基于版本jdk1.7.0_80
java.util.LinkedList
代码如下
/* * Copyright (c) 1997, 2011, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */package java.util;/** * Doubly-linked list implementation of the { @code List} and { @code Deque} * interfaces. Implements all optional list operations, and permits all * elements (including { @code null}). * * All of the operations perform as could be expected for a doubly-linked * list. Operations that index into the list will traverse the list from * the beginning or the end, whichever is closer to the specified index. * *
Note that this implementation is not synchronized. * If multiple threads access a linked list concurrently, and at least * one of the threads modifies the list structurally, it must be * synchronized externally. (A structural modification is any operation * that adds or deletes one or more elements; merely setting the value of * an element is not a structural modification.) This is typically * accomplished by synchronizing on some object that naturally * encapsulates the list. * * If no such object exists, the list should be "wrapped" using the * {
@link Collections#synchronizedList Collections.synchronizedList} * method. This is best done at creation time, to prevent accidental * unsynchronized access to the list: * List list = Collections.synchronizedList(new LinkedList(...));
* * The iterators returned by this class's {
@code iterator} and * { @code listIterator} methods are fail-fast: if the list is * structurally modified at any time after the iterator is created, in * any way except through the Iterator's own { @code remove} or * { @code add} methods, the iterator will throw a { @link * ConcurrentModificationException}. Thus, in the face of concurrent * modification, the iterator fails quickly and cleanly, rather than * risking arbitrary, non-deterministic behavior at an undetermined * time in the future. * * Note that the fail-fast behavior of an iterator cannot be guaranteed * as it is, generally speaking, impossible to make any hard guarantees in the * presence of unsynchronized concurrent modification. Fail-fast iterators * throw {
@code ConcurrentModificationException} on a best-effort basis. * Therefore, it would be wrong to write a program that depended on this * exception for its correctness: the fail-fast behavior of iterators * should be used only to detect bugs. * * This class is a member of the * * Java Collections Framework. * * @author Josh Bloch * @see List * @see ArrayList * @since 1.2 * @param
the type of elements held in this collection */public class LinkedList extends AbstractSequentialList implements List , Deque , Cloneable, java.io.Serializable{ transient int size = 0; /** * Pointer to first node. * Invariant: (first == null && last == null) || * (first.prev == null && first.item != null) */ transient Node first; /** * Pointer to last node. * Invariant: (first == null && last == null) || * (last.next == null && last.item != null) */ transient Node last; /** * Constructs an empty list. */ public LinkedList() { } /** * Constructs a list containing the elements of the specified * collection, in the order they are returned by the collection's * iterator. * * @param c the collection whose elements are to be placed into this list * @throws NullPointerException if the specified collection is null */ public LinkedList(Collection c) { this(); addAll(c); } /** * Links e as first element. */ private void linkFirst(E e) { final Node f = first; final Node newNode = new Node<>(null, e, f); first = newNode; if (f == null) last = newNode; else f.prev = newNode; size++; modCount++; } /** * Links e as last element. */ void linkLast(E e) { final Node l = last; final Node newNode = new Node<>(l, e, null); last = newNode; if (l == null) first = newNode; else l.next = newNode; size++; modCount++; } /** * Inserts element e before non-null Node succ. */ void linkBefore(E e, Node succ) { // assert succ != null; final Node pred = succ.prev; final Node newNode = new Node<>(pred, e, succ); succ.prev = newNode; if (pred == null) first = newNode; else pred.next = newNode; size++; modCount++; } /** * Unlinks non-null first node f. */ private E unlinkFirst(Node f) { // assert f == first && f != null; final E element = f.item; final Node 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; } /** * Unlinks non-null last node l. */ private E unlinkLast(Node l) { // assert l == last && l != null; final E element = l.item; final Node 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; } /** * Unlinks non-null node x. */ E unlink(Node x) { // assert x != null; final E element = x.item; final Node next = x.next; final Node 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; } /** * Returns the first element in this list. * * @return the first element in this list * @throws NoSuchElementException if this list is empty */ public E getFirst() { final Node f = first; if (f == null) throw new NoSuchElementException(); return f.item; } /** * Returns the last element in this list. * * @return the last element in this list * @throws NoSuchElementException if this list is empty */ public E getLast() { final Node l = last; if (l == null) throw