LinkedList源码详解
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LinkedList也和ArrayList一样实现了List接口，因为它是基于双向链表实现的，所以其插入和删除效率比ArrayList高。同时基于链表也决定了它在随机访问方面要比ArrayList逊色(因为要移动指针)。除此之外，LinkedList还提供了一些可以使其作为栈、队列、双端队列的方法。
LinkedList同样是非线程安全的，只适合在单线程下使用。LinkedList实现了Serializable接口，故它支持序列化，能够通过序列化传输，实现了Cloneable接口，能被克隆。
import java.util.function.Consumer;import java.util.*;public class LinkedList<E> extends AbstractSequentialList<E> implements List<E>, Deque<E>, Cloneable, java.io.Serializable{//LinkedList中元素个数 transient int size = 0; //头结点 transient Node<E> first; //尾结点 transient Node<E> last; //默认构造函数：创建一个空的链表 public LinkedList() { } //包含指定集合的构造函数 public LinkedList(Collection<? extends E> c) { this(); addAll(c); } private void linkFirst(E e) { final Node<E> f = first; final Node<E> newNode = new Node<>(null, e, f); first = newNode; if (f == null) last = newNode; else f.prev = newNode; size++; modCount++; } void linkLast(E e) { final Node<E> l = last; final Node<E> newNode = new Node<>(l, e, null); last = newNode; if (l == null) first = newNode; else l.next = newNode; size++; modCount++; } //在非空结点前插入指定元素 void linkBefore(E e, Node<E> succ) { // assert succ != null; final Node<E> pred = succ.prev; final Node<E> newNode = new Node<>(pred, e, succ); succ.prev = newNode; if (pred == null) first = newNode; else pred.next = newNode; size++; modCount++; } private E unlinkFirst(Node<E> f) { // assert f == first && f != null; final E element = f.item; final Node<E> next = f.next; f.item = null; f.next = null; // help GC first = next; if (next == null) last = null; else next.prev = null; size--; modCount++; return element; } private E unlinkLast(Node<E> l) { // assert l == last && l != null; final E element = l.item; final Node<E> prev = l.prev; l.item = null; l.prev = null; // help GC last = prev; if (prev == null) first = null; else prev.next = null; size--; modCount++; return element; } E unlink(Node<E> x) { // assert x != null; final E element = x.item; final Node<E> next = x.next; final Node<E> prev = x.prev; if (prev == null) { first = next; } else { prev.next = next; x.prev = null; } if (next == null) { last = prev; } else { next.prev = prev; x.next = null; } x.item = null; size--; modCount++; return element; } //获得LinkedList的第一个元素 public E getFirst() { final Node<E> f = first; if (f == null) throw new NoSuchElementException(); return f.item; } //获得LinkedList的最后一个元素 public E getLast() { final Node<E> l = last; if (l == null) throw new NoSuchElementException(); return l.item; } //删除并返回第一个元素 public E removeFirst() { final Node<E> f = first; if (f == null) throw new NoSuchElementException(); return unlinkFirst(f); } //删除并返回最后一个元素 public E removeLast() { final Node<E> l = last; if (l == null) throw new NoSuchElementException(); return unlinkLast(l); } //将元素添加到LinkedList的起始位置 public void addFirst(E e) { linkFirst(e); } //将元素添加到LinkedList的结束位置 public void addLast(E e) { linkLast(e); } //判断LinkedList是否包含指定元素 public boolean contains(Object o) { return indexOf(o) != -1; } //返回LinkedList的大小 public int size() { return size; } //将元素添加到LinkedList(末尾） public boolean add(E e) { linkLast(e); return true; } //从LinkedList中删除指定的值 public boolean remove(Object o) { //指定元素为null的情况 if (o == null) { for (Node<E> x = first; x != null; x = x.next) { if (x.item == null) { unlink(x); return true; } } } else { for (Node<E> x = first; x != null; x = x.next) { if (o.equals(x.item)) { unlink(x); return true; } } } return false; } //将指定集合添加到LinkedList中 //实际上，是添加到双向链表的末尾 public boolean