JDK源码阅览之AbstractList

JDK源码阅读之AbstractList

此类提供 List 接口的骨干实现，以最大限度地减少实现“随机访问”数据存储（如数组）支持的该接口所需的工作。对于连续的访问数据（如链表），应优先使用AbstractSequentialList，而不是此类。与其他抽象 collection 实现不同，编程人员不必 提供迭代器实现；迭代器和列表迭代器由此类在以下“随机访问”方法上实现：get(int)、set(int, E)、add(int, E) 和remove(int)，下面看看其实现。

//继承于AbstractCollection接口,实现了List接口
public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {
    protected AbstractList() {//提供一个空的构造函数
    }

    public boolean add(E e) {//在尾部添加元素e
        add(size(), e);//在尾部执行添加操作
        return true;
    }
    //抽象方法
    abstract public E get(int index);
    //未实现
    public E set(int index, E element) {
        throw new UnsupportedOperationException();
    }
    //未实现
    public void add(int index, E element) {
        throw new UnsupportedOperationException();
    }
    //未实现
    public E remove(int index) {
        throw new UnsupportedOperationException();
    }
    //从头开始，定位值为o的元素
    public int indexOf(Object o) {
        ListIterator<E> it = listIterator();//获得双向迭代器
        if (o==null) {//如果o为空元素
            while (it.hasNext())//迭代器遍历
                if (it.next()==null)//元素为空
                    return it.previousIndex();//返回其编号
        } else {//如果元素为o
            while (it.hasNext())
                if (o.equals(it.next()))
                    return it.previousIndex();
        }
        return -1;
    }
    //从尾开始，定位值为o的元素
    public int lastIndexOf(Object o) {
        ListIterator<E> it = listIterator(size());//获得双向迭代器，迭代器是从最后一个元素开始的
        if (o==null) {//如果o为空元素
            while (it.hasPrevious())//迭代器遍历
                if (it.previous()==null)//元素为空
                    return it.nextIndex();//返回其编号
        } else {
            while (it.hasPrevious())
                if (o.equals(it.previous()))//比较内容
                    return it.nextIndex();//返回其编号
        }
        return -1;
    }

    public void clear() {//清除所有元素
        removeRange(0, size());
    }

    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);//参数有效性检查
        boolean modified = false;
        for (E e : c) {
            add(index++, e);//尾部添加元素
            modified = true;
        }
        return modified;
    }

    public Iterator<E> iterator() {//返回单向迭代器
        return new Itr();
    }

    public ListIterator<E> listIterator() {//返回双向迭代器
        return listIterator(0);
    }

    public ListIterator<E> listIterator(final int index) {//返回第index位置开始的迭代器
        rangeCheckForAdd(index);//参数有效性检查
        return new ListItr(index);//返回双向迭代器
    }
    //单向迭代器的实现，继承了Iterator接口
    private class Itr implements Iterator<E> {
        int cursor = 0;//迭代器当期标记位
        int lastRet = -1;//上次访问的迭代器的标记位
        int expectedModCount = modCount;//迭代器迭代期间修改的次数

        public boolean hasNext() {//是否还有元素
            return cursor != size();//判断标记位是否到达列表末尾
        }
        //返回下一个元素
        public E next() {
            checkForComodification();//检查在迭代期间列表大小是否发生变化
            try {
                int i = cursor;//当前标记位
                E next = get(i);//通过列表的get函数获得元素
                lastRet = i;//记录老的标记位
                cursor = i + 1;//标记位+1
                return next;
            } catch (IndexOutOfBoundsException e) {
                checkForComodification();//检查迭代期间是否发生变化，如果发生变化了，会优先抛出并发修改的异常
                throw new NoSuchElementException();
            }
        }
        //删除当前标记位标记的元素
        public void remove() {
            if (lastRet < 0)//参数有效行检查，从这里也可以看出，删除只能删除迭代过的元素
                throw new IllegalStateException();
            checkForComodification();

            try {
                AbstractList.this.remove(lastRet);//调用父类的删除元素接口
                if (lastRet < cursor)
                    cursor--;//当前元素删除之后，当前标记位回退1
                lastRet = -1;//老的标记位置位
                expectedModCount = modCount;//修正修改次数
            } catch (IndexOutOfBoundsException e) {
                throw new ConcurrentModificationException();
            }
        }
        //并发修改检查
        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }
    //双向迭代器，继承了单向迭代器，实现了列表迭代器接口
    private class ListItr extends Itr implements ListIterator<E> {
        ListItr(int index) {//通过索引位来初始化
            cursor = index;//当前标记位置为index
        }
        //是否有前一个元素
        public boolean hasPrevious() {
            return cursor != 0;
        }
        //返回前一个元素
        public E previous() {
            checkForComodification();//并发检查
            try {
                int i = cursor - 1;
                E previous = get(i);
                lastRet = cursor = i;//当前标记位，lastRet永远缓存前一个迭代过的元素
                return previous;
            } catch (IndexOutOfBoundsException e) {
                checkForComodification();
                throw new NoSuchElementException();
            }
        }
        //返回当前标记位
        public int nextIndex() {
            return cursor;
        }
        //返回前一个标记位
        public int previousIndex() {
            return cursor-1;
        }
        //设置元素内容
        public void set(E e) {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                AbstractList.this.set(lastRet, e);//设置元素内容
                expectedModCount = modCount;//并发修改修正
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }
        //添加元素
        public void add(E e) {
            checkForComodification();

            try {
                int i = cursor;
                AbstractList.this.add(i, e);//添加元素
                lastRet = -1;//之前迭代过的元素失效
                cursor = i + 1;//前向迭代器向前移动
                expectedModCount = modCount;//并发修改修正
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }
    }
    //返回列表视图，返回从fromIndex开始到toIndex结束的列表视图，按列表的属性，返回不同的列表
    public List<E> subList(int fromIndex, int toIndex) {
        return (this instanceof RandomAccess ?new RandomAccessSubList<>(this, fromIndex, toIndex) :
                new SubList<>(this, fromIndex, toIndex));
    }
    //比较List，内部是通过迭代器来比较的
    public boolean equals(Object o) {
        if (o == this)
            return true;
        if (!(o instanceof List))
            return false;

