android 集合架构四- LinkedList

直接上源码，通过阅读源码，我们自然就知道LinkedList实现的原理

private static final class Link<ET> {
        ET data;

        Link<ET> previous, next;

        Link(ET o, Link<ET> p, Link<ET> n) {
            data = o;
            previous = p;
            next = n;
        }
    }

使用结点来存储数据

private static final class LinkIterator<ET> implements ListIterator<ET> {
        int pos, expectedModCount;

        final LinkedList<ET> list;

        Link<ET> link, lastLink;

        LinkIterator(LinkedList<ET> object, int location) {
            list = object;
            expectedModCount = list.modCount;
            if (location >= 0 && location <= list.size) {
                // pos ends up as -1 if list is empty, it ranges from -1 to
                // list.size - 1
                // if link == voidLink then pos must == -1
                link = list.voidLink;
                if (location < list.size / 2) {
                    for (pos = -1; pos + 1 < location; pos++) {
                        link = link.next;
                    }
                } else {
                    for (pos = list.size; pos >= location; pos--) {
                        link = link.previous;
                    }
                }
            } else {
                throw new IndexOutOfBoundsException();
            }
        }

        public void add(ET object) {
            if (expectedModCount == list.modCount) {
                Link<ET> next = link.next;
                Link<ET> newLink = new Link<ET>(object, link, next);
                link.next = newLink;
                next.previous = newLink;
                link = newLink;
                lastLink = null;
                pos++;
                expectedModCount++;
                list.size++;
                list.modCount++;
            } else {
                throw new ConcurrentModificationException();
            }
        }

        public boolean hasNext() {
            return link.next != list.voidLink;
        }

        public boolean hasPrevious() {
            return link != list.voidLink;
        }

        public ET next() {
            if (expectedModCount == list.modCount) {
                LinkedList.Link<ET> next = link.next;
                if (next != list.voidLink) {
                    lastLink = link = next;
                    pos++;
                    return link.data;
                }
                throw new NoSuchElementException();
            }
            throw new ConcurrentModificationException();
        }

        public int nextIndex() {
            return pos + 1;
        }

        public ET previous() {
            if (expectedModCount == list.modCount) {
                if (link != list.voidLink) {
                    lastLink = link;
                    link = link.previous;
                    pos--;
                    return lastLink.data;
                }
                throw new NoSuchElementException();
            }
            throw new ConcurrentModificationException();
        }

        public int previousIndex() {
            return pos;
        }

        public void remove() {
            if (expectedModCount == list.modCount) {
                if (lastLink != null) {
                    Link<ET> next = lastLink.next;
                    Link<ET> previous = lastLink.previous;
                    next.previous = previous;
                    previous.next = next;
                    if (lastLink == link) {
                        pos--;
                    }
                    link = previous;
                    lastLink = null;
                    expectedModCount++;
                    list.size--;
                    list.modCount++;
                } else {
                    throw new IllegalStateException();
                }
            } else {
                throw new ConcurrentModificationException();
            }
        }

        public void set(ET object) {
            if (expectedModCount == list.modCount) {
                if (lastLink != null) {
                    lastLink.data = object;
                } else {
                    throw new IllegalStateException();
                }
            } else {
                throw new ConcurrentModificationException();
            }
        }
    }

上面是实现了自己的遍历Iterator，有点类似指针

private class ReverseLinkIterator<ET> implements Iterator<ET> {
        private int expectedModCount;

        private final LinkedList<ET> list;

        private Link<ET> link;

        private boolean canRemove;

        ReverseLinkIterator(LinkedList<ET> linkedList) {
            list = linkedList;
            expectedModCount = list.modCount;
            link = list.voidLink;
            canRemove = false;
        }

        public boolean hasNext() {
            return link.previous != list.voidLink;
        }

        public ET next() {
            if (expectedModCount == list.modCount) {
                if (hasNext()) {
                    link = link.previous;
                    canRemove = true;
                    return link.data;
                }
                throw new NoSuchElementException();
            }
            throw new ConcurrentModificationException();

        }

        public void remove() {
            if (expectedModCount == list.modCount) {
                if (canRemove) {
                    Link<ET> next = link.previous;
                    Link<ET> previous = link.next;
                    next.next = previous;
                    previous.previous = next;
                    link = previous;
                    list.size--;
                    list.modCount++;
                    expectedModCount++;
                    canRemove = false;
                    return;
                }
                throw new IllegalStateException();
            }
            throw new ConcurrentModificationException();
        }
    }

此类是倒叙遍历，倒叙删除集合

/**
     * Constructs a new empty instance of {@code LinkedList}.
     */
    public LinkedList() {
        voidLink = new Link<E>(null, null, null);
        voidLink.previous = voidLink;
        voidLink.next = voidLink;
    }

给最后一个节点赋值

public LinkedList(Collection<? extends E> collection) {
        this();
        addAll(collection);
    }

