元素，并增加index的值
             for (Node<E> x = first; x != null; x = x.next) {
                 if (x.item == null)
                     return index;
                 index++;
             }
         } else {
             //遍历链表，查找元素，并增加index的值
             for (Node<E> x = first; x != null; x = x.next) {
                 if (o.equals(x.item))
                     return index;
                 index++;
             }
         }
         return -1;
     }

     /**
      * 从链表的尾节点开始查找指定的元素
      * 并返回指定元素的位置
      * 没有找到返回-1
      */
     public int lastIndexOf(Object o) {
         int index = size;
         //指定的元素是否为null
         if (o == null) {
             //遍历查找null元素
             for (Node<E> x = last; x != null; x = x.prev) {
                 index--;
                 if (x.item == null)
                     return index;
             }
         } else {
             //遍历查找指定的元素
             for (Node<E> x = last; x != null; x = x.prev) {
                 index--;
                 if (o.equals(x.item))
                     return index;
             }
         }
         return -1;
     }

     /**
      * 进行出队列的操作
      */
     public E peek() {
         final Node<E> f = first;
         return (f == null) ? null : f.item;
     }

     /**
      * 获取链表中的一个元素
      * 默认返回头结点的元素
      */
     public E element() {
         return getFirst();
     }

     /**
      * 进行出队列的操作
      * 并移除元素
      */
     public E poll() {
         final Node<E> f = first;
         return (f == null) ? null : unlinkFirst(f);
     }

     /**
      * 移除链表中的元素
      * 默认为头结点的元素
      */
     public E remove() {
         return removeFirst();
     }

     /**
      *  添加元素，默认在尾部添加
      */
     public boolean offer(E e) {
         return add(e);
     }

     /**
      * 在链表的头部添加元素
      * 成功则返回true
      */
     public boolean offerFirst(E e) {
         addFirst(e);
         return true;
     }

     /**
      * 在尾部添加元素
      */
     public boolean offerLast(E e) {
         addLast(e);
         return true;
     }

     /**
      * 获取第一个元素
      */
     public E peekFirst() {
         final Node<E> f = first;
         return (f == null) ? null : f.item;
      }

     /**
      * 获取最后一个元素
      */
     public E peekLast() {
         final Node<E> l = last;
         return (l == null) ? null : l.item;
     }

     /**
      * 获取第一个元素
      * 并移除
      */
     public E pollFirst() {
         final Node<E> f = first;
         return (f == null) ? null : unlinkFirst(f);
     }

     /**
      * 获取最后一个元素
      * 并移除
      */
     public E pollLast() {
         final Node<E> l = last;
         return (l == null) ? null : unlinkLast(l);
     }

     /**
      * 头部添加元素
      */
     public void push(E e) {
         addFirst(e);
     }

     /**
      * 移除头部元素
      */
     public E pop() {
         return removeFirst();
     }

     /**
      * 从头查找，并移除指定的元素
      * 成功返回true
      */
     public boolean removeFirstOccurrence(Object o) {
         return remove(o);
     }

     /**
      * 从尾部查找，并移除指定的元素
      * 成功返回true
      */
     public boolean removeLastOccurrence(Object o) {
         if (o == null) {
             //遍历查找指定的null元素
             for (Node<E> x = last; x != null; x = x.prev) {
                 if (x.item == null) {
                     unlink(x);
                     return true;
                 }
             }
         } else {
             //遍历查找指定的元素
             for (Node<E> x = last; x != null; x = x.prev) {
                 if (o.equals(x.item)) {
                     unlink(x);
                     return true;
                 }
             }
         }
         return false;
     }

     /**
      * 从指定位置开始获取链表的迭代器
      */
     public ListIterator<E> listIterator(int index) {
         checkPositionIndex(index);
         return new ListItr(index);
     }

     /**
      * 链表的内部类
      * 一个迭代器对象
      * @author Administrator
      *
      */
     private class ListItr implements ListIterator<E> {
         private Node<E> lastReturned;
         private Node<E> next;
         private int nextIndex;
         private int expectedModCount = modCount;