        ListIterator<E> e1 = listIterator();
        ListIterator e2 = ((List) o).listIterator();
        while (e1.hasNext() && e2.hasNext()) {
            E o1 = e1.next();
            Object o2 = e2.next();
            if (!(o1==null ? o2==null : o1.equals(o2)))
                return false;
        }
        return !(e1.hasNext() || e2.hasNext());
    }
    //Time31算法
    public int hashCode() {
        int hashCode = 1;
        for (E e : this)
            hashCode = 31*hashCode + (e==null ? 0 : e.hashCode());
        return hashCode;
    }
    //删除元素，通过迭代器删除
    protected void removeRange(int fromIndex, int toIndex) {
        ListIterator<E> it = listIterator(fromIndex);
        for (int i=0, n=toIndex-fromIndex; i<n; i++) {
            it.next();
            it.remove();
        }
    }

    protected transient int modCount = 0;
    //参数有效性检查
    private void rangeCheckForAdd(int index) {
        if (index < 0 || index > size())
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size();
    }
}
//子列表，继承了AbstractList
class SubList<E> extends AbstractList<E> {
    private final AbstractList<E> l;//内部通过AbstractList来存储元素
    private final int offset;//偏移量
    private int size;//元素个数

    SubList(AbstractList<E> list, int fromIndex, int toIndex) {
        if (fromIndex < 0)
            throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
        if (toIndex > list.size())
            throw new IndexOutOfBoundsException("toIndex = " + toIndex);
        if (fromIndex > toIndex)
            throw new IllegalArgumentException("fromIndex(" + fromIndex +
                                               ") > toIndex(" + toIndex + ")");
        l = list;
        offset = fromIndex;
        size = toIndex - fromIndex;
        this.modCount = l.modCount;
    }
    //第index位添加元素
    public E set(int index, E element) {
        rangeCheck(index);
        checkForComodification();
        return l.set(index+offset, element);
    }
    //获取第index个元素
    public E get(int index) {
        rangeCheck(index);
        checkForComodification();
        return l.get(index+offset);
    }
    //返回元素个数
    public int size() {
        checkForComodification();
        return size;
    }
    //第index位置添加元素
    public void add(int index, E element) {
        rangeCheckForAdd(index);//参数有效性检查
        checkForComodification();//并发修改检查
        l.add(index+offset, element);//按偏移量修正index，之后添加元素
        this.modCount = l.modCount;//修改并发修改量
        size++;//元素个数+1
    }
    //删除第index个元素
    public E remove(int index) {
        rangeCheck(index);
        checkForComodification();
        E result = l.remove(index+offset);
        this.modCount = l.modCount;
        size--;
        return result;
    }
    //删除元素
    protected void removeRange(int fromIndex, int toIndex) {
        checkForComodification();
        l.removeRange(fromIndex+offset, toIndex+offset);
        this.modCount = l.modCount;
        size -= (toIndex-fromIndex);
    }
    //尾部添加元素
    public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }

    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);
        int cSize = c.size();
        if (cSize==0)
            return false;

        checkForComodification();
        l.addAll(offset+index, c);
        this.modCount = l.modCount;
        size += cSize;
        return true;
    }
    //返回列表迭代器
    public Iterator<E> iterator() {
        return listIterator();
    }
    //返回列表迭代器，这里返回的是经过偏移量修改的列表迭代器，里面的实现和上面描述的一致，这里不详术
    public ListIterator<E> listIterator(final int index) {
        checkForComodification();
        rangeCheckForAdd(index);

        return new ListIterator<E>() {
            private final ListIterator<E> i = l.listIterator(index+offset);

            public boolean hasNext() {
                return nextIndex() < size;
            }

            public E next() {
                if (hasNext())
                    return i.next();
                else
                    throw new NoSuchElementException();
            }

            public boolean hasPrevious() {
                return previousIndex() >= 0;
            }

            public E previous() {
                if (hasPrevious())
                    return i.previous();
                else
                    throw new NoSuchElementException();
            }

            public int nextIndex() {
                return i.nextIndex() - offset;
            }

            public int previousIndex() {
                return i.previousIndex() - offset;
            }

            public void remove() {
                i.remove();
                SubList.this.modCount = l.modCount;
                size--;
            }

            public void set(E e) {
                i.set(e);
            }

            public void add(E e) {
                i.add(e);
                SubList.this.modCount = l.modCount;
                size++;
            }
        };
    }

    public List<E> subList(int fromIndex, int toIndex) {
        return new SubList<>(this, fromIndex, toIndex);
    }

    private void rangeCheck(int index) {
        if (index < 0 || index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    private void rangeCheckForAdd(int index) {
        if (index < 0 || index > size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

    private void checkForComodification() {
        if (this.modCount != l.modCount)
            throw new ConcurrentModificationException();
    }
}
//随机列表,继承了SubList
class RandomAccessSubList<E> extends SubList<E> implements RandomAccess {
    RandomAccessSubList(AbstractList<E> list, int fromIndex, int toIndex) {
        super(list, fromIndex, toIndex);
    }

    public List<E> subList(int fromIndex, int toIndex) {
        return new RandomAccessSubList<>(this, fromIndex, toIndex);
    }
}