LinkedList内部是经过双向链表存储的,提供顺序访问。继承了AbstractSequentialList,实如今迭代器上的随机访问。而且,还实现了List、Deque、Cloneable,Serializable。Deque是双端队列接口,继承自Queue,Queue是队列接口。LinkedList的定义以下:java
public class LinkedList<E> extends AbstractSequentialList<E> implements List<E>, Deque<E>, Cloneable, java.io.Serializable {}
数据存储的结构是链表,定义以下:node
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; } }
ArrayList提供两个构造方法,定义以下:数组
//默认构造方法 public LinkedList() {} //给定初始化集合c的构造方法 public LinkedList(Collection<? extends E> c) { this(); addAll(c); }
LinkedList经过链表结构实现了双端队列、栈等数据结构,下面具体分析源码:数据结构
public class LinkedList<E> extends AbstractSequentialList<E> implements List<E>, Deque<E>, Cloneable, java.io.Serializable { //链表大小 transient int size = 0; /** * 指向第一个节点指针 */ transient Node<E> first; /** *指向最后一个节点的指针 */ transient Node<E> last; /** * 构造一个空的list. */ public LinkedList() { } /** * 构造一个有特定元素的list */ public LinkedList(Collection<? extends E> c) { this(); addAll(c); } /** * 添加一个元素,做为第一个节点 */ 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++; } /** * 添加一个节点,做为最后一个元素 */ 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++; } /** * 在非空节点succ前插入一个元素 */ 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++; } /** * 删除一个非空的first节点,将其next节点指向first,若是next节点为空,则last节点也为空 */ 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; } /** * 删除一个非空的last节点,将其prev节点指向last,若是prev节点为空,则first节点也为空 */ 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; } /** * 删除非空节点x,将其next节点的prev节点指向其prev节点,prev节点的next节点指向其next节点 */ 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; } /** * 返回first节点,若是节点为空,抛出NoSuchElementException 异常 */ public E getFirst() { final Node<E> f = first; if (f == null) throw new NoSuchElementException(); return f.item; } /** * 返回last节点,若是节点为空,抛出NoSuchElementException 异常 */ public E getLast() { final Node<E> l = last; if (l == null) throw new NoSuchElementException(); return l.item; } /** * 删除第一个节点,若是节点为空,抛出NoSuchElementException 异常 */ public E removeFirst() { final Node<E> f = first; if (f == null) throw new NoSuchElementException(); return unlinkFirst(f); } /** * 删除最后一个节点,若是节点为空,抛出NoSuchElementException 异常 */ public E removeLast() { final Node<E> l = last; if (l == null) throw new NoSuchElementException(); return unlinkLast(l); } /** * 在链表的头部插入节点e */ public void addFirst(E e) { linkFirst(e); } /** * 在链表的头部插入节点e */ public void addLast(E e) { linkLast(e); } /** * 返回链表中是否包含元素e */ public boolean contains(Object o) { return indexOf(o) != -1; } /** * 返回链表的大小 */ public int size() { return size; } /** * 在链表尾部添加一个元素 */ public boolean add(E e) { linkLast(e); return true; } /** * 从头部遍历,移除链表中第一个o元素 */ public boolean remove(Object o) { 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; } /** * 将给定集合中的元素插入到链表尾部 */ public boolean addAll(Collection<? extends E> c) { return addAll(size, c); } /** * 在指定的位置index后面插入集合c中的元素 */ public boolean addAll(int index, Collection<? extends E> c) { checkPositionIndex(index); Object[] a = c.toArray(); int numNew = a.length; if (numNew == 0) return false; Node<E> 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<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; } size += numNew; modCount++; return true; } /** * 删除list中全部的元素. */ public void clear() { //解除节点间的连接是没有必要的,可是,这样作有助于GC和释放内存 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++; } // 下面是位置访问的操做 /** * 返回指定位置节点中的元素. */ public E get(int index) { checkElementIndex(index); return node(index).item; } /** * 