         ListItr(int index) {
             // assert isPositionIndex(index);
             next = (index == size) ? null : node(index);
             nextIndex = index;
         }

         public boolean hasNext() {
             return nextIndex < size;
         }

         public E next() {
             checkForComodification();
             if (!hasNext())
                 throw new NoSuchElementException();

             lastReturned = next;
             next = next.next;
             nextIndex++;
             return lastReturned.item;
         }

         public boolean hasPrevious() {
             return nextIndex > 0;
         }

         public E previous() {
             checkForComodification();
             if (!hasPrevious())
                 throw new NoSuchElementException();

             lastReturned = next = (next == null) ? last : next.prev;
             nextIndex--;
             return lastReturned.item;
         }

         public int nextIndex() {
             return nextIndex;
         }

         public int previousIndex() {
             return nextIndex - 1;
         }

         public void remove() {
             checkForComodification();
             if (lastReturned == null)
                 throw new IllegalStateException();

             Node<E> lastNext = lastReturned.next;
             unlink(lastReturned);
             if (next == lastReturned)
                 next = lastNext;
             else
                 nextIndex--;
             lastReturned = null;
             expectedModCount++;
         }

         public void set(E e) {
             if (lastReturned == null)
                 throw new IllegalStateException();
             checkForComodification();
             lastReturned.item = e;
         }

         public void add(E e) {
             checkForComodification();
             lastReturned = null;
             if (next == null)
                 linkLast(e);
             else
                 linkBefore(e, next);
             nextIndex++;
             expectedModCount++;
         }

         public void forEachRemaining(Consumer<? super E> action) {
             Objects.requireNonNull(action);
             while (modCount == expectedModCount && nextIndex < size) {
                 action.accept(next.item);
                 lastReturned = next;
                 next = next.next;
                 nextIndex++;
             }
             checkForComodification();
         }

         final void checkForComodification() {
             if (modCount != expectedModCount)
                 throw new ConcurrentModificationException();
         }
     }

     /**
      * 链表中保存的节点对象
      * @param <E>
      */
     private static class Node<E> {
         E item;  //当前元素
         Node<E> next; //下一个元素
         Node<E> prev; //上一个元素

         Node(Node<E> prev, E element, Node<E> next) {
             this.item = element;
             this.next = next;
             this.prev = prev;
         }
     }

     /**
      * @since 1.6
      */
     public Iterator<E> descendingIterator() {
         return new DescendingIterator();
     }

     /**
      * Adapter to provide descending iterators via ListItr.previous
      */
     private class DescendingIterator implements Iterator<E> {
         private final ListItr itr = new ListItr(size());
         public boolean hasNext() {
             return itr.hasPrevious();
         }
         public E next() {
             return itr.previous();
         }
         public void remove() {
             itr.remove();
         }
     }

     /**
      * 调用父类进行链表的克隆
      * @return
      */
     @SuppressWarnings("unchecked")
     private LinkedList<E> superClone() {
         try {
             return (LinkedList<E>) super.clone();
         } catch (CloneNotSupportedException e) {
             throw new InternalError(e);
         }
     }

     /**
      * 进行链表的克隆
      */
     public Object clone() {
         LinkedList<E> clone = superClone();

         // Put clone into "virgin" state
         clone.first = clone.last = null;
         clone.size = 0;
         clone.modCount = 0;

         // Initialize clone with our elements
         for (Node<E> x = first; x != null; x = x.next)
             clone.add(x.item);

         return clone;
     }

     /**
      * 将链表转换为Object类型的数组
      */
     public Object[] toArray() {
         Object[] result = new Object[size];
         int i = 0;
         for (Node<E> x = first; x != null; x = x.next)
             result[i++] = x.item;
         return result;
     }

