第一部分：
For-each Loop
Purpose
The basic for loop was extended in Java5 to make iteration over arrays and other collections more convenient. This newer for statement is called the enhanced for or for-each (because it is called this in other programming languages). I‘ve also heard it called the for-in loop.
Use it in preference to the standard for loop if applicable (see last section below) because it‘s much more readable.
Series of values. The for-each loop is used to access each successive value in a collection of values.
Arrays and Collections. It‘s commonly used to iterate over an array or a Collections class (eg, ArrayList).
Iterable<E>. It can also iterate over anything that implements the Iterable<E> interface (must defineiterator() method). Many of the Collections classes (eg, ArrayList) implement Iterable<E>, which makes thefor-each loop very useful. You can also implement Iterable<E> for your own data structures.
General Form
The for-each and equivalent for statements have these forms. The two basic equivalent forms are given, depending one whether it is an array or an Iterable that is being traversed. In both cases an extra variable is required, an index for the array and an iterator for the collection.
这里我们只要知道下面的事实就好了：
- For-each语法内部，对collection是用nested iteratoration来实现的，对数组是用下标遍历来实现。
- Java 5 及以上的编译器隐藏了基于iteration和下标遍历的内部实现。（注意，这里说的是“Java编译器”或Java语言对其实现做了隐藏，而不是某段Java代码对其实现做了隐藏，也就是说，我们在任何一段JDK的Java代码中都找不到这里被隐藏的实现。这里的实现，隐藏在了Java 编译器中，我们可能只能像这篇帖子中说的那样，查看一段For-each的Java代码编译成的字节码，从中揣测它到底是怎么实现的了）
下面对“For-each”和“其对等的iteration/index实现”的对比再简洁明了不过了。
| For-each loop | Equivalent for loop |
|---|---|
for (type var : arr) {
body-of-loop
}
|
for (int i = 0; i < arr.length; i++) {
type var = arr[i];
body-of-loop
}
|
for (type var : coll) {
body-of-loop
}
|
for (Iterator<type> iter = coll.iterator(); iter.hasNext(); ) {
type var = iter.next();
body-of-loop
}
|
Example - Adding all elements of an array
Here is a loop written as both a for-each loop and a basic for loop.
double[] ar = {1.2, 3.0, 0.8};
int sum = 0;
for (double d : ar) { // d gets successively each value in ar.
sum += d;
}
And here is the same loop using the basic for. It requires an extra iteration variable.
double[] ar = {1.2, 3.0, 0.8};
int sum = 0;
for (int i = 0; i < ar.length; i++) { // i indexes each element successively.
sum += ar[i];
}
Where the for-each is appropriate
一定要注意For-each不是万能的，下面的场合是不适宜使用For-each的
Altho the enhanced for loop can make code much clearer, it can‘t be used in some common situations.
使用For-each时对collection或数组中的元素不能做赋值操作
- Only access. Elements can not be assigned to, eg, not to increment each element in a collection.
同时只能遍历一个collection或数组，不能同时遍历多余一个collection或数组
- Only single structure. It‘s not possible to traverse two structures at once, eg, to compare two arrays.
遍历过程中，collection或数组中同时只有一个元素可见，即只有“当前遍历到的元素”可见，而前一个或后一个元素是不可见的。
- Only single element. Use only for single element access, eg, not to compare successive elements.
只能正向遍历，不能反向遍历（相比之下，C++ STL中还有reverse_iterator, rbegin(), rend()之类的东西，可以反向遍历）
- Only forward. It‘s possible to iterate only forward by single steps.
如果要兼容Java 5之前的Java版本，就不能使用For-each
- At least Java 5. Don‘t use it if you need compatibility with versions before Java 5.