替换index位置节点中的元素 */ 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index处插入元素,若是index==size,则在最后位置插入 */ public void add(int index, E element) { checkPositionIndex(index); if (index == size) linkLast(element); else linkBefore(element, node(index)); } /** * 删除指定位置的元素 */ public E remove(int index) { checkElementIndex(index); return unlink(node(index)); } /** * index处是否有节点 */ private boolean isElementIndex(int index) { return index >= 0 && index < size; } /** * 返回index是不是有效的位置 */ private boolean isPositionIndex(int index) { return index >= 0 && index <= size; } /** * 数组越界的异常详细信息 */ 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)); } /** * 返回指定位置的元素 */ Node<E> node(int index) { // assert isElementIndex(index); //若是元素在前半部分,从前查找,不然,从后面开始查找 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; } } // 下面是搜索操做 /** * 从list头部开始搜索,返回遇到的第一个给定元素的下标,若是不存在,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; } /** * 返回最后一个指定元素的下标,若是不存在,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; } // 下面是队列操做 /** * 取第一个元素,可是不删除 */ public E peek() { final Node<E> f = first; return (f == null) ? null : f.item; } /** * 去第一个元素,可是不删除 */ public E element() { return getFirst(); } /** * 取第一个元素,而且删除 */ public E poll() { final Node<E> f = first; return (f == null) ? null : unlinkFirst(f); } /** * 返回第一个元素,而且删除 */ public E remove() { return removeFirst(); } /** * 在尾部插入元素e */ public boolean offer(E e) { return add(e); } // 下面是双端队列的操做 /** * 在list的前面插入元素e */ public boolean offerFirst(E e) { addFirst(e); return true; } /** * 在list的后面插入元素e */ public boolean offerLast(E e) { addLast(e); return true; } /** * 返回list中的第一个元素 */ public E peekFirst() { final Node<E> f = first; return (f == null) ? null : f.item; } /** * 返回list中的最后一个元素, */ public E peekLast() { final Node<E> l = last; return (l == null) ? null : l.item; } /** * 返回第一个元素,而且删除 */ public E pollFirst() { final Node<E> f = first; return (f == null) ? null : unlinkFirst(f); } /** * 返回最后一个元素,而且删除 */ public E pollLast() { final Node<E> l = last; return (l == null) ? null : unlinkLast(l); } /** * 在list头部添加节点 */ public void push(E e) { addFirst(e); } /** * 删除头部节点 */ public E pop() { return removeFirst(); } /** * 从头部开始遍历,删除第一个遇到的元素 */ public boolean removeFirstOccurrence(Object o) { return remove(o); } /** * 从尾部开始遍历,删除第一个遇到的元素 */ 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; } /** * List迭代器 */ public ListIterator<E> listIterator(int index) { checkPositionIndex(index); return new ListItr(index); } //迭代器内部类 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(); } } //节点定义 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; } } /** * 降序迭代器,从后向前遍历 */ public Iterator<E> descendingIterator() { return new DescendingIterator(); } /** * 降序迭代器实现 */ 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(); } } @SuppressWarnings("unchecked") private LinkedList<E> superClone() { try { return (LinkedList<E>) super.clone(); } catch (CloneNotSupportedException e) { throw new InternalError(e); } } /** * 克隆list对象 */ 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list转换为数组,顺序遍历,取每个节点元素,放入数组 */ 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list转换为数组,使用示例:String[] y = x.toArray(new String[0]) * @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> 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; /** * 将list写入输出流 */ 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); } /** * 从输出流构造list */ @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()); } /** *如下方法为1.8新增 */ /** * * @since 1.8 */ @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; } } }
