Iterator、Iteratable与ListIterator

• Iteratable:

public interface Iterable<T> {

    Iterator<T> iterator();

    default void forEach(Consumer<? super T> action) {
        Objects.requireNonNull(action);
        for (T t : this) {
            action.accept(t);
        }
    }

    default Spliterator<T> spliterator() {
        return Spliterators.spliteratorUnknownSize(iterator(), 0);
    }
}

• • Iteratable接口提供了iterator()方法。
• • Collection接口继承了Iteratable，由实现Collection的ArrayList、Hashset等来实现方法。

• Iterator：

public interface Iterator<E> {
 
    boolean hasNext();

    E next();

    default void remove() {
        throw new UnsupportedOperationException("remove");
    }

    default void forEachRemaining(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        while (hasNext())
            action.accept(next());
    }
}

• • Iterator接口在ArrayList、LinkedList等类中都有内部类实现
• • Iterator接口的不当使用会导致抛出ConcurrentModificationException:

    List<String> famousList = new ArrayList<>();
    famousList.add("Sheldon");
    famousList.add("Sherlock");
    famousList.add("Batman");
    famousList.add("Optimus Prime");
        
    for (String famous :
        famousList) {
        famousList.remove(famous);
    }

上面的代码之所以会抛出ConcurrentModificationException异常的原因：

• 增强for循环其实在编译生成字节码后会发现，就是转化为通过调用iterator的hasNext()、next()函数来遍历集合，查看ArrayList的iterator的实现代码：

    private class Itr implements Iterator<E> {
        int cursor;       // index of next element to return
        int lastRet = -1; // index of last element returned; -1 if no such
        int expectedModCount = modCount;

        public boolean hasNext() {
            return cursor != size;
        }

        @SuppressWarnings("unchecked")
        public E next() {
            checkForComodification();
            int i = cursor;
            if (i >= size)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i + 1;
            return (E) elementData[lastRet = i];
        }

        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                ArrayList.this.remove(lastRet);
                cursor = lastRet;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        @Override
        @SuppressWarnings("unchecked")
        public void forEachRemaining(Consumer<? super E> consumer) {
            Objects.requireNonNull(consumer);
            final int size = ArrayList.this.size;
            int i = cursor;
            if (i >= size) {
                return;
            }
            final Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length) {
                throw new ConcurrentModificationException();
            }
            while (i != size && modCount == expectedModCount) {
                consumer.accept((E) elementData[i++]);
            }
            // update once at end of iteration to reduce heap write traffic
            cursor = i;
            lastRet = i - 1;
            checkForComodification();
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

从以上代码可知，当调用next()函数的时候，会先调用checkForComodification函数来检查集合的modCount与expectedModCount是否相等，若不相等则抛出错误。来到这里就能知道，在使用iterator遍历集合的时候，使用集合的remove、add等函数，就会导致modCount和expectedModCount不一致，从而导致异常抛出。

• 正确的做法：

    Iterator<String> iterator = famousList.iterator();
    for (;iterator.hasNext();){
        String famous=iterator.next();
        System.out.println(famous);
        if ("Batman".equals(famous)){
            iterator.remove();
            break;
        }
    }
    System.out.println(famousList.size());

或者：

    for (int i = 0; i < famousList.size(); i++) {
        System.out.println(famousList.get(i));
        if ("Batman".equals(famousList.get(i))){
            famousList.remove(famousList.get(i));
            break;
        }
    }
    System.out.println(famousList.size());

• ListIterator：

public interface ListIterator<E> extends Iterator<E> {

    boolean hasNext();
    
    E next();

    //判断cursor前是否有元素
    boolean hasPrevious();

    //获得cursor前一个元素，并且cursor后退一位
    E previous();

    //返回cursor元素的index
    int nextIndex();

    //返回cursor前一个元素的index
    int previousIndex();

    void remove();

    //更新上一次调用next、previous返回的元素，也就是iterator最后一次操作的元素,没有调用next、previous前调用的话会抛出IllegalStateExceptiony异常
    void set(E e);

    //向cursor前插入元素
    void add(E e);

}

• ListIterator由实现List接口的集合类通过以下两种方法返回：

    public ListIterator<E> listIterator() {
        return new ListItr(0);
    }

    //返回指定cursor位置的listIterator
    public ListIterator<E> listIterator(int index) {
        if (index < 0 || index > size)
            throw new IndexOutOfBoundsException("Index: "+index);
        return new ListItr(index);
    }

相比Iterator，多了add、set以及previous等方法。