此文章来自：http://blog.csdn.net/yasi_xi/article/details/25482173#t1
第二部分：
先看一下这么一段代码：
public static void main(String[] args)
{
List<String> list = new ArrayList<String>();
list.add("111");
list.add("222");
for (String str : list)
{
System.out.println(str);
}
}
用foreach循环去遍历这个list，结果就不说了，都知道。看一下Java是如何处理这个foreach循环的，javap反编译一下：
d:\MyEclipse\TestArticle\bin\test29>javap -verbose TestMain.class
反编译出来的内容很多，有类信息、符号引用、字节码信息，截取一段信息：
1 public static void main(java.lang.String[]); 2 flags: ACC_PUBLIC, ACC_STATIC 3 Code: 4 stack=2, locals=4, args_size=1 5 0: new #16 // class java/util/ArrayList 6 3: dup 7 4: invokespecial #18 // Method java/util/ArrayList."<in 8 it>":()V 9 7: astore_1 10 8: aload_1 11 9: ldc #19 // String 111 12 11: invokeinterface #21, 2 // InterfaceMethod java/util/List. 13 add:(Ljava/lang/Object;)Z 14 16: pop 15 17: aload_1 16 18: ldc #27 // String 222 17 20: invokeinterface #21, 2 // InterfaceMethod java/util/List. 18 add:(Ljava/lang/Object;)Z 19 25: pop 20 26: aload_1 21 27: invokeinterface #29, 1 // InterfaceMethod java/util/List. 22 iterator:()Ljava/util/Iterator;
看不懂没关系，new、dup、invokespecial这些本来就是字节码指令表内定义的指令，虚拟机会根据这些指令去执行指定的C++代码，完成每个指令的功能。关键看到21、22这两行就可以了，看到了一个iterator，所以得出结论：在编译的时候编译器会自动将对for这个关键字的使用转化为对目标的迭代器的使用，这就是foreach循环的原理。进而，我们再得出两个结论：
1、ArrayList之所以能使用foreach循环遍历，是因为ArrayList所有的List都是Collection的子接口，而Collection是Iterable的子接口，ArrayList的父类AbstractList正确地实现了Iterable接口的iterator方法。之前我自己写的ArrayList用foreach循环直接报空指针异常是因为我自己写的ArrayList并没有实现Iterable接口
2、任何一个集合，无论是JDK提供的还是自己写的，只要想使用foreach循环遍历，就必须正确地实现Iterable接口
实际上，这种做法就是23中设计模式中的迭代器模式。
数组呢？
上面的讲完了，好理解，但是不知道大家有没有疑问，至少我是有一个疑问的：数组并没有实现Iterable接口啊，为什么数组也可以用foreach循环遍历呢？先给一段代码，再反编译：
public static void main(String[] args)
{
int[] ints = {1,2,3,4,5};
for (int i : ints)
System.out.println(i);
}
同样反编译一下，看一下关键的信息：
1 0: iconst_2 2 1: newarray int 3 3: dup 4 4: iconst_0 5 5: iconst_1 6 6: iastore 7 7: dup 8 8: iconst_1 9 9: iconst_2 10 10: iastore 11 11: astore_1 12 12: aload_1 13 13: dup 14 14: astore 5 15 16: arraylength 16 17: istore 4 17 19: iconst_0 18 20: istore_3 19 21: goto 39 20 24: aload 5 21 26: iload_3 22 27: iaload 23 28: istore_2 24 29: getstatic #16 // Field java/lang/System.out:Ljav 25 a/io/PrintStream; 26 32: iload_2 27 33: invokevirtual #22 // Method java/io/PrintStream.prin 28 tln:(I)V 29 36: iinc 3, 1 30 39: iload_3 31 40: iload 4 32 42: if_icmplt 24 33 45: return
这是完整的这段main函数对应的45个字节码指令，因为这涉及一些压栈、出栈、推送等一些计算机原理性的内容且对于这些字节码指令的知识的理解需要一些C++的知识，所以就不解释了。简单对照字节码指令表之后，我个人对于这45个字节码的理解是Java将对于数组的foreach循环转换为对于这个数组每一个的循环引用。
第二部分内容来自：http://www.cnblogs.com/xrq730/p/4868465.html