//获取第一个元素,不删除节点 public E getFirst() //获取最后一个元素,不删除节点 public E getLast() //删除第一个节点 public E removeFirst() //删除最后一个节点 public E removeLast() //在头部添加一个节点 public void addFirst(E e) //在尾部添加一个节点 public void addLast(E e) //在尾部添加一个节点 public boolean add(E e) //顺序遍历,删除链表中第一个元素的节点 public boolean remove(Object o) //将集合c中的元素添加到尾部 public boolean addAll(Collection<? extends E> c) //将集合c中的元素添加到index位置后面 public boolean addAll(int index, Collection<? extends E> c) //获取位置index处的元素,不删除节点 public E get(int index) //替换位置index处的位置 public E set(int index, E element) //在位置index前插入节点 public void add(int index, E element) //删除位置index处的节点 public E remove(int index) //从头部遍历,返回第一个元素的位置 public int indexOf(Object o) //从尾部遍历,返回最后一个元素的位置 public int lastIndexOf(Object o) //返回第一个节点元素,不删除节点 public E peek() //返回第一个节点元素 public E element() //返回第一个节点元素,而且删除节点 public E poll() //删除第一个节点 public E remove() //在尾部添加节点 public boolean offer(E e) //在头部添加节点 public boolean offerFirst(E e) //在尾部添加节点 public boolean offerLast(E e) //获取第一个节点元素,不删除节点 public E peekFirst() //获取最后一个节点元素,不删除节点 public E peekLast() //获取第一个节点元素,删除节点 public E pollFirst() //获取最后一个节点元素,删除节点 public E pollLast() //在头部添加节点 public void push(E e) //返回第一个节点元素,删除节点 public E pop()
因为LikedList实现了接口List、Queue、Deque,内部经过链表存储,因此支持链表、队列、双端队列和栈等数据结构,下面分别介绍做为这几种数据结构的使用方法:less
队列是一种先进先出的数据结构,能够经过add()和poll()两个方法实现,具体代码以下:ide
/** * LinkedList做为队列的使用 */ public static void queueTest(){ Queue<Integer> queue = new LinkedList<>(); Integer[] arrays = new Integer[10]; int size = 10; //队尾加入元素 for (int i = 0; i < size; i++){ queue.add(i); } System.out.println("queue:" + queue.toString()); //取队头元素--不删除 for (int i = 0; i < size; i++){ arrays[i] = queue.peek(); } System.out.println("peek:" + Arrays.toString(arrays)); System.out.println("queue:" + queue.toString()); arrays = new Integer[10]; //取队头元素--删除 for (int i = 0; i < size; i++){ arrays[i] = queue.poll(); } System.out.println("poll" + Arrays.toString(arrays)); System.out.println("queue:" + queue.toString()); }
运行结果以下:ui
栈是一种先进后出的数据结构,能够经过push()和pop()两个方法实现,具体代码以下:this
/** * LinkedList做为栈的使用 */ public static void stackTest(){ Deque<Integer> stack = new LinkedList<>(); Integer[] arrays = new Integer[10]; int size = 10; //元素入栈 for (int i = 0; i < size; i++){ stack.push(i); } System.out.println("stack:" + stack.toString()); //元素出栈 for (int i = 0; i < size; i++){ arrays[i] = stack.pop(); } System.out.println("pop:" + Arrays.toString(arrays)); System.out.println("stack:" + stack.toString()); }
运行结果以下:spa
双端队列是一种在两端均可以进出的数据结构,能够经过offerFirst()、offerLast() 和pollFirst()、pollLast()等方法实现,具体代码以下:指针
/** * LinkedList做为双端队列的使用 */ public static void dequeTest(){ Deque<Integer> deque = new LinkedList<>(); Integer[] arrays = new Integer[10]; int size = 5; //从队头和队尾各入队五个 for (int i = 0; i < size; i++){ deque.offerFirst(i); deque.offerLast(i); } System.out.println("deque:" + deque.toString()); int n = 0; //从队头和队尾各出队五个 for (; n < 10;){ arrays[n++] = deque.pollFirst(); arrays[n++] = deque.pollLast(); } System.out.println("poll:" + Arrays.toString(arrays)); System.out.println("deque:" + deque.toString()); }
运行结果以下:
和ArrayList同样,进行四种遍历方式的比较,遍历代码和ArrayList同样,运行结果以下:
从上图中的结果能够看出,经过下标遍历LinkedList效率是很是低的。遍历中,get(i)方法每次都从头部或者尾部遍历,找到位置i的节点,取出节点中的元素,因此致使效率低。
本节分析了LinkedList的源码的用法。LinkedList实现了List、Queue、Deque接口,内部经过链表实现,可以实现链表、队列、栈和双端队列等数据结构的功能。