new NoSuchElementException(); return l.item; } /** * Removes and returns the first element from this list. * * @return the first element from this list * @throws NoSuchElementException if this list is empty */ public E removeFirst() { final Node f = first; if (f == null) throw new NoSuchElementException(); return unlinkFirst(f); } /** * Removes and returns the last element from this list. * * @return the last element from this list * @throws NoSuchElementException if this list is empty */ public E removeLast() { final Node l = last; if (l == null) throw new NoSuchElementException(); return unlinkLast(l); } /** * Inserts the specified element at the beginning of this list. * * @param e the element to add */ public void addFirst(E e) { linkFirst(e); } /** * Appends the specified element to the end of this list. * * This method is equivalent to {
@link #add}. * * @param e the element to add */ public void addLast(E e) { linkLast(e); } /** * Returns { @code true} if this list contains the specified element. * More formally, returns { @code true} if and only if this list contains * at least one element { @code e} such that * (o==null ? e==null : o.equals(e)). * * @param o element whose presence in this list is to be tested * @return { @code true} if this list contains the specified element */ public boolean contains(Object o) { return indexOf(o) != -1; } /** * Returns the number of elements in this list. * * @return the number of elements in this list */ public int size() { return size; } /** * Appends the specified element to the end of this list. * * This method is equivalent to {
@link #addLast}. * * @param e element to be appended to this list * @return { @code true} (as specified by { @link Collection#add}) */ public boolean add(E e) { linkLast(e); return true; } /** * Removes the first occurrence of the specified element from this list, * if it is present. If this list does not contain the element, it is * unchanged. More formally, removes the element with the lowest index * { @code i} such that * (o==null ? get(i)==null : o.equals(get(i))) * (if such an element exists). Returns { @code true} if this list * contained the specified element (or equivalently, if this list * changed as a result of the call). * * @param o element to be removed from this list, if present * @return { @code true} if this list contained the specified element */ public boolean remove(Object o) { if (o == null) { for (Node x = first; x != null; x = x.next) { if (x.item == null) { unlink(x); return true; } } } else { for (Node x = first; x != null; x = x.next) { if (o.equals(x.item)) { unlink(x); return true; } } } return false; } /** * Appends all of the elements in the specified collection to the end of * this list, in the order that they are returned by the specified * collection's iterator. The behavior of this operation is undefined if * the specified collection is modified while the operation is in * progress. (Note that this will occur if the specified collection is * this list, and it's nonempty.) * * @param c collection containing elements to be added to this list * @return { @code true} if this list changed as a result of the call * @throws NullPointerException if the specified collection is null */ public boolean addAll(Collection c) { return addAll(size, c); } /** * Inserts all of the elements in the specified collection into this * list, starting at the specified position. Shifts the element * currently at that position (if any) and any subsequent elements to * the right (increases their indices). The new elements will appear * in the list in the order that they are returned by the * specified collection's iterator. * * @param index index at which to insert the first element * from the specified collection * @param c collection containing elements to be added to this list * @return { @code true} if this list changed as a result of the call * @throws IndexOutOfBoundsException { @inheritDoc} * @throws NullPointerException if the specified collection is null */ public boolean addAll(int index, Collection c) { checkPositionIndex(index); Object[] a = c.toArray(); int numNew = a.length; if (numNew == 0) return false; Node pred, succ; 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 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; } size += numNew; modCount++; return true; } /** * Removes all of the elements from this list. * The list will be empty after this call returns. */ public void clear() { // Clearing all of the links between nodes is "unnecessary", but: // - helps a generational GC if the discarded nodes inhabit // more than one generation // - is sure to free memory even if there is a reachable Iterator for (Node x = first; x != null; ) { Node next = x.next; x.item = null; x.next = null; x.prev = null; x = next; } first = last = null; size = 0; modCount++; } // Positional Access Operations /** * Returns the element at the specified position in this list. * * @param index index of the element to return * @return the element at the specified position in this list * @throws IndexOutOfBoundsException { @inheritDoc} */ public E get(int index) { checkElementIndex(index); return