addAll(Collection<? extends E> c) { return addAll(size, c); } //从双向链表的index开始，将指定集合添加到双向链表中 public boolean addAll(int index, Collection<? extends E> c) { checkPositionIndex(index); //将集合转化为Object对象数组 Object[] a = c.toArray(); int numNew = a.length; if (numNew == 0) return false; Node<E> pred, succ; //如果index等于size if (index == size) { succ = null; pred = last; } else { succ = node(index); pred = succ.prev; } for (Object o : a) { @SuppressWarnings("unchecked") E e = (E) o; Node<E> newNode = new Node<>(pred, e, null); if (pred == null) first = newNode; else pred.next = newNode; pred = newNode; } if (succ == null) { last = pred; } else { pred.next = succ; succ.prev = pred; } //调整大小 size += numNew; modCount++; return true; } //清空双向链表 public void clear() { 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++; } //返回指定位置上的结点的值 public E get(int index) { checkElementIndex(index); return node(index).item; } Node<E> node(int index) { // assert isElementIndex(index); //若index < 双向链表长度的1/2，则从前往后查找 //否则，从后往前查找 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; } } //设置指定位置上的值 public E set(int index, E element) { checkElementIndex(index); Node<E> x = node(index); E oldVal = x.item; x.item = element; return oldVal; } //在指定位置添加结点，且结点的值为element public void add(int index, E element) { checkPositionIndex(index); if (index == size) linkLast(element); else linkBefore(element, node(index)); } //删除index位置上的结点 public E remove(int index) { checkElementIndex(index); return unlink(node(index)); } //判断是否越界 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; } 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)); } /*查找操作*/ //从前往后查找，返回值为对象(o)的结点对应的索引 //不存在就返回-1 public int indexOf(Object o) { int index = 0; if (o == null) { for (Node<E> x = first; x != null; x = x.next) { if (x.item == null) return index; index++; } } else { for (Node<E> x = first; x != null; x = x.next) { if (o.equals(x.item)) return index; index++; } } return -1; } //从前往后查找，返回值为对象(o)的结点对应的索引 //不存在就返回-1 public int lastIndexOf(Object o) { int index = size; if (o == null) { for (Node<E> x = last; x != null; x = x.prev) { index--; if (x.item == null) return index; } } else { for (Node<E> x = last; x != null; x = x.prev) { index--; if (o.equals(x.item)) return index; } } return -1; } // 队列操作 //返回第一个结点 //若LinkedList的大小为0，返回null public E peek() { final Node<E> f = first; return (f == null) ? null : f.item; } //返回第一个结点 //若LinkedList的大小为0，则抛出异常 public E element() { return getFirst(); } //删除并返回第一个结点 //若LinkedList的大小为0，返回null public E poll() { final Node<E> f = first; return (f == null) ? null : unlinkFirst(f); } //删除并返回第一个结点 //若LinkedList的大小为0，则抛出异常 public E remove() { return removeFirst(); } //将指定元素添加到链表的末尾 public boolean offer(E e) { return add(e); } //将指定元素添加到链表的开头 public boolean offerFirst(E e) { addFirst(e); return true; } //将指定元素添加到链表的末尾 public boolean offerLast(E e) { addLast(e); return true; } //返回(并不删除)第一个结点 //若LinkedList的大小为0，则返回null public E peekFirst() { final Node<E> f = first; return (f == null) ? null : f.item; } //返回(并不删除)最后一个结点 //若LinkedList的大小为0，则返回null public E peekLast() { final Node<E> l = last; return (l == null) ? null : l.item; } //删除并返回第一个结点 //若LinkedList的大小为0，则返回null public E pollFirst() { final Node<E> f = first; return (f == null) ? null : unlinkFirst(f); } public E pollLast() { final Node<E> l = last; return (l == null) ? null : unlinkLast(l); } //将e插入到双向链表开头 public void push(E e) { addFirst(e); } //删除并返回第一个结点 public E pop() { return removeFirst(); } //从前往后查找，删除第一个值为o的结点 