为什么java中有两种不同的for循环? [英] why are there two different kinds of for loops in java?

问题描述

我有一个 for 循环，但我不确定它是如何工作的.我熟悉:

I have a for loop, and I am not sure how it works. I am familiar with:

for(int i = 0; i <= 9; i++)
{
    /* implementation */
}

我对以下形式的 for 循环感到困惑:

I am confused about a for loop in the following form:

String[] myString = new String[] {"one", "two", "three", "some other stuff"};

String str1 = "", str2 = "";  

for (String s :  myString)
{
    /* implementation */
}

这些类型的 for 循环如何工作?它们与常规 for 循环有何不同?

How do these types of for loops work? what do they do differently then regular for loops?

推荐答案

第一个是原来的 for 循环.你初始化一个变量，设置一个终止条件，并提供一个状态递增/递减计数器(有例外，但这是经典)

The first is the original for loop. You initialize a variable, set a terminating condition, and provide a state incrementing/decrementing counter (There are exceptions, but this is the classic)

为此，

for (int i=0;i<myString.length;i++) { 
  System.out.println(myString[i]); 
}

是正确的.

对于 Java 5，提出了一个替代方案.任何实现 iterable 的东西都可以得到支持.这在 Collections 中特别好.例如，您可以像这样迭代列表

For Java 5 an alternative was proposed. Any thing that implements iterable can be supported. This is particularly nice in Collections. For example you can iterate the list like this

List<String> list = ....load up with stuff

for (String string : list) {
  System.out.println(string);
}

代替

for (int i=0; i<list.size();i++) {
  System.out.println(list.get(i));
}

所以它实际上只是一种替代符号.任何实现 Iterable(即可以返回迭代器)的项都可以这样编写.

So it's just an alternative notation really. Any item that implements Iterable (i.e. can return an iterator) can be written that way.

幕后发生的事情是这样的:(效率更高，但我写得很明确)

What's happening behind the scenes is somethig like this: (more efficient, but I'm writing it explicitly)

Iterator<String> it = list.iterator();
while (it.hasNext()) {
  String string=it.next();
  System.out.println(string);
}

最后它只是语法糖，但相当方便.

In the end it's just syntactic sugar, but rather convenient.

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