node(index).item; } /** * Replaces the element at the specified position in this list with the * specified element. * * @param index index of the element to replace * @param element element to be stored at the specified position * @return the element previously at the specified position * @throws IndexOutOfBoundsException { @inheritDoc} */ public E set(int index, E element) { checkElementIndex(index); Node x = node(index); E oldVal = x.item; x.item = element; return oldVal; } /** * Inserts the specified element at the specified position in this list. * Shifts the element currently at that position (if any) and any * subsequent elements to the right (adds one to their indices). * * @param index index at which the specified element is to be inserted * @param element element to be inserted * @throws IndexOutOfBoundsException { @inheritDoc} */ public void add(int index, E element) { checkPositionIndex(index); if (index == size) linkLast(element); else linkBefore(element, node(index)); } /** * Removes the element at the specified position in this list. Shifts any * subsequent elements to the left (subtracts one from their indices). * Returns the element that was removed from the list. * * @param index the index of the element to be removed * @return the element previously at the specified position * @throws IndexOutOfBoundsException { @inheritDoc} */ public E remove(int index) { checkElementIndex(index); return unlink(node(index)); } /** * Tells if the argument is the index of an existing element. */ private boolean isElementIndex(int index) { return index >= 0 && index < size; } /** * Tells if the argument is the index of a valid position for an * iterator or an add operation. */ private boolean isPositionIndex(int index) { return index >= 0 && index <= size; } /** * Constructs an IndexOutOfBoundsException detail message. * Of the many possible refactorings of the error handling code, * this "outlining" performs best with both server and client VMs. */ 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)); } /** * Returns the (non-null) Node at the specified element index. */ Node node(int index) { // assert isElementIndex(index); if (index < (size >> 1)) { Node x = first; for (int i = 0; i < index; i++) x = x.next; return x; } else { Node x = last; for (int i = size - 1; i > index; i--) x = x.prev; return x; } } // Search Operations /** * Returns the index of the first occurrence of the specified element * in this list, or -1 if this list does not contain the element. * More formally, returns the lowest index { @code i} such that * (o==null ? get(i)==null : o.equals(get(i))), * or -1 if there is no such index. * * @param o element to search for * @return the index of the first occurrence of the specified element in * this list, or -1 if this list does not contain the element */ public int indexOf(Object o) { int index = 0; if (o == null) { for (Node x = first; x != null; x = x.next) { if (x.item == null) return index; index++; } } else { for (Node x = first; x != null; x = x.next) { if (o.equals(x.item)) return index; index++; } } return -1; } /** * Returns the index of the last occurrence of the specified element * in this list, or -1 if this list does not contain the element. * More formally, returns the highest index { @code i} such that * (o==null ? get(i)==null : o.equals(get(i))), * or -1 if there is no such index. * * @param o element to search for * @return the index of the last occurrence of the specified element in * this list, or -1 if this list does not contain the element */ public int lastIndexOf(Object o) { int index = size; if (o == null) { for (Node x = last; x != null; x = x.prev) { index--; if (x.item == null) return index; } } else { for (Node x = last; x != null; x = x.prev) { index--; if (o.equals(x.item)) return index; } } return -1; } // Queue operations. /** * Retrieves, but does not remove, the head (first element) of this list. * * @return the head of this list, or { @code null} if this list is empty * @since 1.5 */ public E peek() { final Node f = first; return (f == null) ? null : f.item; } /** * Retrieves, but does not remove, the head (first element) of this list. * * @return the head of this list * @throws NoSuchElementException if this list is empty * @since 1.5 */ public E element() { return getFirst(); } /** * Retrieves and removes the head (first element) of this list. * * @return the head of this list, or { @code null} if this list is empty * @since 1.5 */ public E poll() { final Node f = first; return (f == null) ? null : unlinkFirst(f); } /** * Retrieves and removes the head (first element) of this list. * * @return the head of this list * @throws NoSuchElementException if this list is empty * @since 1.5 */ public E remove() { return removeFirst(); } /** * Adds the specified element as the tail (last element) of this list. * * @param e the element to add * @return { @code true} (as specified by { @link Queue#offer}) * @since 1.5 */ public boolean offer(E e) { return add(e); } // Deque operations /** * Inserts the specified element at the front of this list. * * @param e the element to insert * @return { @code true} (as specified by { @link