for(:)增强，Iterator迭代器

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我是靠谱客的博主留胡子寒风，这篇文章主要介绍for(:)增强，Iterator迭代器，现在分享给大家，希望可以做个参考。

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1、在JDK5之前对于数组或者list的遍历需要根据长度，以及下标进行遍历于是就有了其遍历方式

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List<String> list = new ArrayList<String>();
for (int index = 0; index < list.size(); index++ ) {
    String str = list.get(index);
    System.out.println("str: " + str);
}

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2、在JDK5之后引入了：增强特性使list的遍历，进行改变

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List<String> list = new ArrayList<String>();
for (String str : list){
    System.out.println("str:" + str);
}

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3、反编译class文件可以看到增强for循环会被编译器自动处理如下代码：

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List<String> list = new ArrayList<String>();
Iterator var4 = list.iterator();

while(var4.hasNext()) {
    str = (String)var4.next();
    System.out.println("str:" + str);
}

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即JDK在编译java文件的时候会将：符号以JDK的特性进行转换成list的迭代器，接着通过获取迭代器的下一元素处理数据

在ArrayList类的里面存着一个内部类Itr 实现了Iterator的接口

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

/**
     * An optimized version of AbstractList.Itr
     */
    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();
            }
        }

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

    /**
     * An optimized version of AbstractList.Itr
     */
    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();
            }
        }