     /**
      * 将链表转换为指定类型的数组
      */
     @SuppressWarnings("unchecked")
     public <T> T[] toArray(T[] a) {
         if (a.length < size)
             a = (T[])java.lang.reflect.Array.newInstance(
                                 a.getClass().getComponentType(), size);
         int i = 0;
         Object[] result = a;
         for (Node<E> x = first; x != null; x = x.next)
             result[i++] = x.item;

         if (a.length > size)
             a[size] = null;

         return a;
     }

     private static final long serialVersionUID = 876323262645176354L;

     /**
      * 序列化链表中的元素
      */
     private void writeObject(java.io.ObjectOutputStream s)
         throws java.io.IOException {
         // Write out any hidden serialization magic
         s.defaultWriteObject();

         // Write out size
         s.writeInt(size);

         // Write out all elements in the proper order.
         for (Node<E> x = first; x != null; x = x.next)
             s.writeObject(x.item);
     }

     /**
      * 反序列化链表中的元素
      */
     @SuppressWarnings("unchecked")
     private void readObject(java.io.ObjectInputStream s)
         throws java.io.IOException, ClassNotFoundException {
         // Read in any hidden serialization magic
         s.defaultReadObject();

         // Read in size
         int size = s.readInt();

         // Read in all elements in the proper order.
         for (int i = 0; i < size; i++)
             linkLast((E)s.readObject());
     }

     /**
      * java8新增的返回
      * 返回一个并行的分片迭代器
      */
     @Override
     public Spliterator<E> spliterator() {
         return new LLSpliterator<E>(this, -1, 0);
     }

     /** A customized variant of Spliterators.IteratorSpliterator */
     static final class LLSpliterator<E> implements Spliterator<E> {
         static final int BATCH_UNIT = 1 << 10;  // batch array size increment
         static final int MAX_BATCH = 1 << 25;  // max batch array size;
         final LinkedList<E> list; // null OK unless traversed
         Node<E> current;      // current node; null until initialized
         int est;              // size estimate; -1 until first needed
         int expectedModCount; // initialized when est set
         int batch;            // batch size for splits

         LLSpliterator(LinkedList<E> list, int est, int expectedModCount) {
             this.list = list;
             this.est = est;
             this.expectedModCount = expectedModCount;
         }

         final int getEst() {
             int s; // force initialization
             final LinkedList<E> lst;
             if ((s = est) < 0) {
                 if ((lst = list) == null)
                     s = est = 0;
                 else {
                     expectedModCount = lst.modCount;
                     current = lst.first;
                     s = est = lst.size;
                 }
             }
             return s;
         }

         public long estimateSize() { return (long) getEst(); }

         public Spliterator<E> trySplit() {
             Node<E> p;
             int s = getEst();
             if (s > 1 && (p = current) != null) {
                 int n = batch + BATCH_UNIT;
                 if (n > s)
                     n = s;
                 if (n > MAX_BATCH)
                     n = MAX_BATCH;
                 Object[] a = new Object[n];
                 int j = 0;
                 do { a[j++] = p.item; } while ((p = p.next) != null && j < n);
                 current = p;
                 batch = j;
                 est = s - j;
                 return Spliterators.spliterator(a, 0, j, Spliterator.ORDERED);
             }
             return null;
         }

         public void forEachRemaining(Consumer<? super E> action) {
             Node<E> p; int n;
             if (action == null) throw new NullPointerException();
             if ((n = getEst()) > 0 && (p = current) != null) {
                 current = null;
                 est = 0;
                 do {
                     E e = p.item;
                     p = p.next;
                     action.accept(e);
                 } while (p != null && --n > 0);
             }
             if (list.modCount != expectedModCount)
                 throw new ConcurrentModificationException();
         }

         public boolean tryAdvance(Consumer<? super E> action) {
             Node<E> p;
             if (action == null) throw new NullPointerException();
             if (getEst() > 0 && (p = current) != null) {
                 --est;
                 E e = p.item;
                 current = p.next;
                 action.accept(e);
                 if (list.modCount != expectedModCount)
                     throw new ConcurrentModificationException();
                 return true;
             }
             return false;
         }

         public int characteristics() {
             return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
         }
     }

 }