LinkedList阅读

package java.util;

import java.util.function.Consumer;

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
    transient int size = 0;

    transient Node<E> first;

    transient Node<E> last;

    public LinkedList() {
    }
　　
　　//　传入一个Collection类型的集合，转换为linkedlist类型
    public LinkedList(Collection<? extends E> c) {
        this();
        addAll(c);
    }

    /**
     * 将一个element添加为第一个节点 该方法提供给addFirst调用
     */
    private void linkFirst(E e) {
　　　　  //当前第一个节点暂存
        final Node<E> f = first;
　　　　　//新建一个节点，pre为空，next为之前的first节点
        final Node<E> newNode = new Node<>(null, e, f);
　　　　　//将当前节点设置为第一个节点，并且判断原来第一个节点是否存在，不存在，则新增节点既是first，也是last节点，
　　　　　//如果添加之前存在第一个节点，（因为linkedList是一个双向列表，所以需要将原第一个节点的pre设置为新增节点），添加完成之后size，modCount（改变次数）自增
        first = newNode;
        if (f == null)
            last = newNode;
        else
            f.prev = newNode;
        size++;
        modCount++;
    }

    /**
     * 同linkFirst，将添加的节点设置为最后一个
     */
    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++;
    }

    /**
     * 在指定节点之前添加一个节点
     */
    void linkBefore(E e, Node<E> succ) {
        // assert succ != null;
　　　　　//暂存指定节点的前一个节点
        final Node<E> pred = succ.prev;
　　　　　//创建新节点，且pre设置为pred，next设置为指定的节点（该新建只是指定了当前节点的pre和next，尚没有指定前一个节点的next和后一个节点的pre）
        final Node<E> newNode = new Node<>(pred, e, succ);
　　　　　//一下步骤是指定前一个节点的next和后一个节点的pre
        succ.prev = newNode;
        if (pred == null)
            first = newNode;
        else
            pred.next = newNode;
        size++;
        modCount++;
    }

    /**
     * 清掉第一个非空的节点.
     */
    private E unlinkFirst(Node<E> f) {
        // assert f == first && f != null;
　　　　　//暂存要清掉节点的item和next，并将f节点的item和next置空（置空是为了帮助清理无用的空间）
        final E element = f.item;
        final Node<E> next = f.next;
        f.item = null;
        f.next = null; // help GC
　　　　　//将下一节点置为第一个节点（因是双向列表，所以需要设置节点的pre为空，同时size减小，modCount改变次数增加）
        first = next;
        if (next == null)
            last = null;
        else
            next.prev = null;
        size--;
        modCount++;
　　　　　//返回删除节点的item
        return element;
    }

    /**
     * 同unlinkFirst方法，删除最后一个非空节点.
     */
    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;
    }

    /**
     * 删除指定节点.
     */
    E unlink(Node<E> x) {
        // assert x != null;
　　　　　//暂存要删除节点的 值：item/next/pre
        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;
    }

    /**
     *　返回列表的第一个节点*/
    public E getFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return f.item;
    }

    /**
     * 返回节点的最后一个节点*/
    public E getLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return l.item;
    }

    /**
     * 删除第一个节点，（方法调用私有方法unlinkFirst）*/
    public E removeFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return unlinkFirst(f);
    }

    /**
     * 删除最有一个节点*/
    public E removeLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return unlinkLast(l);
    }

    /**
     * 添加一个首节点*/
    public void addFirst(E e) {
        linkFirst(e);
    }

    /**
     * 添加尾节点
     *
     * @param e the element to add
     */
    public void addLast(E e) {
        linkLast(e);
    }

    /**
     * 判断列表中是否存在指定节点*/
    public boolean contains(Object o) {
        return indexOf(o) != -1;
    }

    /**
     * 列表大小*/
    public int size() {
        return size;
    }

    /**
     * 添加尾节点
     **/
    public boolean add(E e) {
        linkLast(e);
        return true;
    }

    /**删除链表中的指定值*/
    public boolean remove(Object o) {
        if (o == null) {
　　　　　　//循环列表，将列表中null删除
            for (Node<E> x = first; x != null; x = x.next) {
                if (x.item == null) {
                    unlink(x);
                    return true;
                }
            }
        } else {
　　　　　　//循环列表，删除列表中item为o的节点
            for (Node<E> x = first; x != null; x = x.next) {
                if (o.equals(x.item)) {
                    unlink(x);
                    return true;
                }
            }
        }
        return false;
    }

    /**
     * 构造函数调用，将一个collection类型的集合转换成一个linkedlist类型
*/
    public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }

    /**
     *　index：范围在【0-size】，当为0是，将集合中的节点添加的linkedlist的开头
　　　　不为0是，则添加到第index位置*/
    public boolean addAll(int index, Collection<? extends E> c) {
　　　　　//校验index是否在[0，size]之间，不再则报错
        checkPositionIndex(index);

        Object[] a = c.toArray();
        int numNew = a.length;
        if (numNew == 0)
            return false;
　　　　　//index==size是 添加到最后
        Node<E> pred, succ;
        if (index == size) {
            succ = null;
            pred = last;
        } else {
　　　　　　　//暂存指定位置的节点，之后将该节点设置到新增集合节点中的后边
            succ = node(index);
            pred = succ.prev;
        }

　　　　　//循环设置新节点，并将pre的next设置为新增节点
        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;
　　　　　　　//为下一此循环判断，赋值 准备
            pred = newNode;
        }

        if (succ == null) {
            last = pred;
        } else {
            pred.next = succ;
            succ.prev = pred;
        }