Deque#offerFirst}) * @since 1.6 */ public boolean offerFirst(E e) { addFirst(e); return true; } /** * Inserts the specified element at the end of this list. * * @param e the element to insert * @return { @code true} (as specified by { @link Deque#offerLast}) * @since 1.6 */ public boolean offerLast(E e) { addLast(e); return true; } /** * Retrieves, but does not remove, the first element of this list, * or returns { @code null} if this list is empty. * * @return the first element of this list, or { @code null} * if this list is empty * @since 1.6 */ public E peekFirst() { final Node f = first; return (f == null) ? null : f.item; } /** * Retrieves, but does not remove, the last element of this list, * or returns { @code null} if this list is empty. * * @return the last element of this list, or { @code null} * if this list is empty * @since 1.6 */ public E peekLast() { final Node l = last; return (l == null) ? null : l.item; } /** * Retrieves and removes the first element of this list, * or returns { @code null} if this list is empty. * * @return the first element of this list, or { @code null} if * this list is empty * @since 1.6 */ public E pollFirst() { final Node f = first; return (f == null) ? null : unlinkFirst(f); } /** * Retrieves and removes the last element of this list, * or returns { @code null} if this list is empty. * * @return the last element of this list, or { @code null} if * this list is empty * @since 1.6 */ public E pollLast() { final Node l = last; return (l == null) ? null : unlinkLast(l); } /** * Pushes an element onto the stack represented by this list. In other * words, inserts the element at the front of this list. * * This method is equivalent to {
@link #addFirst}. * * @param e the element to push * @since 1.6 */ public void push(E e) { addFirst(e); } /** * Pops an element from the stack represented by this list. In other * words, removes and returns the first element of this list. * * This method is equivalent to {
@link #removeFirst()}. * * @return the element at the front of this list (which is the top * of the stack represented by this list) * @throws NoSuchElementException if this list is empty * @since 1.6 */ public E pop() { return removeFirst(); } /** * Removes the first occurrence of the specified element in this * list (when traversing the list from head to tail). If the list * does not contain the element, it is unchanged. * * @param o element to be removed from this list, if present * @return { @code true} if the list contained the specified element * @since 1.6 */ public boolean removeFirstOccurrence(Object o) { return remove(o); } /** * Removes the last occurrence of the specified element in this * list (when traversing the list from head to tail). If the list * does not contain the element, it is unchanged. * * @param o element to be removed from this list, if present * @return { @code true} if the list contained the specified element * @since 1.6 */ public boolean removeLastOccurrence(Object o) { if (o == null) { for (Node x = last; x != null; x = x.prev) { if (x.item == null) { unlink(x); return true; } } } else { for (Node x = last; x != null; x = x.prev) { if (o.equals(x.item)) { unlink(x); return true; } } } return false; } /** * Returns a list-iterator of the elements in this list (in proper * sequence), starting at the specified position in the list. * Obeys the general contract of { @code List.listIterator(int)}. * * The list-iterator is fail-fast: if the list is structurally * modified at any time after the Iterator is created, in any way except * through the list-iterator's own {
@code remove} or { @code add} * methods, the list-iterator will throw a * { @code ConcurrentModificationException}. Thus, in the face of * concurrent modification, the iterator fails quickly and cleanly, rather * than risking arbitrary, non-deterministic behavior at an undetermined * time in the future. * * @param index index of the first element to be returned from the * list-iterator (by a call to { @code next}) * @return a ListIterator of the elements in this list (in proper * sequence), starting at the specified position in the list * @throws IndexOutOfBoundsException { @inheritDoc} * @see List#listIterator(int) */ public ListIterator listIterator(int index) { checkPositionIndex(index); return new ListItr(index); } private class ListItr implements ListIterator { private Node lastReturned = null; private Node 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 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++; } final void checkForComodification() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); } } private static class Node { E item; Node next; Node prev; Node(Node prev, E element, Node next) { this.item = element; this.next = next; this.prev = prev; } } /** * @since 1.6 */ public Iterator descendingIterator() { return new DescendingIterator(); } /** * Adapter to provide descending iterators via ListItr.previous */ private class DescendingIterator implements Iterator { private final ListItr itr = new ListItr(size()); public boolean hasNext() { return itr.hasPrevious(); } public E next() { return itr.previous(); } public void