ArrayList源码分析2

主要谈下fail-fast机制。
fail-fast机制是一种迭代器在迭代集合的时候，不允许对集合的结构发生改变，如果改变，那么就报错。并发修改异常ConcurrentModificationException。

private class Itr implements Iterator<E> {
    
    
        int cursor;       // 返回下一个元素的位置
        int lastRet = -1; // 返回最后一个元素的位置。-1表示没有
        //expectedModCount 是期望值。也就是集合的期望修改的值。ArrayList每次元素增加或者减少都会变化modCount。
        int expectedModCount = modCount;

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

        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
        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();
            }
        }
		//检查如果modCount！=expectedModCount。就开始抛出异常
        final void checkForComodification() {
    
    
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }