LinkedList源码分析

public class LinkedList<E>    extends AbstractSequentialList<E>    implements List<E>, Deque<E>, Cloneable, java.io.Serializable{    transient int size = 0;    /**     * 链表第一个节点     */    transient Node<E> first;    /**     * 链表最后一个节点     */    transient Node<E> last;    /**     * 构造一个空列表     */    public LinkedList() {    }    /**     * 构造一个包含指定 collection 中的元素的列表，这些元素按其 collection 的迭代器返回的顺序排列     */    public LinkedList(Collection<? extends E> c) {        this();        addAll(c);    }    /**     * 私有方法     * 向链表首部插入一个元素     * 如果是空链表，给last赋值     */    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++;    }    /**     * 包访问权限     * 向链表末尾插入一个节点     * 如果是空链表，给first赋     */    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++;    }    /**     * 包访问权限的方法     * 向节点succ前插入一个节点e     * 如果succ是first节点，则重新定义first节点     */    void linkBefore(E e, Node<E> succ) {        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++;    }    /**     * 删除一个非空的开始节点     *      */    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;    }    /**     * 删除一个非空的末尾节点     */    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;    }    /**     * 移除指定节点     */    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;    }    /**     * 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<E> 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<E> 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<E> 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<E> 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.     *     * <p>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     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.     *     * @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;    }    /**     * 将指定元素添加到链表的结尾     */    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     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>     * (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<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;    }    public boolean addAll(Collection<? extends E> c) {        return addAll(size, c);    }    /**     *  将指定 collection 中的所有元素从指定位置开始插入此列表。其中index表示在其中插入指定collection中第一个元素的索引     */    public boolean addAll(int index, Collection<? extends E> c) {        //检查索引位置是否合法        checkPositionIndex(index);        Object[] a = c.toArray();        int numNew = a.length;        //如果插入元素个数为空，返回false        if (numNew == 0)            return false;        Node<E> pred, succ;        if (index == size) {            //在链表末尾插入,succ定义为null,pred定义为last            succ = null;            pred = last;        } else {            //在链表其他位置插入，succ定义为后移一位的元素node[index],pred定义为succ.prev            succ = node(index);            pred = succ.prev;        }        //遍历将要插入的元素集合        for (Object o : a) {            @SuppressWarnings("unchecked")             //实例化一个元素（包括preNode的节点信息）            E e = (E) o;            Node<E> newNode = new Node<>(pred, e, null);            if (pred == null)                //当前插入位置是第一个位置                first = newNode;            else                //如果当前插入位置不是第一个位置，需要修改插入位置上一个节点的nextNode信息                pred.next = newNode;            //节点插入成功，重新定义下一个节点的pred为新插入的节点            pred = newNode;        }        if (succ == null) {            //更新last为新插入的最后一个节点            last = pred;        } else {            //更新新插入的最后一个节点nextNode信息            pred.next = succ;            //更新succ节点的prevNode信息            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<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    /**     * 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<E> x = node(index);        E oldVal = x.item;        x.item = element;        return oldVal;    }    /**     * 向指定元素前插入元素     */    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<E> node(int index) {        // assert isElementIndex(index);        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    /**     * 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     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,     * 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<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;    }    /**     * 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     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,     * 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<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.    /**     * 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<E> 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<E> 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<E> 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<E> 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<E> 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<E> 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.     *     * <p>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.     *     * <p>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<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;    }    /**     * 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)}.<p>     *     * The list-iterator is <i>fail-fast</i>: 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<E> listIterator(int index) {        checkPositionIndex(index);        return new ListItr(index);    }    private class ListItr implements ListIterator<E> {        private Node<E> lastReturned = null;        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++;        }        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();        }    }    /**     * 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;    }    /**     *      */    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;    }    /**     * 返回以适当顺序（从第一个元素到最后一个元素）包含此列表中所有元素的数组；     * 返回数组的运行时类型为指定数组的类型。如果指定数组能容纳列表，则在其中返回该列表。     * 否则，分配具有指定数组的运行时类型和此列表大小的新数组。      * 如果指定数组能容纳列表，并有剩余空间（即数组比列表元素多），则紧跟在列表末尾的数组元素会被设置为 null。     *（只有 在调用者知道列表不包含任何 null 元素时，才可使用此方法来确定列表的长度。）      * 像 toArray() 方法一样，此方法充当基于数组的 API 与基于 collection 的 API 之间的桥梁。     * 更进一步说，此方法允许对输出数组的运行时类型上进行精确控制，在某些情况下，可以用来节省分配开销。      * 假定 x 是只包含字符串的一个已知列表。以下代码可用来将该列表转储到一个新分配的 String 数组：      * String[] y = x.toArray(new String[0]);注意，toArray(new Object[0]) 和 toArray() 在功能上是相同的。      *      */    @SuppressWarnings("unchecked")    public <T> T[] toArray(T[] a) {        //如果指定数组长度不能容纳列表，配具有指定数组的运行时类型和列表大小的新数组        //a.getClass().getComponentType():如果此类是数组，则返回表示此类组件类型的 Class        if (a.length < size)            a = (T[])java.lang.reflect.Array.newInstance(                                a.getClass().getComponentType(), size);        int i = 0;        //不知道为什么将a转给result后再赋值        //数组是对象引用，修改a和result的结果是一样的        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());    }}