why are there two different kinds of for loops in java?

Question

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 */
}

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 */
}

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

Answer 1

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)

For that,

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

is correct.

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);
}

instead of

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

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

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);
}

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