java Iterator源码

Iterator接口

java的Iterator是个接口,其定义了四个方法.主要是hasNext,Next方法.

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接口的实现

比如在ArrayList中有个内部类Itr实现了Iterator接口,如下

 private class Itr implements Iterator<E> {
 		//下一个元素的索引位置
        int cursor;      
        //上一个元素的索引位置,初始值为-1
        int lastRet = -1;
        int expectedModCount = modCount;

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

        public boolean hasNext() {
        //szie为ArrayList的大小,因为Itr是内部类,所以可以访问ArrayList的size field
        //如果索引的数组不等于size的大小,就返回false,表明还有下一个
            return cursor != size;
        }


        public E next() {
            checkForComodification();
            int i = cursor;
            //如果索引的值大于ArrayList的元素数量,抛出异常
            if (i >= size)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            //如果索引大于数组容量,报并发异常,why?
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
                //游标往后移动一位,为后面做准备
            cursor = i + 1;
            //返回第i个元素
            return (E) elementData[lastRet = i];
        }

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

            try {
            //移除第i号元素
                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();
            }
        }

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

本文地址：https://blog.csdn.net/u010711495/article/details/109576379