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

    /**清空链表*/
    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
　　　　　//节点全都置null，是为了方便释放内存
        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++;
    }


    // Positional Access Operations

    /**
     * 获取指定位置的节点值*/
    public E get(int index) {
　　　　　//判断index是否合法
        checkElementIndex(index);
　　　　　//node方法内部循环获取节点
        return node(index).item;
    }

    /**
     * 替换指定位置的节点，并返回旧节点的item*/
    public E set(int index, E element) {
        checkElementIndex(index);
        Node<E> x = node(index);
        E oldVal = x.item;
        x.item = element;
        return oldVal;
    }

    /**在指定位置添加节点*/
    public void add(int index, E element) {
　　　　　//校验位置是否合法
        checkPositionIndex(index);
　　　　　//index==size的话，表明指定的位置是在列表的最后
        if (index == size)
            linkLast(element);
        else
            linkBefore(element, node(index));
    }

    /**删除指定位置的节点*/
    public E remove(int index) {
        checkElementIndex(index);
        return unlink(node(index));
    }

    /**
     * 判断指定位置是否存在一个节点（因为size表明链表的大小，只有index在[0,size]之间，表明指定index存在一个节点）.
     */
    private boolean isElementIndex(int index) {
        return index >= 0 && index < size;
    }

    /**判断指定位置是否合法（加入要新增节点，节点的位置只能在0-size之间，首尾位置也是可插入位置，合法）
     */
    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));
    }

    /**
     * 返回指定位置的节点
     */
    Node<E> node(int index) {
        // assert isElementIndex(index);
　　　　　//使用二分模式 （减少内存耗费）
        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;
        }
    }

    // Search Operations

    /**
     * 返回指定对象在链表中第一次出现的位置，没有则返回-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;
    }

    /**
     * 返回一个对象在链表中最后一次出现的位置*/
    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;
    }

    // Queue operations.

    /**
     * 返回一个链表的第一个节点
     * @since 1.5
     */
    public E peek() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
    }

    /**
     * 获取第一个节点  如果链表为空则会爆出异常*/
    public E element() {
        return getFirst();
    }

    /**
     * 返回第一个节点，并且将此在链表中删除*/
    public E poll() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }

    /**
     * 删除第一个节点*/
    public E remove() {
        return removeFirst();
    }

    /**
     * 在链表末尾添加一个节点*/
    public boolean offer(E e) {
        return add(e);
    }

    // Deque operations
    /**
     * 在链表开头添加一个节点*/
    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;
    }

    /**
     * 返回第一个节点的item*/
    public E peekFirst() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
     }

    /**
     * 返回最有一个节点的item*/
    public E peekLast() {
        final Node<E> l = last;
        return (l == null) ? null : l.item;
    }

    /**
     * 删除第一个节点，并返会删除的节点*/
    public E pollFirst() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }

    /**
     * 删除最有一个节点，并返回节点*/
    public E pollLast() {
        final Node<E> l = last;
        return (l == null) ? null : unlinkLast(l);
    }

    public void push(E e) {
        addFirst(e);
    }

    public E pop() {
        return removeFirst();
    }

    /**
     * 删除链表中的第一个o*/
    public boolean removeFirstOccurrence(Object o) {
        return remove(o);
    }

    /**
     * 删除链表中最后一个*/
    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;
    }

    /**
     * 返回指定位置之后的一个迭代器*/
    public ListIterator<E> listIterator(int index) {
        checkPositionIndex(index);
        return new ListItr(index);
    }

    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();
        }
    }

    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
     */
    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();
        }
    }

    @SuppressWarnings("unchecked")
    private LinkedList<E> superClone() {
        try {
            return (LinkedList<E>) super.clone();
        } catch (CloneNotSupportedException e) {
            throw new InternalError(e);
        }
    }

    /**
     * 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<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;
    }

    /**
     * Returns an array containing all of the elements in this list
     * in proper sequence (from first to last element).
     *
     * <p>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.
     *
     * <p>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<E> 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.
     *
     * <p>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 <i>only</i> if
     * the caller knows that the list does not contain any null elements.)
     *
     * <p>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.
     *
     * <p>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}:
     *
     * <pre>
     *     String[] y = x.toArray(new String[0]);</pre>
     *
     * 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> 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;

    /**
     * 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<E> 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());
    }

    /**
     * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
     * and <em>fail-fast</em> {@link Spliterator} over the elements in this
     * list.
     *
     * <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
     * {@link Spliterator#ORDERED}.  Overriding implementations should document
     * the reporting of additional characteristic values.
     *
     * @implNote
     * The {@code Spliterator} additionally reports {@link Spliterator#SUBSIZED}
     * and implements {@code trySplit} to permit limited parallelism..
     *
     * @return a {@code Spliterator} over the elements in this list
     * @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;
        }
    }

}

遍历：

链表的遍历过程也很简单，和查找过程类似，我们从头节点往后遍历就行了。但对于 LinkedList 的遍历还是需要注意一些，不然可能会导致代码效率低下。通常情况下，我们会使用 foreach 遍历 LinkedList，而 foreach 最终转换成迭代器形式。所以分析 LinkedList 的遍历的核心就是它的迭代器实现

我们都知道 LinkedList 不擅长随机位置访问，如果大家用随机访问的方式遍历 LinkedList，效率会很差。比如下面的代码：

List<Integet> list = new LinkedList<>();
list.add(1)
list.add(2)
......
for (int i = 0; i < list.size(); i++) {
    Integet item = list.get(i);  //这种遍历方式  每次获取指定索引的元素 都会从开头重新寻找到指定位置，验证影响效率
    // do something
}