public boolean removeFirstOccurrence(Object o) { return remove(o); } //从后往前查找，删除第一个值为o的结点 public boolean removeLastOccurrence(Object o) { if (o == null) { for (Node<E> x = last; x != null; x = x.prev) { if (x.item == null) { unlink(x); return true; } } } else { for (Node<E> x = last; x != null; x = x.prev) { if (o.equals(x.item)) { unlink(x); return true; } } } return false; } //返回index到末尾的全部结点对应的ListIterator对象(List迭代器） public ListIterator<E> listIterator(int index) { checkPositionIndex(index); return new ListItr(index); } //List迭代器 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; } //判断是否存在下一个元素 public boolean hasNext() { return nextIndex < size; } //获取下一个元素 public E next() { //fail-fast机制 checkForComodification(); if (!hasNext()) throw new NoSuchElementException(); lastReturned = next; next = next.next; nextIndex++; return lastReturned.item; } //判断是否存在上一个元素 public boolean hasPrevious() { return nextIndex > 0; } //获取上一个元素 public E previous() { checkForComodification(); if (!hasPrevious()) throw new NoSuchElementException(); lastReturned = next = (next == null) ? last : next.prev; nextIndex--; return lastReturned.item; } //获取下一个元素的索引 public int nextIndex() { return nextIndex; } //获取上一个元素的索引 public int previousIndex() { return nextIndex - 1; } //删除当前结点 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++; } //设置当前结点为e public void set(E e) { if (lastReturned == null) throw new IllegalStateException(); checkForComodification(); lastReturned.item = e; } //将e添加到当前结点的前面 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(); } //fail-fast机制 final void checkForComodification() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); } } //双向链表采用的数据结构 private static class Node<E> { E item; Node<E> next; Node<E> prev; Node(Node<E> prev, E element, Node<E> next) { this.item = element; this.next = next; this.prev = prev; } } /** * @since 1.6 */ //反向迭代器 public Iterator<E> descendingIterator() { return new DescendingIterator(); } //反向迭代器实现类 private class DescendingIterator implements Iterator<E> { private final ListItr itr = new ListItr(size()); //反向迭代是否有下一个元素 public boolean hasNext() { return itr.hasPrevious(); } public E next() { return itr.previous(); } public void remove() { itr.remove(); } } @SuppressWarnings("unchecked") private LinkedList<E> superClone() { try { return (LinkedList<E>) super.clone(); } catch (CloneNotSupportedException e) { throw new InternalError(e); } } //返回LinkedList的浅克隆 public Object clone() { LinkedList<E> clone = superClone(); // Put clone into "virgin" state clone.first = clone.last = null; clone.size = 0; clone.modCount = 0; //将链表中所有结点的数据添加到克隆对象 for (Node<E> x = first; x != null; x = x.next) clone.add(x.item); return clone; } //返回LinkedList的Object对象数组 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; } //返回LinkedList的模板数组 @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; } //版本号 private static final long serialVersionUID = 876323262645176354L; //将LinkedList的大小以及所有元素值都写入到输出流中 private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException { // Write out any hidden serialization magic s.defaultWriteObject(); 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); } //先将LinkedList的容量读出，然后将所有的元素对象读出 @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()); }}几点总结：1.从源码中很明显可以看出，LinkedList的实现是基于双向链表的。
2.在查找和删除某元素时，源码中都划分为该元素为null和不为null两种情况来处理，LinkedList中允许元素为null。
3.LinkedList是基于链表实现的，因此不存在容量不足的问题，所以这里没有扩容的方法。
4.LinkedList是基于链表实现的，因此插入删除效率高，查找效率低。
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