Iterator：迭代器实现？ArrayList, Linked

• ArrayList中迭代器的实现
  • Iterator的实现

    /**
     * ArrayList中迭代器Iterator的实现
     */
    private class Itr implements Iterator<E> {
        int cursor;       // 游标，下个元素的索引
        int lastRet = -1; // 当前元素的索引
        int expectedModCount = modCount; 

        Itr() {}

        public boolean hasNext() {
            return cursor != size; // 如果游标<数组的长度，则还有元素
        }

        @SuppressWarnings("unchecked")
        public E next() {
            // 删除了校验代码
            cursor = i + 1; //
            return (E) elementData[lastRet = i];
        }

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

        @Override
        @SuppressWarnings("unchecked") 
        //遍历数组，并对数组中的每个元素按照consumer中的动作进行处理
        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;
            
            while (i != size && modCount == expectedModCount) {
                //将元素e传入闭包consumer中进行处理
                consumer.accept((E) elementData[i++]);
            }
            cursor = i;
            lastRet = i - 1;
            checkForComodification();
        }

        
    }

• ListIterator的实现

    /**
     * An optimized version of AbstractList.ListItr
     */
    private class ListItr extends Itr implements ListIterator<E> {
        ListItr(int index) {
            super();
            cursor = index;
        }
            
        public boolean hasPrevious() {
            return cursor != 0;
        }

        // nextIndex == cursor
        public int nextIndex() {
            return cursor;
        }

        // previousIndex = cursor - 1 不使用lastRet是因为父类remove的时候lastRet会被置位-1
        public int previousIndex() {
            return cursor - 1;
        }

        @SuppressWarnings("unchecked")
        public E previous() {
            // 删除了校验的代码，只剩下核心代码
            int i = cursor - 1;
            Object[] elementData = ArrayList.this.elementData;
            cursor = i;
            return (E) elementData[lastRet = i];
        }

        public void set(E e) {
            //设置元素
            try {
                ArrayList.this.set(lastRet, e);
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        public void add(E e) {
            checkForComodification();

            try {
                int i = cursor;
                ArrayList.this.add(i, e);
                cursor = i + 1;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }
    }

• LinkedList

private class ListItr implements ListIterator<E> {
        private Node<E> lastReturned;
        private Node<E> next;
        private int nextIndex;
        private int expectedModCount = modCount;

        ListItr(int index) {
            // assert isPositionIndex(index);
            next = (index == size) ? null : node(index);
            nextIndex = index;
        }

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

        public E next() {
            checkForComodification();
            if (!hasNext())
                throw new NoSuchElementException();

            lastReturned = next;
            next = next.next;
            nextIndex++;
            return lastReturned.item;
        }

        public boolean hasPrevious() {
            return nextIndex > 0;
        }

        public E previous() {
            checkForComodification();
            if (!hasPrevious())
                throw new NoSuchElementException();

            lastReturned = next = (next == null) ? last : next.prev;
            nextIndex--;
            return lastReturned.item;
        }

        public int nextIndex() {
            return nextIndex;
        }

        public int previousIndex() {
            return nextIndex - 1;
        }

        public void remove() {
            checkForComodification();
            if (lastReturned == null)
                throw new IllegalStateException();

            Node<E> lastNext = lastReturned.next;
            unlink(lastReturned);
            if (next == lastReturned)
                next = lastNext;
            else
                nextIndex--;
            lastReturned = null;
            expectedModCount++;
        }

        public void set(E e) {
            if (lastReturned == null)
                throw new IllegalStateException();
            checkForComodification();
            lastReturned.item = e;
        }

        public void add(E e) {
            checkForComodification();
            lastReturned = null;
            if (next == null)
                linkLast(e);
            else
                linkBefore(e, next);
            nextIndex++;
            expectedModCount++;
        }

        public void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            while (modCount == expectedModCount && nextIndex < size) {
                action.accept(next.item);
                lastReturned = next;
                next = next.next;
                nextIndex++;
            }
            checkForComodification();
        }

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