迭代器

  • ArrayList有返回迭代器的方法

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    public Iterator<E> iterator() {
        return new Itr();
    }

  • Itr实现Iterator接口

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    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;

        // prevent creating a synthetic constructor
        Itr() {}

        public boolean hasNext();   //判断是否还有值
        public E next() ；          //返回下一个
        public void remove() ；     //删除
        public void forEachRemaining();
        final void checkForComodification();

hasNext()

  • 只要cursor不等于size还存在元素
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    public boolean hasNext() {
        return cursor != size;
    }

next()

  • cursor一开始默认是0的，在数组的首元素开始
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     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];
    }

remove()

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  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();
    }
}

forEachRemaining()

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@Override
public void forEachRemaining(Consumer<? super E> action) {
    Objects.requireNonNull(action);
    final int size = ArrayList.this.size;
    int i = cursor;
    if (i < size) {
        final Object[] es = elementData;
        if (i >= es.length)
            throw new ConcurrentModificationException();
        for (; i < size && modCount == expectedModCount; i++)
            action.accept(elementAt(es, i));
        // update once at end to reduce heap write traffic
        cursor = i;
        lastRet = i - 1;
        checkForComodification();
    }
}

checkForComodification()

  • modCount只要不被修改，代表原来容器没被修改，那么使用已经创建的迭代器进行操作也是允许的
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    final void checkForComodification() {
        if (modCount != expectedModCount)
            throw new ConcurrentModificationException();
    }