默认添加一个集合主要还是看addAll 方法

@Override
    public boolean addAll(Collection<? extends E> collection) {
        int adding = collection.size();
        if (adding == 0) {
            return false;
        }
        Collection<? extends E> elements = (collection == this) ?
                new ArrayList<E>(collection) : collection;

        Link<E> previous = voidLink.previous;
        for (E e : elements) {
            Link<E> newLink = new Link<E>(e, previous, null);
            previous.next = newLink;
            previous = newLink;
        }
        previous.next = voidLink;
        voidLink.previous = previous;
        size += adding;
        modCount++;
        return true;
    }

添加集合，首先检验这个集合的长度，假如为0 返回false，取最后一个不为null的previous ，然后在 循环里面，new 一个新的newLink，把 newlink 赋值为 previous.next，然后再把newlink赋值给previous ，这样就能再下次循环中给newlink的next复制了，循环结束后，赋值最后一个为voidLink,此时size 以及相关变量也修改。

public void add(int location, E object) {
        if (location >= 0 && location <= size) {
            Link<E> link = voidLink;
            if (location < (size / 2)) {
                for (int i = 0; i <= location; i++) {
                    link = link.next;
                }
            } else {
                for (int i = size; i > location; i--) {
                    link = link.previous;
                }
            }
            Link<E> previous = link.previous;
            Link<E> newLink = new Link<E>(object, previous, link);
            previous.next = newLink;
            link.previous = newLink;
            size++;
            modCount++;
        } else {
            throw new IndexOutOfBoundsException();
        }
    }

首先要明白，LinkedList 是一环形的链表，最后的voidLink.next 指向首link，所以计算一下location的位置，是偏前还是偏后，假如是在前半列，直接使用voidLink的next 去寻找要添加位置当前的link，假如是在后半列，使用voidLink的 previous，这样可以减少寻找的次数。

public boolean add(E object) {
        return addLastImpl(object);
    }

    private boolean addLastImpl(E object) {
        Link<E> oldLast = voidLink.previous;
        Link<E> newLink = new Link<E>(object, oldLast, voidLink);
        voidLink.previous = newLink;
        oldLast.next = newLink;
        size++;
        modCount++;
        return true;
    }

默认添加数据，是添加在列表的末尾的位置

<pre name="code" class="java"> public boolean addAll(int location, Collection<? extends E> collection) {
        if (location < 0 || location > size) {
            throw new IndexOutOfBoundsException();
        }
        int adding = collection.size();
        if (adding == 0) {
            return false;
        }
        Collection<? extends E> elements = (collection == this) ?
                new ArrayList<E>(collection) : collection;

        Link<E> previous = voidLink;
        if (location < (size / 2)) {
            for (int i = 0; i < location; i++) {
                previous = previous.next;
            }
        } else {
            for (int i = size; i >= location; i--) {
                previous = previous.previous;
            }
        }
        Link<E> next = previous.next;
        for (E e : elements) {
            Link<E> newLink = new Link<E>(e, previous, null);
            previous.next = newLink;
            previous = newLink;
        }
        previous.next = next;
        next.previous = previous;
        size += adding;
        modCount++;
        return true;
    }

在特定位置添加集合，还是得找到此刻特定位置的link，逻辑和上面相同，略过...

public void addFirst(E object) {
        addFirstImpl(object);
    }

    private boolean addFirstImpl(E object) {
        Link<E> oldFirst = voidLink.next;
        Link<E> newLink = new Link<E>(object, voidLink, oldFirst);
        voidLink.next = newLink;
        oldFirst.previous = newLink;
        size++;
        modCount++;
        return true;
    }

在列表头加入数据，上面已经提过，此列表是一个环形的列表，应该利用 voieLink.next 就是第一条数据

@Override
    public void clear() {
        if (size > 0) {
            size = 0;
            voidLink.next = voidLink;
            voidLink.previous = voidLink;
            modCount++;
        }
    }

清理数据，只需要把voidLink 的previous 和next 赋值为null 就可以了

@Override
    public E remove(int location) {
        if (location >= 0 && location < size) {
            Link<E> link = voidLink;
            if (location < (size / 2)) {
                for (int i = 0; i <= location; i++) {
                    link = link.next;
                }
            } else {
                for (int i = size; i > location; i--) {
                    link = link.previous;
                }
            }
            Link<E> previous = link.previous;
            Link<E> next = link.next;
            previous.next = next;
            next.previous = previous;
            size--;
            modCount++;
            return link.data;
        }
        throw new IndexOutOfBoundsException();
    }

    @Override
    public boolean remove(Object object) {
        return removeFirstOccurrenceImpl(object);
    }

删除特定位置的元素，这个也是根据voidLink 来寻找在特定位置的数据

方法就不再一一列举了，LinkedList 原理是一个环形的闭合链表，并且我们是通过最后一条数据来寻找特定位置的数据，无论是增删改查都是通过voidLink这个元素来遍历元素的，和ArrsyList比较 查找数据比较慢，但是添加和删除数据是较快的。