remove() { itr.remove(); } } @SuppressWarnings("unchecked") private LinkedList superClone() { try { return (LinkedList ) super.clone(); } catch (CloneNotSupportedException e) { throw new InternalError(); } } /** * Returns a shallow copy of this { @code LinkedList}. (The elements * themselves are not cloned.) * * @return a shallow copy of this { @code LinkedList} instance */ public Object clone() { LinkedList 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 x = first; x != null; x = x.next) clone.add(x.item); return clone; } /** * Returns an array containing all of the elements in this list * in proper sequence (from first to last element). * * The returned array will be "safe" in that no references to it are * maintained by this list. (In other words, this method must allocate * a new array). The caller is thus free to modify the returned array. * *
This method acts as bridge between array-based and collection-based * APIs. * * @return an array containing all of the elements in this list * in proper sequence */ public Object[] toArray() { Object[] result = new Object[size]; int i = 0; for (Node
x = first; x != null; x = x.next) result[i++] = x.item; return result; } /** * Returns an array containing all of the elements in this list in * proper sequence (from first to last element); the runtime type of * the returned array is that of the specified array. If the list fits * in the specified array, it is returned therein. Otherwise, a new * array is allocated with the runtime type of the specified array and * the size of this list. * * If the list fits in the specified array with room to spare (i.e., * the array has more elements than the list), the element in the array * immediately following the end of the list is set to {
@code null}. * (This is useful in determining the length of the list only if * the caller knows that the list does not contain any null elements.) * * Like the {
@link #toArray()} method, this method acts as bridge between * array-based and collection-based APIs. Further, this method allows * precise control over the runtime type of the output array, and may, * under certain circumstances, be used to save allocation costs. * * Suppose {
@code x} is a list known to contain only strings. * The following code can be used to dump the list into a newly * allocated array of { @code String}: * * * String[] y = x.toArray(new String[0]);
* * Note that { @code toArray(new Object[0])} is identical in function to * { @code toArray()}. * * @param a the array into which the elements of the list are to * be stored, if it is big enough; otherwise, a new array of the * same runtime type is allocated for this purpose. * @return an array containing the elements of the list * @throws ArrayStoreException if the runtime type of the specified array * is not a supertype of the runtime type of every element in * this list * @throws NullPointerException if the specified array is null */ @SuppressWarnings("unchecked") public 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 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; /** * Saves the state of this { @code LinkedList} instance to a stream * (that is, serializes it). * * @serialData The size of the list (the number of elements it * contains) is emitted (int), followed by all of its * elements (each an Object) in the proper order. */ 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 x = first; x != null; x = x.next) s.writeObject(x.item); } /** * Reconstitutes this { @code LinkedList} instance from a stream * (that is, deserializes it). */ @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()); }}
代码量还是一千行出头，读起来并不费力，还是只简单分析一下要点
1. 接口分析
LinkedList继承于AbstractSequentialList抽象类（说明这是一个顺序访问的数据结构，与ArrayList继承的RandomAccess接口不同）
List，Deque（LinkedList原理跟Deque完全相同，所以顺手支持了），Cloneable，java.io.Serializable接口
2. 实现原理
其实现原理是标准的双向链表，节点定义如下
private static class Node{ E item; Node next; Node prev; Node(Node prev, E element, Node next) { this.item = element; this.next = next; this.prev = prev; } }
向前向后指针+元素指针，无需过多解释
3. ConcurrentModificationException
与ArrayList相同，LinkedList也是线程不安全的，但是LinkedList的listIterator方法上有一段很好的注释，清晰的说明了它的设计思路，我且引用一下
* The list-iterator is fail-fast: if the list is structurally * modified at any time after the Iterator is created, in any way except * through the list-iterator's own {@code remove} or {@code add} * methods, the list-iterator will throw a * {@code ConcurrentModificationException}. Thus, in the face of * concurrent modification, the iterator fails quickly and cleanly, rather * than risking arbitrary, non-deterministic behavior at an undetermined * time in the future. 大意是说LinkedList的迭代器是快速失败（fail-fast）的，只要迭代器在被创建之后所属的list受到了改动，无论这个改动是什么原因，迭代器都会抛出ConcurrentModificationException异常，逼迫程序员进行处理，避免在未来发生一些难以追查的异常行为。
4. LinkedList的遍历问题
ArrayList可以在循环里通过下标遍历，但是这个操作不能移植到LinkedList里来
因为LinkedList的get(int index)方法会调用node(int index)方法，而这个方法会试图从头部或者尾部遍历链表来定位元素，时间复杂度高。源码如下
Nodenode(int index) { // assert isElementIndex(index); if (index < (size >> 1)) { Node x = first; for (int i = 0; i < index; i++) x = x.next; return x; } else { Node x = last; for (int i = size - 1; i > index; i--) x = x.prev; return x; } }
所以最好还是用迭代器来遍历数组元素